T W 4 W W
Beretning Nr. 150
C. MUIILE LARSEN:
EXPERIMENTS WITH SOFTWOOD CUTTINGS OF FOREST TREES
(FORSØG MED URTEAGTIGE STIKLINGER AF SKOVTRÆER)
MEDDELELSE NR. 18 FRA SKOVTRÆFORÆDLINGEN, ARBORETET, HØRSHOLM.
(Særtryk af Det forstlige Forsøgsvæsen i Danmark, XVII.) MCMXLVI
'±1
Bd. XI. Nr. 96. C. H. BORNEBUSCH: The F a u n a of Forest Soil (Skovbundens Dyreverden), S. 1. — Nr. 98. A. OPPERMANN
og C. H. BORNEBUSCH: Nørholm Skov og Hede (La foret et la lande de Norholm), S. 257. — Nr. 99. Hedeskovenes Foryngelse I—II (Verjüngung der Heidewälder I—II), S. 361. — Nr. 100.
A. OPPERMANN: Lawsoniens Vækst i Danmark (Chamaecyparis Lawsoniana Pari. in Denmark), S. 377. — Nr. 101. A. O P P E R -
MANN: Bøgekvas (Reisholz der Rötbuche), S. 395.
Bd. XII. Nr. 104. A. OPPERMANN: Egens Træformer og Racer (Les configurations et races du chéne).
Bd. XIII, H. 1: Nr. 102. G. H. BORNEBUSCH: Dybtgaaende Jordbundsundersøgelser, Hedeskovenes Foryngelse III (Tiefgeh- ende Bodenuntersuchungen), S. 1. — Nr. 103. A. OPPERMANN:
Nordmannsgranens Vækst i Danmark (Abies Nordmanniana in Dänemark), S. 51. H. 2 : Nr. 105. C. H. BORNEBUSCH: Skovbunds- floraen i Mølleskoven (The flora in »Mølleskoven«), S. 57. — Nr. 106. F R . W E I S : Beplantningsforsøg paa et afføgent Sande (Boisement d'un terrain du sable mouvant éventé), S. 63. — Nr. 107. C. H. BORNEBUSCH: Et Udhugningsforsøg i Rødgran (Ein Durchforstungsversuch in Fichte), S. 117. — Nr. 108. MATH.
THOMSEN: Sprøjtemidler til Bekæmpelse af Chermes paa Ædel- gran (Spritzmitteln gegen Chermes auf Weisstannen), S. 215.
H. 3: Nr. 109. C. H. BORNEBUSCH og FOLKE H O L M : Kultur paa
trametesinnceret B u n d med forskellige Træarter (Replanting of areas infected with Polyporus annosus), S. 225. — Nr. 110.
C. MUHLE LARSEN: T O gamle fynske Egeprøveflader (Zwei alte Eichenprobeflächen auf Fünen), S. 2Ö5. H. 4 : Nr. 111. E. C. L.
LØFTING: Bjergfyrbevoksninger paa Hedebund og deres Foryn- gelse, Hedeskovenes Foryngelse IV (Mountain pine plantations in Jutland and their conversion into forests of more valuable tree-species), S. 305. H. 5: Nr. 112. C. H. BORNEBUSCH: Proveniens- forsøg med Rødgran (Ein Provenienzversuch mit Fichte), S. 325.
— Nr. 113. FOLKE H O L M : Abies grandis i D a n m a r k (Abies grandis in Denmark), S. 379. — Nr. 114. C. H. BORNEBUSCH: Forsøgs- væsenets Ordning og Ledelse, IX, S. 409.
Bd. XIV, H. 1: Nr. 115. E. G. L. L Ø F T I N G : Bevaring af storm- fældet Gran (Aufbewahrung von sturmgeschlagenem Fichten- holz), S. 1. — Nr. 116. POUL LARSEN: Regenererende Kulsyre- assimilation hos Askegrene (Regenerierende Kohlensäureassimi- lation bei Eschenästen), S. 13. — Nr. 117. C. H. BORNEBUSCH:
Thuja som dansk Skovtræ (Thuja plicata as a Danish Forest Tree), S. 53. H. 2 : Nr. 118. C. H. BORNEBUSCH: Sommerplant- ning af Naaletræer (Sommerpflanzung von Nadelhölzern), S. 97.
— Nr. 119. E. C. L. LØFTING: Rodfordærverangrebenes Betyd- ning for Sitkagrans Anvendelighed i Klitter og Heder, Hede- skovenes Foryngelse V (The significance of the attacks of Po- lyporus annosus to the suitability of the Sitka spruce for Dunes and Heaths), S. 133. — Nr. 120. C. H. BORNEBUSCH: Stormskaden paa Udhugningsforsøget i Hastrup Plantage (Sturmschaden in dem Hastruper Durchforstungsversuch), S. 161. — Nr. 121.
C. H. BORNEBUSCH: Iagttagelser over Rødgranens Naalefald (Chute
EXPERIMENTS WITH SOFTWOOD CUTTINGS OF FOREST TREES
BY
C. MUHLE LARSEN
Report No. 18 from the Institute of Forest Tree Breeding at the Arboretum.
I n 1940 Det Kongelige Danske Videnskabernes Selskab proposed a prize essay, the expenses of which were to be defrayed by the T H O T T Foundation and which was i. a. intended to comprise an investigation of the vegetative propagation of Danish forest trees. While working at this prize essay I un- dertook, in the years 1940—42, a number of experiments with cuttings which were carried out at the Arboretum, and the present work is based upon these experiments and their results, supplemented with continued observations in 1943 and 1944.
T h e possibility of carrying out the experiments to their present extent is first and foremost due to Dr. C. SYRACH LARSEN,
Director of the Arboretum, who with unfailing kindness has permitted me to undertake them as part of my work at the Institute of Forest Tree Breeding. In this manner the expenses involved were defrayed by the institution, chiefly by means of a grant awarded by the THOMAS B. THRIGE Foundation. In addition I received a personal grant from the RABEN-LEVETZAU
Foundation, which has also been a very valuable help for me in the execution of this work. The cost of the translation, undertaken by Mrs. ASLAUG MIKKELSEN M. A., has been defrayed by the RASK-ØRSTED Fund.
T h e material is arranged in the following m a n n e r : 1. Introduction.
2. T h e Experiments and their Methodics.
3. T h e Influence of External Factors on the Rooting of the Cuttings.
Det forstlige Forsøgsvæsen. XVII. 3. Maj 1946. 19
4. T h e Dependence of Rooting upon the Nature of the Cuttings.
5. T h e Application of Growth-promoting Substances.
6. T h e Individual Species of Trees.
7. Summary
and in connection with the above a number of general tables XXII—XXXII of the principal data of the experi- mental material.
