PLANT SOCIETIES 317 



(Fig. 245). The epidermis, even on the younger portions 

 of the stem, is highly cutinized (Fig. 121), and this structure 

 makes any evaporation very slow. 



387. Leaves of Xerophytes. In regions where the 

 greatest dangers to vegetation arise from long droughts 

 and the excessive heat of the sun, the leaves of plants 

 usually offer much less surface to the sun and air than is 

 the case in temper- 



ate climates. Some- 

 times the blade of 

 the leaf is absent 

 and the expanded 

 petiole answers the 

 purpose of a blade, 



Or, again, foliage FIG. 224. Cross-Section of Rolled-Up Leaf of Crow- 

 leaveS are altO- Berry (Empetrum nigruin). (Magnified.) 



gether lacking, as in the cactuses (Fig. 222), and the green 

 outer layers of the stem do the work of the leaves. 



388. Rolled-Up Leaves. Leaves which receive but a 

 scanty supply of water are often protected from losing it 

 too rapidly by being rolled up, so that the evaporating, 

 i.e., stomata-containing, surface is on the inside of the roll. 

 Sometimes, as in the crow-berry (Fig. 224), the curled con- 

 dition is permanent. In other plants, as in such grasses 

 as Stipa (Fig. 225), and in Indian corn, the leaf rolls up 

 when the weather is very dry and unrolls again when it 

 receives a better supply of water. 



389. Mesophytes. A mesophyte is a plant which 

 thrives best with a moderate supply of water. The great 

 majority of the wild and the cultivated plants of the 

 United States are mesophytes, and what has been learned 



