Influence of Environment on Plant-organs 235 



requires experimental proof, which in the case of the egg-cells of 

 flowering plants hardly appears possible ; but it derives considerable 

 support from the fact that in herbaceous plants, e.g. Sempervivum 1 , 

 rosettes or flower-shoots are formed in response to external con- 

 ditions at the base, in the middle, or at the apex of the stem, so that 

 polarity as it occurs under normal conditions cannot be the result of 

 unalterable hereditary factors. On the other hand, the lower plants 

 should furnish decisive evidence on this question, and the experi- 

 ments of Stahl, Winkler, Kniep, and others indicate the right method 

 of attacking the problem. 



The relation of leaf-form to environment has often been investi- 

 gated and is well known. The leaves of bog and water plants 2 afford 

 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 on transplantation from 

 the plains to the mountains 3 or vice versa. Such variations are by 

 no means isolated examples. All plants exhibit 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 different 

 composition and strength. 



Every individual which is exposed to definite combinations of 

 external factors exhibits eventually the same type of modification. 

 This is the type of variation which Darwin termed "definite." It is 

 easy to realise that indefinite or fluctuating variations belong essenti- 

 ally to the same class of phenomena ; both are reactions to changes 

 in environment. In the production of individual variations two 

 different influences undoubtedly cooperate. One set of variations 

 is caused by different external conditions, during the production, 

 either of sexual cells or of vegetative primordia ; another set is the 

 result of varying external conditions during the development of the 

 embryo into an adult plant. The two sets of influences cannot as yet 

 be sharply differentiated. If, for purposes of vegetative reproduction, 

 we select pieces of the same parent-plant of a pure species, the 

 second type of variation predominates. Individual fluctuations de- 

 pend essentially in such cases on small variations in environment 

 during development. 



These relations must be borne in mind if we wish to understand 

 the results of statistical methods. Since the work of Quetelet, 



1 Klebs, " Variationen der Bliiten," Jahrb. Wiss. Bot. 1905, p. 260. 



2 Cf. Goebel, loc. cit. chap. n. ; also Gliick, Untersuchungen iiber Wasser-'und Sumpf- 

 gewdchse, Jena, Vols. i. n. 1905 06. 



3 Bonnier, Recherches sur V Anatomic experimentale des Vegttaux, Corbeil, 1895. 



