PROTECTION AGAINST TRANSPIRATION 



267 



piring surface. {Fig. 240.) By reducing the number of stoniata, 

 as in many Grasses, or by sinking the stomata in special epider- 

 mal -cavities, as in the Carnation, transpiration is reduced. 



Fig. 240. — A globular cactus, an example of a plant having leaves 

 replaced by spines. After J. M. Coulter. 



Sometimes, as in the Corn, the rolling of the leaves decreases 

 the surface exposed and lessens transpiration. {Fig. 2^1 •) The 



Fig. 241. — Cross section of a Com leaf. I, lower epidermis; ;/, upper 

 epidermis. Notice that the cells are larger on the upper side than on the 

 lower side of the leaf. The cells of the upper epidermis, being larger, shrink 

 more than those of the lower epidermis, and thus cause the rolling of the 

 leaf in dry weather. Much enlarged. 



leaves may have an edgewise position and thereby avoid the 

 direct rays of the midday sun, as Wild Lettuce illustrates. 



The shedding of leaves from the plant is an important means of 

 protection. Many of our trees shed some of their leaves during a 



