Febbuaby 3, 1911] 



SCIENCE 



169 



the submerged leaves of both are linear or ribbon- 

 like. In all cases the submerged leaves are of a 

 more delicate texture, more or leas translucent, 

 and of a brighter green color than the others. 



This is a much more complex case than 

 Vernon realizes. Submerged leaves do not 

 develop the supporting frame of the aerial 

 leaves. The delicate texture is therefore to 

 a great extent a result of the supporting 

 power of the water and not of its wetness. 

 Since the supporting structure of the aerial 

 leaf is not developed when the leaf is sub- 

 merged, the leaf grows longer, just as the 

 stem of the plant grows longer in the dark 

 because the leaves develop but slightly. 

 The thinness of the leaf is probably chiefly 

 a result of the decreased intensity of light. 

 This is the best analysis that can be given 

 at present; but it brings out clearly how 

 slack people have been in controlling con- 

 ditions. 



The effect of excessive moisture or aridity 

 upon plants is similar in type'^ to the ef- 

 fect of heat or cold on the pupse of butter- 

 flies. 



The relation of leaf form to environment has 

 often been investigated and is well known. The 

 leaves of bogs and water-plants aflford the most 

 striking examples of modifications: according as 

 they are grown in water, moist or dry air, the 

 form of the species characteristic of the particular 

 habitat is produced, since the stems are also modi- 

 fied. To the same group of phenomena belongs 

 the modification of the forms of leaves and stems 

 in plants or transplantation from the plains to 

 the mountains or vice versa. Such variations are 

 by no means isolated examples. All plants ex- 

 hibit a definite alteration in form as the result 

 of prolonged cultivation in moist or dry air, in 

 strong or feeble light or in darkness, or in salt 

 solutions of difi'erent composition and strength. 



The last sentence in the preceamg para- 

 graph is interesting for what it does not 

 say. There is no indication that Klebs has 

 any inkling of the universal law underly- 

 ing all these changes. 



"" Klebs, " Darwin and Modern Science," 235, 

 1909. 



FOOD AND FERTILIZERS 



It seems evident that an exuberant 

 growth would be favorable to variability 

 and to the development of sports. This is 

 universally recognized to be the case. 

 Thus Darwin"" says : 



Of all the causes which induce variability, ex- 

 cess of food, whether or not changed in nature, 

 is probably the most powerful. This view was 

 held with regard to plants by Andrew Knight and 

 is now held by Schleiden, more especially in refer- 

 ence to the inorganic elements of the food. In 

 order to give a plant more food, it sufiSces in 

 most cases to grow it separately, and thus pre- 

 vent other plants robbing its roots. It is sur- 

 prising, as I have often seen, how vigorously our 

 common wild species flourish when planted by 

 themselves, though not in highly manured land; 

 separate growth is, in fact, the first step in cul- 

 tivation, we see the converse of the belief that 

 excess of food induces variability in the following 

 statement by a great raiser of seeds of all kinds: 

 " It is a rule invariably with us, when we desire 

 to keep a true stock of any one kind of seed, to 

 grow it on poor land without dung; but when we 

 grow for quantity we act contrary, and some- 

 times have dearly to repent of it." According 

 also to Carri&re, who had great experience with 

 flower-garden seeds, " On remarque en gSnfiral les 

 plantes de vigeur moyenne sont celles qui con- 

 servent le mieux leurs caractferes." 



Under the heading of effect of cultiva- 

 tion, Klebs*" says : 



It is however a fact that if a plant is removed 

 from natural conditions into cultivation, a well- 

 marked variation occurs. The well-known plant 

 breeder, L. de Vilmorin, speaking from his own 

 experience, states that a plant is induced to 

 affoler, that is, to exhibit all possible variations 

 from which the breeder may make a further selec- 

 tion only after cultivation for several generations. 

 The effect of cultivation was particularly striking 

 in Veronica chamcedrys, which, in spite of its 

 wide distribution in nature, varies very little. 

 After a few years of cultivation this "good" and 

 constant species becomes highly variable. The 

 specimens on which the experiments were made 

 were three modified inflorescence cuttings, the 



"° " Animals and Plants under Domestication," 

 2d ed., 2, 214, 1890. 



'" " Darwin and Modern Science," 245, 1909. 