T h e subjoined illustrations are all from photographs taken by the author, with the exception of figs. 2 a—b, for which C. H. BORNEBUSCH Ph. D., Director of the Danish Forest Ex- perimental Station, is responsible.
1. INTRODUCTION
It is a well-known fact that some genera, as e. g. willows and poplars, are in practice almost exclusively propagated by cuttings, whereas others, as for instance beech, oak and ash, are propagated by seeds. Between these extreme groups there are genera, such as alder, elm, and plane tree, where vegetative propagation takes or has taken place to a limited extent. This gives the impression t h a t propagation by cuttings is restricted to the individual genera. Within genera such as willow and poplar, there may as is also well-known be excep- tions, seeing that goat willow, Salix caprea, L. and European Aspen, Populus tremula L. — as well as other closely related species — cannot in practice be propagated by cuttings. As to the conifers similar conditions make themselves felt, and the first general impression is that the forest tree species cannot to any large extent be propagated b y cuttings.
After subjecting this problem to a more detailed investiga- tion it quickly appears that this first impression does not hold good. Partly, it is based upon experiences with a material consisting of hardwood cuttings, and partly it turns out that the practical work done with the various species of trees is not sufficiently intensive to permit of saying that the problem h a s been clearly elucidated in the case of all species.
Further, we realize that the work done in this field during the last decennia has greatly altered the picture. It is only in recent years that the need for the propagation of forest trees by means of
[3] 291 cuttings has made itself strongly felt, it having been realized that in forestry too it is important to use plants with a clonal origin.
By the vegetative propagation of an individual tree, be it by means of cutting, grafting or in some other way, a clone formation takes place, i. e. the mother individual is divided into a number of new individuals, each of which is genetically identical with the mother plant and contains the same amount of heridity matter.
T h u s , the clone formation makes it possible to transfer certain qualities or combinations of qualities unchanged from the mother individual to any number of new individuals. By this means the type desired can be preserved, whereas propa- gation by seed, based upon the same mother individual, or even by self-pollination, will alter the present combination of qualities through the splitting, which takes place in the individual cases owing to the existing combinations of sexual cells. In the case of forest trees it is nearly always a question of heterozygotics (cf. i. a. JOHANNSEN 1909, p. 215 and W I N G E
1931, p. 237). Add to this that in practice the forest trees are generally cross-pollinators, which fact frequently enhances the possibilities of combinations of qualities.
The importance of the clones in the preservation of the heridity qualities of the mother tree is universal, seeing that all these qualities are transmitted from the mother individual to the progeny. Not only the external form and the intensity of growth, but also the quality characteristics of all kinds, susceptibility or resistance towards attacks of fungi or parasites, resistibility towards frost and drought, the cellulose-contents of the wood etc., in so far as they are qualities, which are not exclusively associated with the phenotype. If it is a question of the value of the mother tree, especially from the point of view of breeding tree, this quality will also recur unchanged in the clone offspring, even though the latter, under unfavourable external conditions, should habitually prove to be of poorer quality than the mother tree.
As there are in nature single individuals, which are ideal from a forestal point of view, and as the deliberate forest breeding going on at the present time constantly raises new and better types, it is obvious that the vegetative formation generally offers such points of advantage that it is impossible not to make it a feature of forestal practice.
19»
Innumerable examples of contemporary types of the same species of tree, one of which is greatly superior in some way or other, are found everywhere in our forests. In illustration of a different fertility of growth can be mentioned two contemporary types of gray poplars, which in former times stood in the same locality on the western side of Vinderød Forest along the high road to Frederiksværk. Both of these types were in the spring of 1939 about seventy years old, but whereas one was a straight, fast-growing manly type, the other was sinuous, slow-growing and of a pronouncedly female character. Fig. 1
Fig. 1. Slices of two contemporary gray poplars from Vinderød. They illustrate individual differences in development under similar external conditions.
Photographed October 8, 1942.
shows basal slices of these two types, which belong to the material collection of the Arboretum, brought together in the spring of 1939 at a time when the trees had just been felled on account of the extension of the road. The diameter of the large slice was about 62 cm, whereas the smaller one measured about 24 cm in diameter. This means that by propagating the fast-growing type in this locality an increase of diameter may be obtained, amounting to about 160 per cent more than that of the slow-growing type, and also in this case an increased growth in height and a better form and quality of the tree.
Difference in growth form is also illustrated by the two young gray poplar types from Bornholm, which are represented by figs. 2a—b.
It should be borne in mind that individual differences, caused by
[5] 293 the influence of external conditions, may naturally occur in clones, and that this will be of great importance in the future selection of clones for definite purposes. A testing of the material under different conditions is more necessary in the case of the clones than when dealing with plants raised by seeds, where the qualities fluctuate from one plant to another, and where the mean value af a sowing of seeds (f. i. as regards the height) may be the same in two different places, even though it is not the same biotypes which will dominate the picture of the future growth.
a b Fig. 2 a—b. Gray poplars from Rømersdal, Bornholm. To the left, an ordinary
badly shaped type (a); to t h e right, a well-shaped type (b).
C. H. BORNEBUSCH phot.
Therefore it is necessary in many cases to make tests, before an extensive use is made of one single clone in places of growth differing as regards climate and soil from those of the mother individual.
That altered conditions may greatly influence the development of a certain clone material is naturally not to be considered as a depre- ciation of the qualities of the clone. It only means that a more intensive testing must be undertaken, than is generally the case in the arrangement of stands raised by seeds. Recourse may naturally also be had to the use of clone mixtures in the cultivated areas at hand, thus diminishing the risk which is at present associated with the cultivation of a single clone, so that all the individuals are damaged in the same manner under the same unfavourable conditions.
But at the same time that the use of clones requires a certain attention, the clones themselves are a means towards a final testing of the mother trees, whether dependent upon the climate or the soil or other factors, which may be imagined to play a part in the appraisement of the individual trees and their value as timber. Through the medium of the clones it will be possible to give an exact charac- teristic of the individual tree types and the different forest localities, and to do so in a far more exhaustive manner than is the case at present (SYRACH LARSEN 1936a, 1942a; LANTELMÉ 1938). This testing of the reaction of the mother trees under different external conditions gives in reality a much better characteristic of the genetic qualities of trees than can otherwise be obtained in forestry by the valuation of the offspring.
The importance of the clones in forestry, both directly and indirectly, has in this country first and foremost been emphasized by SYRACH LARSEN, who in forest tree breeding in various ways makes a rational use of the clone qualities (1934, 1936 a, 1937, 1940, 1942 a & c, 1944, 1945), and also in other countries the forestal importance of the vegetative propagation has been emphasized time after time.
