THE STEM AND THE LEAF 



81 



79. Structure of leaves in relation to water supply. Leaves 

 of plants which grow in very moist earth usually have a 

 moderately thin epidermis l and are of a rather loose interior 

 structure, with abundant air spaces even more than are 

 shown in figure 26. 



The leaves of 

 plants which usu- 

 ally grow in places 

 where they are some- 

 times in great dan- 

 ger of dying for lack 

 of water commonly 

 have a thick, nearly 

 moisture-proof epi- 

 dermis and closely 

 packed cells in the 

 interior of the leaf. 

 This insures slow 

 evaporation of what- 

 ever water is car- 

 ried into the leaf, 

 since the thick 

 epidermis prevents 

 rapid drying up of 

 the cells near the 

 exterior, and the 

 scanty air spaces 

 render the circula- 

 tion of dry air com- 

 ing into the leaf 

 slow and difficult. Such leaves are often densely covered 

 with hairs, especially on the lower surface, and this coating 

 of hair has been found greatly to hinder the escape of moisture 

 through the stomata. 



1 It is not necessary to discuss here the marsh plants and halophytes 

 treated in works on the ecology of plants. 



FIG. 66. The giant cactus 



This specimen is about 40 feet high. The holes seen in 

 one of the stems are birds' nests. Photograph by the 

 Desert Botanical Laboratory of the Carnegie Institution 



