PROTECTION AGAINST TRANSPIRATION 267 
piring surface. (Fig. 240.) By reducing the number of stomata, 
as in many Grasses, or by sinking the stomata in special epider- 
mal cavities, as in the Carnation, transpiration is reduced. 
Fic. 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. 241.) The 
Fic. 241. — Cross section of a Corn leaf. 1, lower epidermis; u, 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 