The importance of the propagation by cuttings in forestal work will in the future chiefly be associated with the qualities of the clones. The application of cuttings can take place under various forms, and a distinction must be made between the application of the cuttings, directly as objects of use and indirectly as standard and experimental material for compara- tive investigations and experiments. Another group of vegetative propagations, viz. graftings, are proved to be of invaluable importance in the production of seed a n d tree breeding, because of their special qualities as regards flowering at an early age, but in this respect it seems as if the cuttings must be considered on a plane with seed plants, which can only be made to flower quickly by external influence.
The importance of using material in the shape of c u t t i n g s f o r f o r e s t a l u s e naturally consists in the propagation of suitable indi- viduals, which both as regards rapidity and mode of growth, health and other quality characters are superior to the ordinarj' individuals in our populations of forest trees. The idea is — after testing — to set out cultures with cuttings of particularly valuable individual trees, taken from the very best of the already existing stands.
Add to this that vegetative propagation may be the means of rapidly distributing good individuals of a hybrid character, the qualities of which cannot be preserved through seed propagation.
[7] 2 9 5 Of particular interest is the propagation of good individuals in the Fi-generation, where the heterosis effect makes itself felt. By this means the powerful growth and good qualities of such types can b e preserved and spread in the cultivation of forests, whereas the seed offspring of t h e Ft cannot continue being characterized by this luxu- riance of crossings.
It is t r u e that in t h e cultivation of forests it is possible to lay out stands o r cultures, specially raised for seed production and breeding, which for a long time are capable of supplying t h e desired Fi generation (SYRACH LARSEN i. a. 1937 and 1940 p p . 437—439), but it may often be a case of preserving Fi individuals w h i c h have proved desirable, though they are no longer capable of recreating, e. g. because the parents n o longer exist.
The direct possibilities of applying vegetative propagations a n d more particularly cuttings of the forest trees have frequently been demonstrated in print, a n d a series of experiments with cuttings, as e. g. in Denmark SYRACH LARSEN'S experimental work in this field, have been undertaken for direct forestal purposes. It is interesting to notice that in Germany, where the problem of t h e seed propagation of poplars during recent years has come to play a prominent part, BAUR (1938, p . 18) calls attention to the fact that »Die Pappel als Trägerin des Wertzuwachses wird ausschliesslich auf vegetativem Wege nachgezogen«, and further that for t h e cultivation of veneer poplar individuals, raised by seeds although capable of a good growth, a r e not by far as interesting as poplars of a vegetatively propagated material, their quality as timber being unknown and their value there- fore uncertain.
From America we have a very instructive example of the impor- tance a n d practical application of clone formations, which example in the course of time probably may prove classical, viz: the so-called shipmast locust, which is a variety of the common locust, and w h i c h for about two centuries and a half have been vegetatively propagated (RABER 1938). As the locust is the m o s t important tree in the planting of the erosion areas of U. S. A., the shipmast locust for this purpose is of great interest and also the object of propagation by cuttings on a large scale. As to this interesting subject the reader is referred to DETWILER (1937), HALL (1937), HIRT (1938),HOPP (1941), and RABER (1936), cf. TOOLE (1938).
There are a n u m b e r of general statements as regards the use of vegetatively propagated individuals. I shall not enter into details, b u t only briefly mention the work done by Australian foresters towards propagation b y cuttings of Pinas radiata. It is h e r e a case of propagation of good individual trees for direct forest planting, a n d the experiments with Pinus radiata were carried out on the basis of good results obtained by FIELD in New Zealand. In the report on t h e Australian experiments it is said: "Actually the 1937 cuttings planted in field in 1938 have been more successful than the seedlings, b o t h as to success in establishment and vigour". (Commonwealth Forestry
Bureau, 1939). It may be added that FIELD began his experiments several years ago, and his oldest propagation from 1924 succeeded by means of planting torn-off pine twigs in field (cf. MIROV 1938).
As to the vegatative propagation of t h e Fi individuals for direct forest planting, its importance has been emphasized on several occa- sions, not only by SYRACH LARSEN (1936 etc.), but also by American authorities. SCHREINER (1939) makes particular mention of this first generation in connection with a quick testing of crossing products generally, whereas JOHNSON states (1939), on the strength of experiments which are being carried out with propagation by cuttings of conifers etc.: "vegetative propagation holds good possibilities as a convenient, efficient, and commercially adoptable means of increasing Fi-hybrids in certain forest-tree genera not ordinarily propagated vegetatively".
The same opinion is set forth by SNOW & DUFFIELD (1940): "Advances in methods of vegetative propagation of forest-tree species will enable excellent types obtained in the first generation to be made available for general distribution as clones in a relatively short time". These views, which express the possibility of a practical future application of vege- tatively propagated material of valuable types of the forest trees, are what I myself shall support in the following work.
The use of clone material will bring about a greater homogeneity in the plantations, in which manner t h e r e will be a possibility of obtaining a homogeneous treatment of the individual plantation groups.
On the other hand, a complete planting with a single clone will diminish t h e frequently desired differentiation of t h e crown roofing. This may be avoided by merely using the clone material for intermixture in an ordinary plantation, and this will also make it possible to use more sparingly t h e valuable material of plants. There is, however, a diffe- rence in the amount of this intermixture in the various tree species, according as emphasis should be placed on mass production or quality production.
In t h e case of our principal foliferous trees and e. g. larch, w h e r e the value of the timber first and foremost depends on the shape and quality of the tree, it is principally a question of improving the quality of the timber production and so increasing the profit. In such stands the n u m b e r of plants is, to begin with, very large, and the first thin- nings can only yield a comparatively small economic profit. There is consequently no reason to be lavish with a great intermixture of valuable clone material, which must be removed in early years.
What should be aimed at is undoubtedly a comparatively small intermixture, the object being to secure the necessary number of main trees, so t h a t the effect of the clone on t h e plantation and the profit derived from it only makes itself felt in the middle-aged forest, when the yield normally begins to be valuable.
The conifer plantations, especially those consisting of Norway spruce, are as a rule somewhat more homogeneous in shape, and here the mass production is generally of far greater importance for the ecomomic profit. In the case of the coniferous tree cultures, whose
[9] 297 number of plants is considerably smaller at the start, there is conse- quently reason to use clone material to a greater extent than in the foliferous cultures, and t h u s it is really here a case of a shortening of the production period through the increased growth, which h a s been made possible by the clone material. A too small intermixture of a good clone material will only make itself economically felt at too late a period.
As formerly mentioned, an unfortunate selection of clones can do more h a r m than a wrong selection of provenience of seed plants, and this because of the very monotonousness of the clone as opposed to the lot of seed plants with its many biotypes. Consequently, there is often reason to use clone mixtures, not only as recommended by SCHREINER (1939) in o r d e r to eliminate or diminish the danger of the trees being damaged by insects, but also to safeguard as m u c h as possible against other catastrophes of an unforeseen nature arising out of extreme climatic constellations, for which the clone material used had perhaps not been tested beforehand. Clone mixtures will surely often be necessary on account of the variation in the conditions of growth, which may exist even within the single compartments.
