82 TEXTBOOK OF BOTANY 



south plane. In shade, these leaves have the usual oblique or horizontal 

 position. 



Some plants tliat grow partly submerged have finely divided leaves 

 under water and nearly entire leaves above water. The mermaid weed 

 ( Proserpinaca ) , for example, grows well both on moist soil and in water. 

 It is what may be called a highly plastic species since its leaf types are 

 readily changed. As long as the leaf primordia are under water the leaves 

 that develop are divided. If the leaf primordia are raised above the 



WARM MOIST 



COOL MOIST 



Fig. 41. EflFects of temperatiue and drought on leaf size and structure of the 

 sour dock. In the warm dry culture the plants wilted daily. The temperature of the 

 substrate in the "cool moist" culture was on the average 10 degrees below that of 

 the "warm moist" culture. 



water, undivided serrate leaves result (Fig. 40). Moreover, if the stem 

 tip is held alternately above and below the water, successive tiers of 

 divided and undivided leaves may develop on the same stem. The 

 epidermis of submerged leaves may have paired guard cells which do 

 not separate and form stomates. Internally the submerged leaves have 

 considerably less differentiation of cells and tissues than do the aerial 

 leaves. 



In a preceding chapter attention was called to the chemical effects of 

 light and temperature on the greening of leaves, on autumn coloration, 

 and on abscission of leaves. Some of the effects of drought and tempera- 

 ture on the development of leaves are illustrated in Fig. 41. Drought 

 often increases the spinescence of plants (Fig. 42). 



