The Water Relations of Leaves 55 



reduced, so that evaporation from the mesophyll cells is 

 greatly lessened. In extreme cases the mesophyll cells are 

 all of the compact palisade type, which leaves the minimum of 

 air space within the leaf. 



(3) Small leaf area. A third way in which plants become 

 adjusted to dry conditions is by a decrease in the total leaf 

 area. When a plant is brought into the house in autumn, 

 it drops a number of leaves. The air inside most houses 

 being much drier than the air outside, transpiration is greatly 

 increased. As the water supply remains about the same, 

 the dropping of a few leaves restores the internal water bal- 

 ance of the plant. Some trees, like the cottonwood, shed 

 part of their leaves during a summer drought. If a wet 

 period follows, more leaves may be added, and in this way a 

 nearly uniform water balance is maintained. 



That plants growing under moist conditions have larger 

 leaves and more leaves than the same kinds of plants growing 

 under dry conditions has been noted by every one. The 

 contrast may be observed by comparing weeds that grow along 

 the base of a railroad embankment or the high bank of a stream 

 with those that grow near the top. 



Transplanting and the water balance. Wlien the skillful 

 gardener transplants a tree, he cuts off a number of branches 

 to reduce the number of leaves, in order that the plant may 

 not dry out before new water-absorbing roots are developed. 

 Before lettuce, tomato, and cabbages are lifted for trans- 

 planting, the plants should be watered and allowed to become 

 turgid ; water should be poured into the holes in which they 

 are placed, before the soil is closed in around the plants. It 

 is customary also to cover the plants with boards or paper 

 covers so as to reduce the transpiration. Maintaining the 



