LEAVES 589 



between the structural requirements associated with transpiration and 

 carbohydrate synthesis. The form of leaf best fitted for maximum syn- 

 thesis is least fitted for the reduction of transpiration, and vice versa. 

 Hydrophytes and mesophytes as a class have thin, expanded leaves well 

 fitted for synthetic activity, because their structure is such as to facili- 

 tate the reception of light and the absorption of carbon dioxid. Xero- 

 phytes, on the other hand, including most plants of alpine and arctic 

 regions and also those of salt marshes and peat bogs, have small, thick 

 leaves, or leaves otherwise/ suited for reducing transpiration. Transpira- 

 tion is a minor danger among hydrophytes and mesophytes, because the 

 water supply commonly is adequate, just as insufficient light for synthesis 

 is rarely a danger in xerophytes. To a large extent the features that 

 facilitate synthesis on the one hand, or the reduction of transpiration 

 on the other, are determined by external factors; indeed, in many in- 

 stances transpiration itself occasions the production of the very struc- 

 tures (cutin, wax, hairs, etc.), which minimize the dangers that it causes. 

 It would appear, however, that many plant structures are not thus 

 related to environment. The xerophytic features of such leaves 

 as those of the ericads, conifers, and begonias, features that are 

 equally advantageous with those of other xerophytic leaves in reduc- 

 ing transpiration, appear inflexible when subjected to varying conditions. 

 But as conditions of soil and climate are subject to constant change, 

 those species whose structures become modified accordingly would seem 

 to be best fitted to survive. The extinction of species often may have 

 resulted from a lack of plasticity. 



6. VARIATIONS IN LEAF FORM 



The significance of leaf variation. Distinctions between plant spe- 

 cies commonly are based upon the forms of leaves and of other organs, 

 hence the determination of the causes underlying form is among the 

 most fundamental of problems. In many species (e.g. Sagittaria hetero- 

 phylla, figs. 848-853) there is a wide variation in leaf form which is 

 connected definitely with external causes. When comparable differences 

 in leaf form constitute specific characters, it is a tenable hypothesis 

 that the present species are the fixed descendants of once plastic an- 

 cestors that had a range of variation broad enough to include differ- 

 ences as great as those that- to-day characterize distinct species. For 

 example, among the buttercups there are some species (as Ranunculus 