In the case of the above-mentioned possibilities of applying the cuttings, the importance is conditioned by their clone qualities. There is also a possibility that the peculiar root system of the cutting may become of practical importance, cf. p. 382.
Mention must also be made of t h e p o s s i b i l i t i e s of u s i n g t h e c u t t i n g s i n , a n d t h e i r i m p o r t a n c e for, t h e e x p e r i m e n t a l w o r k and through the results of this w o r k also in practical forestry.
The testing of the qualities of the mother trees, investigations of climatic conditions etc. have already been mentioned, and in many other fields similar applications of the cuttings may also be to the benifit of forestry. The principle of the thing is that by means of a sound standard material it is possible to compare certain individuals and plantations, raised from seeds, and also their reactions both to biological conditions and forestal treatment, and not least it should by this means be possible to clear up the problems connected with the thinning of the stands. T h r o u g h a number of other experiments, where it is of importance to eliminate the individual reactions of the seed plants to this influence, a biologically identical material is also obtained by means of t h e cuttings. In these respects the cuttings are surely superior to the graftings, as the individuals, being own-rooted, are genetically quite homogeneous, whereas the graftings can only to a certain extent fulfil the requirement for biological identity, the root and its different reactions varying from one grafting to another. This drawback can be eliminated by making the scion itself strike root, so that it loses its character of grafting, but in most cases the material of cuttings will be preferred, if such a material is at hand.
The importance of the biogical identity in connection with forest trees has been emphasized in print on several occasions; thus, besides by the authors already mentioned, i. a. MIROV (1937) and KOLESSNI-
KOFF (1930), FERDINANDSEN and JØRGENSEN (1938) mention it in con- nection with the different biological strains of the fungi, and SCHREINER
(1939) in his mention of the forestal importance of the clone says that: "The clone can also be used to excellent advantage as an indi- cator tree, particularly in forest planting experiments", in other words, a statement to the same effect as those which on many occasions have been made with such emphasis by SYRACH LARSEN.
It should be borne in mind that the whole question of the future use of cuttings in forestry, when it becomes practically applicable, cannot be conceived to replace all that is propagated by seeds. It can never in this connection become a question of either — or, but only of both — and.
Through the interest in and the work with cuttings of forest trees the picture of the capacity of the tree species to be propagated by cuttings is greatly altered. For one thing, the use of softwood cuttings which had already been made to advantage by KURDIANI (cf. HUMMEL 1930) has resulted in a number of tree species proving willing to form roots on pieces of shoots, a n d for another the discovery of the root- promoting of the growth substances has caused practical pro- pagation to be possible to a much greater extent than was formerly supposed.
In the following work, which is based upon observations and experiments, I have tried to give a view of the factors playing a part in t h e propagation of our forest trees by means of softwood cuttings. Whether these factors are the external conditions in the rooting period, or the momentary physiolo- gical state of the material of cuttings and the age of the mother individual, they have a decisive influence on the result ob- tainable; but experiments have shown that all the tree species, which have been experimented with more than once, have been capable of being propagated by cuttings at some stage or other of their existence. T h e problem remains to what extent the root-forming capability of the cuttings can be utilized in practical propagation, and here I hope that the present work m a y form a basis for more exhaustive attempts in the future with a view to practical exploitation.
2. T H E EXPERIMENTS AND T H E I R METHODICS As experimental objects were chosen the so-called soft- wood cuttings consisting of young not yet wooded shoots. The
[11] 299 experiences of gardeners and others, as also the experiments made by SYRACH LARSEN at the Arboretum in Hørsholm, show that a number of tree species can more easily be propagated in this manner than by means of hardwood cuttings, and especially forest trees belong to the group, which can only with difficulty be propagated by fully ripe parts of shoots. A number of the conifer experiments have, however, been under- taken during the winter season with fully developed shoots.
In order to exclude fluctuations in the results, caused by individual variation, a number of the principal orientations as to the course of the rooting under different conditions were undertaken by means of genetically homogeneous material.
For this purpose first and foremost clones of poplar and alder were used.
T h e poplar chosen was " H e n r y s poplar", Populus verni- rubens A. Henry, a hybrid, which in this country has been cultivated and mentioned b y SYRACH LARSEN (1936 b, 1942 b).
Already in the summer of 1937 this poplar was successfully propagated by means of softwood cuttings, and it was there- fore possible to work with an easily accessible clone material, about which it was k n o w n that it was capable of rooting under favourable conditions.
T h e alder used was a crossing of Alnus cordata Desf. and a father individual which was not more accurately defined, but in which there seem to have been features both from A.
subcordata and from another Alnus (cf. GRAM and others. 1941, p. 56). This individual, which originated from a sowing in the spring of 1936 had also formerly, with good results, been used for propagation by cuttings at the Arboretum, and the material used in the present experiments was partly taken from the tree itself and partly from its daughter individuals. The alder is in the following designated as S. 225 * (sowing 2251,
cf. SYRACH LARSEN, i. a. 1942 c).
In addition to these two large clones, genetically homo- geneous material has been used wherever possible, and in the chapter on the individual tree species the origin of the material has been communicated in the individual cases.
T h e cuttings generally originate from young plants, but also shoots from older trees have been used in a number of cases. The age problem, which has only been subjected to a
peripheric treatment, will be dealt with later on, i. a. on p p . 342—344.
As regards the treatment of the material of cuttings certain details will be given in the following.
The cuttings forming the material of the experiments have generally been collected in the shape of smaller branches of foliferous trees with many young shoots. Immediately after being collected they were placed in a pail with water so as to prevent their drying, and they were kept there until the preparation could be undertaken indoors. In the pruning of the shoots and the subsequent treatment of the cuttings I aimed all the time at preventing the shoots from drying, so that at no time during the treatment were the cuttings un- necessarily weakened or the leaves made to hang.
The length of the cuttings varied somewhat with the nature of the material, but in the larger lots it was generally 8—10 cm.
The original placing of the cutting on the shoot has varied in m a n y of the experiments, the largest twigs having either been cut as basal, middle or (exceptionally) top pieces. T h e shorter twigs were either cuttings of the full length of the shoot or abbreviated, basal cuttings without a tip. The greater part of the cuttings used were, however, basal cuttings, and where nothing special is indicated, it is to this category of cuttings that reference is made. As will be shown later on the divi- sion of the shoots of the foliferous trees yields cuttings of a greatly differing value. When preparing the cuttings, the lower- most cut was generally made at the base of the shoot and so low down that very little of the wood of the preceding year was removed on the surface of the cut. This was either done so that the shoot was torn off with a heel, cf. fig. 3, which afterwards was smoothed with a knife; or, what was more common, by the cuttings being cut off directly at the base of the shoots with a single, smooth cut or a couple of cuts, the surfaces forming a right angle. This latter method was more particularly used in preparing cuttings from side shoots.
