7io 



ECOLOGY 



by cold. In such regions trees have thick, impermeable bark, and 

 the leaves of sclerophyllous species are heavily cutinized and thus well 

 protected against excessive transpiration (figs. 809, 955). The advan- 

 tages possessed by such trees are extensive foliage display and the pos- 

 sibility of synthetic work at all seasons. However, there are corre- 

 sponding disadvantages, since much energy and material are used in 

 fhe construction of protective tissues, and since the heavily cutinized 

 layers which reduce transpiration also reduce synthesis. SclerophyU 

 lous evergreens may be broad-leaved, as in the olives, oaks, and hollies 

 of warm temperate climates, or narrow-leaved, as in the conifer which 

 have their culmination in cold climates. 



Leafless evergreen trees. Leafless trees, such as Casuarina and the 

 taller cacti (fig. 1035), are well fitted for climates in which all seas >ns 

 are unfavorable, there being a relative minimum of synthetic surface 

 and a relative maximum of protection from excessive transpiration, 

 because of verticality, leaflessness, water accumulation, a highly cu in- 

 ized epidermis, and slight surface exposure in proportion to volu ne. 

 Restriction in foliage display, and hence in synthesis, generally is lis- 

 advantageous, but in arid regions it is of marked advantage since 

 excessive transpiration entails far greater danger than does a scsnty 

 food supply. The cactoid form is illustrated not alone by the cacti 

 of American deserts, but also by wholly unrelated plants of African 

 deserts having almost identical form (e.g. Euphorbia and Stapelid). 

 The four evergreen habits above noted, namely, rain forest evergreens, 

 broad-leaved sclerophylls, narrow-leaved sclerophylls, and leafless ever- 

 greens, are fitted in the order mentioned for conditions that are in- 

 creasingly xerophytic. Evergreen shrubs may be divided into the 

 same four classes although the shrubs in each class extend into much 

 severer climates than do the trees. 



Deciduous trees. Different as are the classes of evergreens anc the 

 conditions for which they are fitted, they agree in not changing heir 

 aspect from season to season. Deciduous trees (figs. 843-845, 356) 

 and shrubs, on the other hand, exhibit leaf abscission at the inception of 

 dry or cold periods, 'thus presenting two seasonal aspects. Such trees 

 in the vegetative period may have leaves that are as expanded and 

 about as little protected as are those of rain forest evergreens, while in 

 the inclement period they are as well protected as are the cacti and 

 better protected than are the sclerophylls. The obvious advantages 

 of the deciduous habit are partially offset by obvious disadvantages; 



