578 



ECOLOGY 



of surface to volume, the former leaves being narrower and thicker 

 than the latter, and thus presenting a smaller transpiring surface (figs. 

 867, 868) ; furthermore, small leaves are much less rapidly heated than 

 are large leaves with equal exposure to the sun. The extremes of 

 divergence in this respect are represented, on the one hand, by hydro- 

 phytic or mesophytic leaves one to three cells thick (as in mosses, filmy 

 ferns, and pondweeds, fig. 763), and, on the 

 other hand, by xerophytic leaves that are 

 round in cross section (figs. 926, 927). Within 

 the same species (as Lactuca scariola) the xero- 

 phytic form has narrower and thicker leaves 

 than has the mesophytic form. In some 

 xerophytes (notably in the Ericaceae) the 

 leaves are revolute {i.e. with edges curved 

 under), a habit that results in a reduced trans- 

 piring surface without a change in volume 

 (figs. 828-830). Frequently sun leaves are 

 concave as viewed from above, hence pre- 

 senting a reduced surface to the sun, and 

 contrasting with the flatness of shade leaves. 

 The internal transpiring surface of a xero- 

 phytic leaf is decreased by the reduction of 

 air spaces and by the consequent compact ar- 

 rangement of the mesophyll ; this feature, like 

 the changes in proportion between surface and 

 volume, can be induced experimentally (figs. 

 768, 769). The most pronounced reductions 

 in the transpiring surface are found in cushion 

 plants and in plants wholly without leaves; 

 these and similar cases will be treated under 

 stems (p. 739). 



Reduction in transpiration due to leaf orientation. — Diaphototropic 

 leaves appear to be placed in a position favorable for maximum synthesis, 

 while the vertical or profile position seems to make possible a reduction in 

 transpiration, by reason of less direct exposure to the sun. The leaf 

 orientation of Lactuca (fig. 785) and of Nicotiana (fig. 786) may be re- 

 garded as advantageous from the standpoint of protection from trans- 

 piration, the latter plant exhibiting, as the transpiration increases, a 

 series of leaf positions varying from horizontal to vertical- 



Figs. 828-830. — Leaves 

 of certain Ericaceae, showing 

 xerophytic features ; such 

 leaves are stiff and leathery, 

 largely by reason of promi- 

 nent cutinization : 828, a leaf 

 of the Labrador tea {Ledum 

 groenlandicum), viewed from 

 above; 829, a leaf of the same 

 plant, viewed from beneath, 

 showing a copious woolly to- 

 mentum and revolute edges; 

 830, a diagrammatic cross 

 section of a revolute leaf, 

 that of the bog rosemary 

 {AtidroTneda glaucophylla). 



