THE LOSS OF WATER BY THE PLANT 



177 



mature seeds, and die. Their whole life cycle occurs during the 

 short moist period, usually in spring. During the remainder of the 

 year, they exist in the form of ripe seeds that cannot be affected by 

 desiccation. In all their other characteristics these plants are 

 typical mesophytes. 



The groups discussed by no means exhaust the various xero- 

 phytes. The types sharply delimited here, moreover, are con- 

 nected by a series of plant forms showing imperceptible transi- 

 tions from one group to another. A detailed examination of 

 these types belongs to the 

 field of ecology. 



57. Correlation between 

 Assimilation and Transpira- 

 tion. Efficiency of Transpira- 

 tion and the Water Require- 

 ment. — The transpiration of 

 plants is closely connected 

 with assimilation by the mere 

 fact that carbon dioxide enters 

 through the same stomatal 

 openings through which water 

 vapor escapes. Moreover, the 

 transpiration current carries 

 with it mineral salts and 

 nitrogen compounds necessary 



for the synthesis of organic substances. The correlation between 

 these two processes is, therefore, very important not merely for the 

 physiological characterization of the plants, but also for the evalua- 

 tion of the external conditions under which the plant has to work. 



This correlation, or the efficiency of transpiration, is usually 

 expressed in grams of dry weight accumulated per liter of water 

 lost. Under rather constant environmental conditions the value 

 thus obtained remains quite stable for each plant. In central 

 Europe, for instance, cereals like wheat have a transpiration effi- 

 ciency of 3; those of the type of maize, about 5. The effi- 

 ciency of transpiration will vary with changes in the external con- 

 ditions. In the hot and dry climate of Tiflis (Caucasus) it is only 

 1.5 for wheat, and 2.5 for corn. If, on the contrary, a plant is 

 moved to the moist atmosphere of a greenhouse, the efficiency of 

 transpiration may be raised to 10 or even higher. 



Fig. 78. — Leaf of Stipa curled into a tube. 



