[Chap. XXV TRANSPIRATION AFFECTS PLANT DEVELOPMENT 243 



root systems were larger in proportion to the leaves and stems above 

 ground, and both their dry weight and salt content were greater. That 

 is, there was a greater amount of photosynthesis in the plants in the 

 drier air, a greater amount of assimilation of foods, and a greater amount 

 of salts absorbed. Here again is evidence of quantitative variations in 

 plant development due in part to differences in transpiration, but such 

 facts are not evidence that transpiration is essential to the plant. One 

 may say, however, that it is essential to certain differences in develop- 

 ment, provided that these same differences cannot be obtained by chang- 

 ing some other process. 



Most plant phvsiologists have come to the conclusion that wet cell 

 walls, exposed either directly to the atmosphere or to the intercellular 

 spaces necessarily lose water to the surrounding air by evaporation. 

 Through a long process of evolution stomates have become an hereditarv 

 structure in all the larger land plants. The opening and closing of these 

 structures have a marked effect upon the inward and outward diffusion 

 of COi', O2, and water vapor. When stomates are closed there is nothing 

 to prevent the slow diffusion of water through all plant surface walls 

 even though the walls contain cutin and suberin. When stomates are 

 open there is nothing to prevent the rapid diffusion of water vapor 

 from the mesophyll cell walls through the intercellular spaces and the 

 stomates to the atmosphere. This rate is greatlv increased in full sun- 

 light when leaf temperatures rise above that of the atmosphere. 



If in the course of plant historv a cell wall substance permeable to 

 oxygen and carbon dioxide but impermeable to water vapor had 

 developed, the present daily waste of water in transpiration from plants 

 might have been eliminated. 



Summary. Transpiration effects are best visualized when excessive 

 water-vapor loss decreases photosynthesis, growth, and the develop- 

 ment of flowers, fruits, and seeds. It is a menace in the life of every land 

 plant except those having roots in a permanent water supply. 



Excessive transpiration is responsible for more crop failures than all 

 otlier factors combined. As new lands were opened in the Central States, 

 centers of production of various crops became stabilized where the crops 

 were most productive, had superior quality, and therefore where 

 they were most profitable. One of the factors involved in this move- 

 ment is transpiration. When agriculture moved during a few favorable 

 years into the short-grass lands, succeeding dry years resulted in exces- 

 sive transpiration, killing of the vegetation bv desiccation, wind erosion, 



