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! 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. 
Fie. 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 
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. 
