486 PEACTICAL BOTANY 



on which they occur most abundantly (usually the lower 

 one, Fig. 57, A). The same species or individual sometimes 

 becomes more hairy when subjected to a drier atmosphere. 

 Experiments show that shearing off the hairs from the surface 

 of the living leaf increases the loss of water by transpiration, 

 sometimes even doubling its amount. 



Rolled-up leaves (6) are familiar in the case of corn (Fig. 2). 

 It would not be easy to perform a field experiment to prove 

 exactly how much the loss of water is lessened in the rolled 

 corn leaves, but it would seem that the surfaces are consid- 

 erably less freely exposed to the air in the rolled condition 

 than when the blades are flat, and free exposure is a well- 

 known factor in increasing transpiration. In some xerophytic 

 grasses there is a complicated arrangement of folds in the 

 leaves by means of which they can close up the transpir- 

 ing surface (almost as in Fig. 363) or open it completely to 

 the air. 



Shedding the leaves (6) is the principal means by which our 

 deciduous trees and shrubs escape the dangers of dry winter 

 weather when no moisture can be absorbed from the ground. 

 It has been found that the larch (which sheds its leaves) is 

 more resistant to such conditions than are most of the ever- 

 green conifers. Some shrubs retain or shed their leaves in 

 a rainless summer according to the amount of soil mois- 

 ture with which they are supplied. The Euphorbia splendens 

 (Fig. 292, A) is a commonly cultivated plant which well illus- 

 trates this capacity to adjust the amount of leaf surface to 

 a varying moisture supply. 



Plants with bulbs (6) are notably common in regions where 

 there is a long rainless summer. To a botanist one of the 

 most interesting sights of the .Mediterranean coast region is 

 the sudden blooming of many bulb-bearing plants toward the 

 close of the summer. Most conspicuous of these is a mem- 

 ber of the Lily family, Urginea SciUa, which sends up its stout 

 flower stalk, almost as tall as a man, out of the earth baked 

 hard by two or more months of hot weather almost without 



