58 



Science of Plant Life 



do not open until the 

 leaf is above water. If 

 the leaves are raised 

 entirely above the sur- 

 face of the water, as 

 sometimes happens 

 when the plants are 

 crowded, both surfaces 

 develop stomata. 



Desert plants and 

 water storage. In the 

 desert, where the air is 

 very dry and the scanty 

 rainfall is confined to 

 one or two periods in 

 the year, plants have 

 very great difficulty in 

 securing water. The 

 pereimial plants have 

 various ways of' con- 

 serving water from one 

 rainy period to the 

 next. The barrel cac- 

 tus has no leaves at 

 all, and the stem is a 

 thick cylinder composed largely of water-storage tissue; it 

 may live without water for 2 years or longer. Some of the 

 desert shrubs have leaves during the rainy periods only, and 

 these are shed as soon as the drought comes. Still others, 

 like the agaves, have very thick, leathery leaves with much 

 internal water-storage tissue and a very low transpiration rate. 



Fig. 38. Desert plants, including forms of Cereus, 

 prickly pear (Opuntia), Yucca, Euphorbia, and cen- 

 tury plant {Agave). 



