July 14, 1905.] 



SCIENCE. 



41 



leaves and stamens. The former Coulter 

 and Chamberlain would regard as leaves, 

 the latter not; where, however, is the line 

 of separation between themi 



From the limited power of transforma- 

 tion possessed by organs it results that in 

 causal morphology the problem is then not 

 a phylogenetic, but an ontogenetic one. 

 Whether sporophylls or foliage leaves are 

 the older phylogenetically may be disre- 

 garded. For it appears more important 

 first to determine why the power of trans- 

 formation is limited, why a shoot-thorn or 

 a shoot-tendril may be transformed only 

 into a shoot, a stamen or a carpel only 

 into a 'leaf; and second, what conditions 

 are determinative thereto. 



The first step toward the solution of the 

 problem is that we learn to call out experi- 

 mentally and at will such transformations 

 as we have heretofore occasionally observed 

 as 'abnormalities.' 



This has been successful in experimental 

 morphology in a great number of cases, and 

 in the future will be still more so. To be 

 sure, we are still unable to induce the trans- 

 formation of stamens into flower leaves at 

 will — we only deceive ourselves when we 

 believe that the art of the plant breeder has 

 succeeded in doing this, for in reality all 

 he has done is to isolate such races which 

 have occurred in nature with more or less 

 doubled flowers — and in this regard we 

 stand in contrast to the fungi and insects, 

 the activities of which, as Peyritsch and 

 others have shown, often— unconsciously 

 of course— call forth such transformations. 

 Yet it has been possible to change scale 

 leaves (cataphylls) and sporophylls into 

 foliage leaves, inflorescences into vegetative 

 shoots and, vice versa, plagiotropous into 

 orthotropous shoots, hypogseous into epi- 

 gasous, not to mention the interesting re- 

 sults which have been obtained by Klebs in 

 his studies of the lower plants. 



Let us take, for example, the just men- 



tioned transformations of scale leaves into 

 foliage leaves and of sporophylls into 

 sterile leaves. Here developmental study 

 and experiment immediately encroach on 

 each other. Development has shown that, 

 e. g., the bud-scales of many trees which 

 in their definitive condition are very dif- 

 ferent from the foliage leaves, yet parallel 

 them developmentally in an extraordinary 

 degree ; and that many bud scales possess 

 the fundament of a leaf blade which has 

 failed to develop and has thus become ves- 

 tigial. Similarly, the fundaments of the 

 foliage leaf and the sporophyll in Onoclea 

 are the same up to a quite late stage of 

 development, beyond which each follows its 

 own course. These facts gave occasion to 

 the question whether or not it were possible 

 to influence the development at will, and so 

 to cause a scale leaf or a sporophyll to 

 grow from a fundament which otherwise 

 would develop into a foliage leaf. It has 

 been shown that such transformations may 

 be occasioned in a simple way, and the de- 

 velopmental correspondence makes such a 

 limited transformation without further 

 difficulty capable of being understood. 

 And since seedlings produce, apart from 

 the cotyledons and certain adaptations in 

 hypogseous germination, only foliage leaves, 

 which are arranged for the work of photo- 

 synthesis; since further it is seen that 

 all foliage leaves of one and the same 

 plant, different as they appear externally, 

 yet in reality follow one and the same 

 course of development, which, as we have 

 seen, is remarked also in scale leaves and 

 sporophylls; I accordingly come to the 

 view that other leaf organs are derived 

 from foliage leaf fundaments through a 

 change in the course of development occur- 

 ring at an- earlier or later period of growth. 

 This conception has found many opponents, 

 some of them for the reason that they have 

 not been able to free themselves from the 

 purely historical conception of the problem. 