The question of the placing of the cut on cuttings of middle or top pieces by cutting above, below or between the nodes (cf. i. a. CHADWICK 1932) has not been subjected to in- vestigation. As a rule such cuttings were taken right below a node.
[13] 301 W h e n the shoots were shortened to the desired length, all leaves were removed, with the exception of one or two which were, however, as a rule trimmed in order to prevent a too rapid evaporation from the surfaces of the leaves. In fig. 3 at the top are seen a row of lateral shoots torn off with heels, and below them a row of prepared cuttings.
In the case of the conifers the work not only comprised softwood cuttings, but also hardwood cuttings, which had been
Fig. 3. Lateral shoots of alder, S. 2251, photographed August 28, 1943. At the top, shoots torn off with heels; at the bottom, prepared cuttings.
cut from fully developed shoots during the winter season. In the experiments undertaken with Picea and Abies a distinction was generally made between the manner in which the shoots were originally placed on the branches, so that the material was divided into cuttings of terminal and lateral shoots, according as the cuttings were taken from the main axis of the branch or from a side shoot of the latter. The shoots were cut off with a single clean cut through the base, and they were almost exclusively used in their full length. As a rule they were inserted with all their needles on them, exceptions being
certain lots of larch and pine, where the basal part was trimmed.
The cut-off and prepared cuttings were normally left for twenty-four hours in water or a growth substance solution in the laboratory. The experiments were generally made with 3-indole acetic acid, mostly used in the concentrations 50 and 100 p. p . m. (part pro million) corresponding with 1/20 and 1/10 o/oo. The preparation of these solutions took place in the usual manner, i. e. by the solution of a suitable quantity of the undiluted acid in a few ml ( = ccm) alcohol, with the subsequent admixture of the necessary quantity of water. No difference in the rate of the rooting was proved whether distilled water or tap water was used. The total alcohol con- centration in the finished growth substance solution never exceeded 1 per cent and was generally much lower. The solutions were used when fresh, or a few day after they had been prepared. Whenever in the course of the experiments mention is made of a unspecified growth-promoting substance, it is 3-indole acetic acid.
It holds good of nearly all the experiments from 1940 that no growth-promoting substances were used, whereas such have generally been applied in the series of experiments during the following years, cf. tables and text. In the individual experiments no quantitative investigations were undertaken of the growth substances absorbed. A treatment with other root- promoting substances has only been used in exceptional cases.
T h u s , no testing of the mutual value of the various root- promoting substances has been aimed at. W i t h a view to answering the question, whether the vegetative capacity of propagation is present in the forest trees, this has not proved necessary, and for the desired orientations of a more practical nature the above-mentioned growth substance was selected, because judging from experiences reaped abroad it is one of the most effective. In individual cases it might, however, have been desirable also to use indole-butyric acid, but this was only possible in the latest experiments with Norway spruce in 1943.
After the cuttings had been packed together in glasses containing the growth-promoting substance, the surface of the liquid being about 2 cm above the lower cut of the cuttings, their nethermost parts were washed in water in order to remove any growth substance adhering to them and to avoid scorching, and then they were planted either in hot beds, cold
[15] 303 frames or in planting boxes and flower pots in greenhouses.
The soil into which the cuttings were planted varied in character. That which was generally used in 1940—42 was a mixture of equal parts of gravel, earth and peat moss. The earth mixtures pH lay chiefly in the interval 6—7. They were
% ">/6 tys V? *h 2Vl Vs /3/s "/, "2/9 'VS ZV9 V/o/9tO.
Fig. 4. The course of the mean temperature under different experimental conditions in 1940.
as a rule covered by a thin layer of pure gravel, but gravel was also now and then used as the only medium for cuttings, and in 1943—44 all cuttings were planted in clean gravel with pH 7—8.
In a few orientating investigations of the relation between root formation and pH a mixture was used consisting of peat moss and washed-out sand, mixed with various quantities of pulverized chalk.
The pn determinations were undertaken by means of the Lautenschläger ionometer.
The cuttings were in the main planted under three different conditions, viz. in hot beds and cold frames and in propagating boxes in greenhouses. To illustrate the temperature conditions in the different localities there are in fig. 4 some curves of the mean temperature in 1940.
In that year the temperature was measured in the morning (about 7 a. m.) and in the afternoon (about 4.30 p. m.), and the mean temperature given is expressed by half the sum of the two tempera- tures. In the curves were also given a series of measurings from a cold frame placed in a forest.
The temperatures during the following years were measured through the reading of maximum-minimum thermometers, and therefore cannot be directly compared with the figures from 1940. The relation between the temperatures of the different localities had, however, a similar mutual course, though with a slightly smaller difference between the hot bed and the cold frame. The reading of the maximum- minimum temperatures yielded a somewhat higher daily difference of temperature than in 1940.
The experiments with conifer cuttings, which were carried out in the winter of 1941—42 in a heated greenhouse, were undertaken under such temperature conditions as appear from the following means of the individual months:
Month Maximum C° Minimum G°
November 1941 18.9 14.6 December „ 19.7 16.7 January 1942 19.8 15.3 February , 22.3 16.0 March „ 26.5 13.6 In the hot beds the heat came from a layer of manure, separated by a cover of earth and peat moss from the earth or gravel, in which the cuttings were planted. In cases where the cover was too thin, i. e. less t h a n 5 cm thick, it caused the putrefaction of the material of cuttings.
In the hot beds and cold frames as well as in the propagating boxes in greenhouses the cuttings were placed under tightly fitting glass. They were protected against the sun by white- washing the surface of the glass panes and by the use of lath shades.
A suitable (estimated) degree of moisture was maintained through frequent watering, but without any automatic regula- tion. It proved of great importance to be able to maintain an optimal moisture of the air, until the rooting h a d set in. Soft cuttings, which were kept too d a m p , sometimes rotted before this had taken place; and on the other hand too little moisture in the air, and more particularly the cold air momentarily coming in through open windows, had weakened the cuttings by evaporation. As to the beds it was more particularly draught which was the disturbing element, and in order to reduce the
[17] 305 evaporation strips of sacking were nailed on to the upper side of the frames, so as to make the windows shut as tightly as possible.
Not all cuttings were equally susceptible to the free supply of air, and a number of conifer cuttings throve surprisingly well throughout a winter, where they stood unprotected in a heated greenhouse with a greatly fluctuating degree of moisture.
As to the cuttings of foliferous trees they are extremely sus- ceptible to fluctuations in the degree of moisture, and un- necessary evaporation, before the rooting h a s stabilized the shoots, is very injurious and may even kill them.
