42 



NA TURE 



[November ii, 1897 



accustomed to do by experiment, and it begins to appear that 

 our final judgments upon many questions of morphology, especi- 

 ally those which relate to variation, homology, &c., must be 

 formed after the evidence is obtained in this higher trial court, 

 that of experimental morphology . While experimental morpho- 

 logy as a designation of one branch of research in plants, or as 

 a distinct and important field of study, is not yet fully taken 

 cognisance of by botanists, we have only to consult our recent 

 literature to find evidence that this great and little explored field 

 has already been entered upon. 



Experimental methods of research in the study of plants have 

 been in vogue for some time, but chiefly by plant physiologists 

 and largely from the standpoint of the physical and chemical 

 activities of the plant, as well as those phases of nutrition and 

 irritability, and of histologic structure, which relate largely to 

 the life processes of the plant, and in which the physiologist is 

 therefore mainly interested. In recent years there has been a 

 tendency in physiological research to limit the special scope of 

 these investigations to those subjects of a physical and chemical 

 nature. At the same time the study of the structure and be- 

 haviour of protoplasm is coming to be regarded as a morpho- 

 logical one, and while experimental methods of research applied 

 to the morphology of protoplasm and the cell is comparatively 

 new, there is already a considerable literature on the subject 

 even upon the side of plant organisms. While certain of the 

 phenomena of irritability and growth are closely related to the 

 physics of plant life, they are essentially morphologic ; and it is 

 here especially that we have a voluminous literature based 

 strictly on the inductions gained by experimentation, and for 

 which we have chiefly to thank the physiologist. 



If we were to write the full history of experimental morpho- 

 logy in its broadest aspect, we could not omit these important 

 experimental researches on the lower plants in determining the 

 ontogeny of polymorphic species of algae and fungi which were 

 so ably begun by De Bary, Tulasne, Pringsheim, and others, 

 and carried on by a host of European and American botanists. 

 The tone which these investigations gave to taxonomic botany 

 has been felt in the study of the higher plants, by using to some 

 extent the opportunities at botanic gardens where plants of a 

 group may be grown under similar conditions for comparison, 

 and in the establishment of alpine, subalpine and tropical 

 stations for the purpose of studying the influence of climate on 

 the form and variations of plants, and in studying the effect of 

 varying external conditions. 



While experimental morphology in its broadest sense also 

 includes in its domain cellular morphology, and the changes 

 resulting from the directive or taxic forces accompanying growth, 

 it is not these phases of morphology with which I wish to deal 

 here. 



The question is rather that of experimental morphology as 

 applied to the interpretation of the rnodes of progress followed 

 by members and organs in attaining their morphologic in- 

 dividuality, in the tracing of homologies, in the relation of 

 members associated by antagonistic or correlative forces, the 

 dependence of diversity of function in homologous members on 

 external and internal forces, as well as the causes which deter- 

 mine the character of certain paternal or maternal structures. 

 I shall deal more especially with the experimental evidence 

 touching the relation of the members of the plant which has 

 been represented under the concept of the leaf, as expressed in 

 the metamorphosis theory of the idealistic morphology. The 

 poetry and mystery of the plant world, which was so beautifully 

 set forth in the writings of Goethe and A. Braun, are interest- 

 ing and entrancing, and poetic communication with nature 

 is elevating to our ethical and spiritual natures. . But fancy or 

 poetry cannot guide us safely to the court of inquiry. We must 

 sometimes lay these instincts aside and deal with nature in a 

 cold, experimental, calculating spirit. 



The beginnings of experimental morphology were made 

 about one century ago, when Knight, celebrated also for the 

 impulse which he gave to experimental physiology, performed 

 some very simple experiments on the potato plant. The under- 

 ground shoots and tubers had been called roots until Hunter 

 pointed out the fact that they were similar to stems. Knight 

 tested the matter by experiment, and demonstrated that the 

 tubers and underground stems could be made to grow into 

 aerial leafy shoots. This he regarded as indicating a com- 

 pensation of growth, and he thought, further, that a com- 

 pensation of growth could be shown to exist between the 

 production of tubers and flowers on the potato plant. He 



NO. 1463, VOL. 57] 



reasoned that by the prevention of the development of the 

 tubers the plant might be made to bloom. An early sort of 

 potato was selected, one which rarely or never set flowers, and 

 the shoots were potted with the earth well heaped up into a 

 mound around the end of the shoot. When growth was well 

 started, the soil was washed away from the shoot and the upper 

 part of the roots, so that the plant was only connected with the 

 soil by the roots. The tubers were prevented from growing, 

 and numbers of flowers were formed. This result he also looked 

 upon as indicating a compensation of growth between the 

 flowers and tubers. 



