ADAPTATION TO WATER 145 



The modifications of the plant which serve to decrease water 

 loss may be grouped under the following heads: (1) position of 

 the leaf; (2) rolling of the leaf; (3) reduction of the leaf or stem 

 surface; (4) epidermal modifications, (a) of epidermal cells, (6) 

 of stomata; (5) changes in the chlorenchym. 



165. Decrease of water loss through leaf position. Horizontal 

 leaves, as a rule, transpire more than those which take a vertical 

 or oblique position. Since the light energy is greatest when the 

 svm is highest, those leaves transpire least which make the smallest 

 angle with the rays of the sun during the middle of the day. A 

 leaf at right angles to the rays of the sun receives almost ten 

 times as much light and heat upon the same surface as one placed 



^^y 



Fig. 43. Plants of the dogbane, Apocynum awlroscbmifolium. The hori- 

 zontal position of the leaves at night or early morning changes toward 

 midday to the vertical, thus protecting the leaves from the direct rays 

 of the sun as well as from the strong radiation from the gravel soil. 



at an angle of 10. Reduction of water loss by means of the 

 vertical or oblique position of the leaves is a frequent occurrence 

 in the erect or hanging leaves of many tropical trees. A similar 

 means is found in " compass plants," such as Silpilium laciniatum, 

 Lactuca scariola, etc., and in all species with more or less erect, 

 hanging, or equitant leaves. The effect, however, is just opposite 

 in the sunflower and other heliotropic species, since the turning 

 of the crown tends to maintain a position at right angles to the 

 rays. In the case of plants that grow in mats, the aggregation of 

 stems brings about the mutual protection of the leaves. In addi- 

 tion, mats often have erect or oblique leaves. 



166, Decrease through the rolUng of the leaf. In a large num- 

 ber of plants, the amount of leaf surface exposed to dry air is re- 

 duced by the rolling or folding of the leaf. Rolling occurs in many 