Under these circumstances where optimal growth conditions were aimed at for the delicate softwood cuttings, other organisms may also develop luxuriantly and so come to exer- cise a disturbing effect on experiments. Besides by rot of various kinds some lots were attacked by aphis, Lachnus fagi on beech and Aphis sp. on birch. Larvæ of Chrysomela ænea and Agelastica alni were often found on the leaves of alder cuttings. Some of the experiments in hot beds were in 1942 greatly disturbed by moles.
These attacks were naturally counteracted as far as possible, a n d as a matter of fact it was only rot which spoilt a number of the experiments to such an extent that it was necessary to condemn them. No absolutely reliable method was found by which to avoid the putrefaction of the cuttings, before they were able to strike root, and there is no doubt that a number of different factors here made themselves felt in the individual cases.
The record of the individual experiments was made at unequal periods after the planting of the cuttings. The individual periods were often chosen for purely practical reasons and did not always coincide with the time, when there were the greatest number of living plants with roots. In several cases the record was only made in the following spring.
In the arranging and recording of the experiments the object aimed at has generally been to supply information of a practical nature, and in the subjoined tables the results have therefore been given in the simplest form, i. e. as a rule by specifying the number of cuttings with roots surviving at the time of the record. The n u m b e r of dead cuttings which had
Det forstlige Forsøgsvæsen. XVII. 3. Maj 1946. 20
managed to root are also recorded, but they are only included in the tables when greatly altering the picture in a definite direction. Neither have the number of fresh cuttings without roots found a place in the tables, in spite of the fact that experience shows that they have frequently formed roots later on. T h u s in the case of the tree species in question the tables generally contain the minimum expression of its capacity for striking root under existing conditions.
In the tables the percentages have been calculated by means of a sliding scale, and as a rule they are given without decimals. W h e n judging the results one must naturally in every given case pay attention to the amount of material used in the experiment. The general tables, XXII—XXXII, are all collected at the end of the treatise, so as not to interrupt the reading of it, and they comprise as much of the material used for experiments as is of interest by its bearing upon the text.
The abbrevations used in the tables as to nature and treatment of the material of cuttings are to be found on p. 390. In certain cases an experiment has only been recorded with a few data of summation, whereas the details are mentioned under the subject in question. In the general tables the material is arranged according to species beginning with the foliferous trees.
The tables tend to show in schematic form that our forest trees under certain conditions are capable of vegetative propa- gation by means of softwood cuttings, furthermore, for some of the species of trees to elucidate the capacity of rooting under different experimental conditions.
3. T H E INFLUENCE O F EXTERNAL FACTORS ON T H E ROOTING OF T H E CUTTINGS
In the course of the work with softwood cuttings it soon appeared that the different external conditions, under which the cuttings were placed during the period before rooting, played a very important part as to their readiness of rooting.
This is a common and well known phenomenon, and among other factors a certain, rather high degree of moisture in soil and air is necessary, in order that the soft herbaceous parts of the shoots may survive, until the rooting has taken place,
[19] 307 and the absorption of water can be regulated in a normal manner through the newly formed root system.
W h a t makes the propagation of our forest trees by cuttings so very difficult is, first and foremost, the interplay of the various factors which frequently veil the very fact that the tree species generally have a pronounced capacity of propa- gating by means of softwood cuttings at some period or other in their existence, as long as the interplay of the factors is favourable.
It is, therefore, natural first of all to elucidate a number of the concurrent factors and first and foremost the purely external conditions under which the propagation of the cuttings takes place.
T h e t e m p e r a t u r e .
One of the external factors which, for practical purposes, exercise a noticeable influence on the propagation of cuttings is the temperature. Experiences reaped by gardeners show that a slightly higher temperature, than the one under which the plants thrive, generally benefits the rooting of the cuttings;
and also in the Arboretum the use of hot beds h a s formerly yielded good results for softwood cuttings of different forest trees.
In the present series of experiments it also proved that the use of hot beds brings about an increased rooting of the cuttings generally speaking, and that cuttings of a number of tree species which did not take root in cold frames were ready to do so, when placed in a hot bed or a greenhouse.
In order further to illustrate this I undertook some expe- riments with a genetically homogeneous material of poplar and alder, which were placed under different temperature conditions, chiefly in cold frames and hot beds, and it proved that an increase of the temperature of the soil amounting to a few degrees greatly precipitated the rooting.
As a case in point one of these experiments will be dealt with in detail.
Some 3300 softwood cuttings of Henrys poplar (Populus ver- nirubens A. Henry) were prepared on June 18, 1941. The material was taken from a 2—4 year old cutting culture with powerful long shoots, and the cuttings were sorted according to the nature of the shoots.
20«
i* xr*
i*SH§
11 *
; " ¥ • -
f .
F.:.'. .«•'•
vv - » , " i
rftVm? jffrX?^ aJWKm,-^*.r >>«**- • - f . . , Ti . \ V * • ü~ - J" - v *
Fig. 5 a—d. Cuttings of Henrys poplar, experiment no. 168, a~b photographed July 15, 1 9 4 1 ;
[21] 309
/
S . # s
.*& '"'"'
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i.
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* - v -? * V *••• ^ - * " * p - &Ü
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planted in cold frames (a and c) and in hot beds (b and d).
c—d photographed September 1, 1941.
cf. table XXII, no. 168, p. 391. Part of the material was treated with 50 p. p. m. 3-indole acetic acid, whereas the remainder were left standing in water for the same length of time. After having been left for about twenty-four hours in growth substance and water the material within the individual groups was distributed fairly equally into two parts, which were planted in a cold frame and a hot bet respectively. As to the reaction of the individual types of shoots and the influence of the growth substance see later pp. 327—329 and p. 360.
Already after a fortnight the material in the two beds differed distinctly in appearance, and on June 15 this difference was still greater, as appears from figs. 5 a—b. In a hot bed the cuttings had thriven well and uniformly, whereas only relatively few cuttings in the cold frame seemed to have taken root. At this time air was gradually supplied to the hot bed, whereas the cold frame for some time still stood behind quite closed windows. The minimum temperature in the rooting period fluctuated in the cold frame between 12 and 15° C, whereas the minimum temperature in the hot bed was between 15 and 180 C.
On September 1, 1941, the lots were again photographed, figs.
5 c—d, and it appears h o w the cuttings in the hot bed stand quite close to one another and are growing rapidly with heigths of up to 1 metre, whereas the cuttings in the cold frame are scattered and only have attained heights of up to Va metre. Medio December that year the results of the experiment were put on a record, and it then proved that of the 1665 cuttings in the hot bed 792, or 48 per cent, had taken root, of which however only 629 or 38 per cent still survived. The number of cuttings placed in the cold frame was 1650, of which 278 or 17 per cent had taken root; of these 256 or 15V2 per cent had survived1).
