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!, 
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? 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® 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 Kiebs, “ Variationen der Bliiten,” Jahrb. Wiss. Bot. 1905, p. 260. 
2 Cf, Goebel, loc. cit. chap. 11.; also Gliick, Untersuchungen tiber Wasser- ‘und Sumpf- 
gewtichse, Jena, Vols. 1.—11. 1905—06. : 
3 Bonnier, Recherches sur UV Anatomie expérimentale des Végétaux, Corbeil, 1895. 