While we recognise Knight's experiments as of great im- 

 portance, yet he erred in his interpretation of the results of this 

 supposed correlation between the tubers and flowers, as 

 Voehting (1887, 1895) has shown. By repeating Knight's ex- 

 periment, and also by growing shoots so that tubers would be 

 prevented from developing, while at the same time the roots 

 would be protected, flowers were obtained in the first case, 

 while they were not in the second ; so that the compensation 

 of growth, or correlation of growth, here exists between the 

 vegetative portion of the plant and the flowers, instead of 

 between the production of tubers and flowers, as Knight 

 supposed. 



The theory of metamorphosis as expressed by Goethe and A. 

 Braun, and applied to to the leaf, regarded the leaf as a coticept 

 or idea. As Goebel points out, Braun did not look upon any 

 one form as the typical one, which through transformation had 

 developed the various leaf forms ; but each one represented a 

 wave in the march of the successive billows of a metamorphosis, 

 the shoot manifesting successive repetitions or renewals of 

 growth each season, presenting in order the " niederblatter, 

 laublatter, hochblatter, kelchblatter, blumenblatter, staub- 

 blatter, fruchtblatter." Though it had been since suggested 

 from time to time, as Goebel remarks, that the foliage leaf must 

 be regarded as the original one from which all the other forms 

 had arisen (at that time Goebel did not think this the correct 

 view). No research, he says, had been carried on, not even in 

 a single case, to determine this point. Goebel plainly showed, 

 in the case of Prumis padus, that axillary buds, which under 

 normal conditions were formed one year with several bud 

 scales, could be made by artificial treatment to develop during 

 the first year. This he accomplished by removing all the leaves 

 from small trees in April, and in some cases also cutting away 

 the terminal shoot. In these cases the axillary shoots, instead 

 of developing buds which remained dormant for one year, as in 

 normal cases, at once began to grow and developed well-formed 

 shoots. Instead of the usual number of bud scales, there were 

 first two stipule-like outgrowths, and then fully expanded leaves 

 were formed ; so that in this case, he says, the metamorphosis of 

 the leaf to bud scales was prevented. For this relation of bud 

 scales to foliage leaves, Goebel proposed the term " correlation 

 of growth." In the case of Vicia faba, removal of the lamina 

 of the leaf of seedlings, when it was very young, caused the 

 stipules to attain a large size, and to perform the function of 

 the assimilating leaf. He points out that experimentation aids 

 us in interpreting certain morphological phenomena which 

 otherwise might remain obscure. He cites the occasional 

 occurrence (" Moquin-Tandon ") in the open of enlarged 

 stipules of this plant, which his experiment aids in interpreting. 

 In the case of Lathyrus apkaca, the stipules are large and leaf- 

 like, while the part which corresponds to the lamina of the leaf 

 is in the form of a tendril, the correlation processes here having 

 brought about the enlargement of the stipules as the lamina of 

 the leaf became adapted to another function. Kronfeld repeated 

 some of Goebel's experiments, obtaining the same results, and 

 extended them to other plants {Pirns mains and Tisum sativum), 

 while negative results attended some other experiments. Hilde- 

 brand, in some experiments on seedlings and cuttings, found 

 that external influences affected the leaves, and in some cases, 

 where the cotyledons were cut, foliage leaves appeared in place 

 of the usual bud scales. In Oxalis rubella, removal of the 

 foliage leaf, which appears after the cotyledons, caused the first 

 of the bulb scales, which normally appear following the foliage 

 leaf, to expand into a foliage leaf. 



In some experiments on the influence of light on the form of 

 the leaves, Goebel has obtained some interesting results. Plants 

 of Campanula rotjindtfolia were used. In this species the lower 

 leaves are petioled and possess broadly-expanded, heart-shaped 

 laminae, while the upper leaves are narrow and sessile, with in- 

 tergrading forms. Plants in diff"erent stages of growth were 