An i n c r e a s e of t h e m i n i m u m t e m p e r a t u r e a m o u n t i n g t o a v e r y few degrees h a s t h u s i n c r e a s e d t h e r o o t e d c u t t i n g s ( H e n r y s p o p l a r ) to m o r e t h a n t w i c e t h e n u m b e r .
Other experiments with Henrys poplar, cf. table XXII nos. 74, 77, 111 and 276 express the same. In no. 276 from 1942, where the material consisted of short lateral shoots, the corresponding percentages of live cuttings with roots before the next season of growth was 58 for a hot bed (minimum temperature in the rooting period 15—18° C) as against 22 per cent for a cold frame (minimum temperature 10—14° C).
The first of the experiments from 1940, no. 74, showed the same great difference in the rooting capacity. In no. 77 the results only deviated slightly, which was due to the fact that the heating medium was too near the cuttings, so that great damage was caused by rot among the
*) The different types of cuttings were very unequal in their readiness to take root. The representation in the two beds was only approximately homogeneous, but when reckoning with the same representation of the types of cuttings in the two beds, the same percentage of surving cuttings with roots is obtained, viz. 151//2-
123] 311 cuttings in the hot bed, and there were only relatively few left, when the result of the experiment was recorded. This was to a still greater extent the case with experiment no. 111. The record in the case of both of these experiments was that after IV2—2 months there were about 90 p e r cent live cuttings in t h e hot beds, whereas t h e figures from the cold frames were considerably lower. T h u s , in no. I l l about 50 cuttings had, without any result, been placed in a cold frame in the forest, w h e r e the temperature was a couple of degrees lower than in an ordinary cold frame. The lot not included in table XXII. At the bottom of table XXII an experiment from 1939 has been recorded without a number, and this experiment also shows a very slight rooting under cool conditions cf. the temperature curve, fig. 4.
The reason why the placing of the cuttings in a hot bed yields so m u c h better results must first and foremost be sought in the acceleration of the rooting process, which is caused by the increased temperature. The rooting of the cuttings begins at a m u c h earlier period, and t h u s the damage caused by rot on the delicate shoots is diminished, seeing that it becomes greater and greater, the longer the cuttings are left without roots and the less vital the parts of the shoots. In cold sur- roundings they are at a standstill; their vital functions are reduced and their power of resisting external injurious effects of rot is therefore smaller. This naturally presupposing that the mixtures of soil into which the cuttings are planted are not unequally infected by rot.
In addition to a quicker rooting the higher temperature also brings about an increased growth of the shoots. In prac- tice this means that air must at a comparatively early period be supplied to the hot bed windows, so that the rooted cut- tings m a y be hardened, before it is necessary entirely to re- move the glass panes because of the growth in length of the cuttings. This, in its turn, entails that cuttings which have not been able to take root, before they were exposed to the free air, are as a rule killed. F r o m the figures men- tioned above it appears that in a hot bed 21 per cent had died after the rooting had set in, and the corresponding figure for the cold frame was only 8 per cent. Therefore it is of great importance, from a practical point of view, that the material of cuttings in the individual bed is so homogeneous as possible, in order that the growth of the cuttings may begin more or less at the same time.
The difference between the figures given for the hot bed and t h e cold frame respectively is explained by the fact t h a t air was supplied to the hot bed at an earlier period than to the cold frame, and as to the plants in the hot bed the growth of the tops had begun a t such an early season t h a t a great n u m b e r of the cuttings, slow to root, suffered much more real damage than the cuttings in the cold frames.
As to the hot beds it must further be mentioned that some of t h e cuttings have perhaps been choked by their more powerful neighbours.
The hot beds also have the advantage that the development of the cuttings becomes stronger, as appears from fig. 6 and
Fig. 6. Cuttings of Henrys poplar, experiment no. 168, phot. May 4, 1942.
From left to right: 4 basal cuttings of different length and thichness from hot beds, 2 of the largest basal cuttings from a cold frame. The largest and the smallest lateral cuttings from a hot bed and the largest from a cold frame.
On the extreme right, the two largest top cuttings from a hot bed and a cold frame, respectively.
the measurements of height in table XXII. In this connection it must, however, be borne in mind that the growth in height attained by the end of the first season of growth to a certain extent has been caused by the manurial value of the heating medium, so that the difference in height would have been less, if for instance the cuttings had been treated in an electric hot bed but otherwise with the same mixture of soil.
With the clone material of the alder hybrid S. 225x, cf.
p. 299, similar experiments have been carried out. In such cases the use of a hot bed offered a still greater advantage,
[25] 313 a s c u t t i n g s of t h i s a l d e r r e q u i r e a little m o r e h e a t i n o r d e r t o r o o t . T h e p r i n c i p a l r e s u l t s of t h e e x p e r i m e n t s a r e t o be f o u n d in t a b l e X X V , n o s . 60, 158, 164 a n d 2 7 0 a n d a r e g i v e n s u m - m a r i l y in t h e following t a b l e I.
Table I. The variation of t h e rooting in clone material of the alder hybrid S. 2251 u n d e r different temperature conditions.
No.
60 158 164 270
Gs p. p. m.
0 0 50 0 & 5 0
P e r cent of cuttings with live roots in Hot beds
35 4—20 2 6 - 4 5
Greenhouses 2 25
Cold Nursery
0 0—1
4 2—7
rames Forest
0
Different types of cuttings have been used for two of the expe- riments in table I, and t h u s two sets of results were b r o u g h t about, which are both recorded in the table. It appears that the cuttings in the greenhouse yield better results than in t h e cold frame, but poorer than in the hot bed, and this agrees with the fact that the temperature conditions in the greenhouse lay between the temperatures of the two types of beds. The cold bed under forest conditions, which represents the coldest locality, yielded no result.
In table XXV there are furthermore a few experiments requiring some supplementary information, as e. g. no. 82 with 0 live cuttings with roots at the time of t h e record. This was an example of poor results in a hot bed, in consequence of there being too thin a cover between t h e heating medium (horse m a n u r e ) and the soil in which the cuttings were placed. The cuttings rotted quickly and to a large extent from the base. They attempted to take root above the place attacked, and from an investigation of September 30, 1940, it appeared that there were roots on some of them. The roots were formed high up on the stalk, most frequently at the surface (in a single case even from the part of the stalk above ground), and the basal parts of the cuttings right up to the place of departure of the roots h a d most fre- quently decayed entirely, i. e. two thirds to three fourths of the original length of t h e cutting. The rooted cuttings, however, died in the course of the winter and are consequently not recorded in the table.
In no. 174 table XXV the word hot is given in parenthesis. This is due to the fact that the degree of heat of the bed was not higher than that of the cold frame. The heating medium had yielded no heat.
In reality this number is not considered as belonging to the group treated in a hot bed, neither is the result better than in the cold frames.
