THE MATERIAL OUTGO OF PLANTS 331 



Measured evaporation from 100 sq. cm. of leaves (200 sq. cm. of surface) 



IN BRIGHT DIFFUSE LIGHT, AT ABOUT 20° C, WITH HUMIDITY ABOUT 

 50 PER CENT 



i hr. 24 hr. 



Phaseolus vulgaris 0.117 gm. 2.81 gm. 



Hedera Helix 0.17 4-09 



Begonia argentea 0.19 4.57 



Coleus Blumci 0.21 1 5.06 



Cucurbita Pepo 0.224 5-39 



Ficus elastica 0.262 6.3 



Helianthus annuus 0.5 12.0 



Lupinus albus 0.594 14.27 



Chrysanthemum frutescens 0.681 *6-35 



Vicia Faba 0.683 l6 -4 



Hemp plants in a season of 140 days were estimated to evaporate (each) 27 kg. 

 and sunflowers 66 kg. of water. It is estimated that if the water evaporated by the 

 following cereals were again condensed on the area occupied by each sort, say 1 

 sq. m., it would cover the ground in the case of rye to a depth of 83 mm., 

 wheat, 118 mm., and oats, 127 mm. The average annual rainfall in the north cen- 

 tral states is in the neighborhood of 1000 mm., so that one twelfth to one eighth of 

 the total passes through such cereals. 



A birch tree with 200,000 leaves is estimated to evaporate on a hot day 300 to 

 400 kg. A beech, 15 years old, is said to average about 75 kg. per day in the months 

 from June to September, inclusive. At that rate a hectare of beech forest contain- 

 ing 400-600 trees would evaporate some 20,000 barrels. In all these calculations 

 and estimates a liberal allowance must be made for errors. 



Reduction of water loss. — Among all the agencies that affect the form 

 and mode of development of plants none has more influence than water, 

 and the relation between the available supply and the loss by evapora- 

 tion. In the peculiarities of form and structure which seem related 

 particularly to water, many see " adaptations " to a habitat with much 

 water, a moderate amount, or a scanty supply. Thus the cutinization 

 of the epidermis, the formation of a waxy or resinous coating, and the 

 development of cork are structures which reduce the loss of water. 

 In other plants the scanty or fleshy foliage, the complete absence of 

 leaves, the development of water-holding tissues, the short cylindric or 

 globular fleshy body, the deep-running roots, and many other peculi- 

 arities ("treated more fully in Part III, Ecology) are considered as 

 " adaptations " to a dry climate. It would be better to look upon 

 them as effects of climate and similar factors, since experiments indicate 

 that such " adaptations" can be produced at will, even in one generation, 

 by cultivation under appropriate conditions. 