The large lot no. 303 from a hot bed represents t h e best of t h e results obtained, viz. 77 p e r cent of rooted cuttings. As appears from
table XXV, however, only 68 per cent survived in the following spring.
It turned out that the percentage of rooted cuttings fluctuated sligthly from one window to another — 600 specimens had been planted per window — though in such a manner that the best result was obtained in the middle of the rows of beds, i. e. in the window which was most protected against draught and loss of heat.
The lot no. 304 which had been planted a week later, also in a hot bed, shows a poorer development and a considerably smaller percentage of cuttings with roots, viz. 60 per cent in all, of which only 43 per cent survived in the following spring. Judging by ordinary experience with alder it is to be supposed that this difference is not so much due to the time of planting as to the altered growth conditions with less bottom heat. The difference in temperature between the two beds can unfortunately not be cleared up, but also in this experiment the greatest number of cuttings with roots were produced in the best protected window, and this further supports the idea that it is the conditions of temperature which here had a decisive influence on the development in the two lots.
In agreement with the quicker start in no. 303 as many as 268 cuttings of this lot had attained heigths of more than 15 cm, while in no. 304 there were only 28 cuttings which had exceeded the same height limit. The maximal height in both lots was about 25 cm.
Also conifer cuttings planted during the winter months reacted distinctly to the temperature, as has been shown in the case of Sitka spruce, no. 235, table XXXI, cf. also Norway spruce in the foot note of the table p. 406.
Besides the results obtained from the experiments mentioned above, where a comparison was directly possible, also a series of experiments with other tree species showed that the intensity of the rooting is greatly increased by the higher temperature in which this process takes place in hot beds and greenhouses.
In a number of cases the rooting only occurred very sparsely and "casually" with lower temperatures — m i n i m u m 10—14° C.
— in cold frames, and in a series of experiments, which have not been included in the tables, all cuttings which had not been subjected to treatment died in cold frames before the rooting began.
As examples m a y be mentioned that cuttings of several
"difficult" tree species, as e. g. ash, oak and birch, have not yet proved sucessful in cold frames, whereas they are ready to take root in greenhouses and hot beds. In no case the soil reached the optimal temperatures, which LADEFOGED (1939) found for the growth of roots. MOEN (1944) mentions 20—25° C as a better temperature area for a great number of cuttings,
[27] 315 and in the summer of 1944 cuttings of Henrys poplar, in the course of 6—7 days, took root in a greenhouse, where the temperature by day was very nearly 30° C. (The experiment is not included in the table).
The orientations and observations made yield the result that the tree species planted under different temperature con- ditions, everything else being equal, root best and quickest where the temperature is highest, and in a number of cases where the ordinary temperature of the soil did not produce rooting, the latter set in when the cuttings had been placed under higher temperature.
For all practical purposes this means increased possibilities of utilization, where clone propagation is desirable, and also that the cutting plants can attain a better development before the end of the season of growth.
M o i s t u r e , l i g h t a n d m i c r o c l i m a t e .
In the propagation by cuttings the temperature factor is closely associated with the two other factors: moisture and light, and together they form the microclimate, under which the rooting takes place. It had been mentioned that a rather high degree of moisture in air and soil is necessary, in order that the soft shoots may live, until the new plant indi- viduals have been stabilized by the rooting. Also the question of the supply of light in the period before the rooting takes place seems to be of great importance. Direct experiments have not been made for the clearing up of these conditions, but the experiences gathered in the course of the work show that there seem to be a couple, not more fully determined optimas for the effect of these factors on the cuttings.
It is the extreme conditions which most easily make themselves known through their effect on the material of cuttings. Too high a degree of moisture in the air and particularly in the soil has the unfortunate effect that the danger of rot is greatly increased, and that the cuttings are very apt to rot from the base. On the other hand, a drying out of the surroundings cause great losses of transpiration and the wilting of the cuttings. As far as the light is concerned, strong, direct sunlight scorches the material, whereas a dimly lighted pro- pagation room favours the growth of fungi and infection by rot. In practice these extremes may, however, be avoided, while the most suitable conditions of light and moiture must for the time being be determined through an estimate based upon general experiences.
On pp. 304—305 a brief statement has been made of the measures ob- served in order to prevent too m u c h evaporation and too strong a supply of light, and here it has also been mentioned that Norway spruce and Douglas fir were possessed of a surprising power of resistance to fluc- 'tuations in the moisture of the air through a longer period.
The microclimate may fluctuate, for instance within a n u m b e r of hot beds, partly on account of draught — as was mentioned u n d e r the alder experiments nos. 303 and 304 (p. 314) — and there m a y also in the same window be localities, w h e r e the cuttings thrive particu- larly well. An example of this is given in table XVIII (p. 361), where the rate of the rooting increases gradually from north to south, probably caused by the light in the southern side of the window sill having been more subdued, at the same time that the moisture h a s been increasing the further south one get on the sill.
Also in greenhouses similar variations have been observed, and as an example of this an experiment with alder cuttings m a y be mentioned, viz. no. 283 in table XXV. The cuttings were placed in a double propagation box in a greenhouse, the light falling both from east and west. They were planted in rows, parallel with the sides of the propagation box in a direction north-south. There were in all 36 rows with 660 cuttings, the greater part of which were treated with 100 p . p . m. growth substance. The cuttings were planted on June 18, 1942, and yielded the following results, the distribution of the cuttings from east to west being shown through the order of succession of the recorded columns:
Treatment with growth substance, p . p . m . . 0 100 100 100 0 Number of series 2 10 10 10 4 cuttings planted 37 183 184 183 73
„ with roots 1 40 77 89 20 Per cent „ „ „ 2.7 21.9 41.8 48.6 27.4 The temperature of the soil was the same in the whole of the propagating box, and the cause of this regular rise in the n u m b e r of rooted cuttings from east to west must first and foremost be looked for in the conditions of light. An any rate the figures recorded are a good expression of the fact that there may be a difference in the microclimate under apparently homogeneous conditions, which causes a great change in the rate of the rooting.
V a r i a t i o n s i n t h e m i c r o c l i m a t e i n h o t b e d s a n d p r o p a g a t i o n b o x e s f r e q u e n t l y o c c u r , e v e n u n d e r a p p a r e n t l y h o m o g e n e o u s e x t e r n a l c o n d i t i o n s . A c o u p l e of e x a m p l e s h a v e b e e n g i v e n a b o v e , a n d t h e y e n c o u r a g e i n t h e h i g h e s t d e g r e e t h e u n d e r - t a k i n g of a w o r k , t h e object of w h i c h is t o create r e a l l y h o m o g e n e o u s c o n d i t i o n s of g r o w t h for t h e c u t t i n g s , b o t h w h e r e it is a case of e x p e r i m e n t s a n d p r a c t i c a l p r o p a g a t i o n .
