Chapter X _ 207 — Loss and Retention 



moisture supply and the almost innumerable adaptations by which plants 

 have left their original aquatic environment and moved across valleys and 

 over mountains and into the remotest wastes of the deserts and frozen arctic 

 regions. One wonders if during the ages of development, plants, by the 

 many ways described in these pages, have not about exhausted the possible 

 means of adaptation. This would impose a definite limit on the extent to 

 which man may be able by plant breeding or other technical means, to in- 

 crease production in the absence of available moisture. Though plant ex- 

 ploration may find plants better able to survive drought, and hybridization 

 may result in plants with more extensive root systems, it seems that the 

 most promise lies in the possibility of conserving and utilizing our existing 

 supplies of water more fully. As forests become depleted, oil reserves 

 fail, and agricultural lands run short, it may become profitable to harvest 

 cellulose and other plant products from the vast desert regions that still 

 remain relatively unoccupied. Production on such areas, however, must 

 remain slow. More hopeful seems to be the efificient utilization of our pro- 

 ductive agricultural lands through control of pests, effective use of fertili- 

 zers, advanced methods of culture and harvest and, at least in our semi- 

 arid regions where irrigation is practiced, the most effective possible use of 

 our water resources. 



In many lands the impounding and distribution of flood waters and 

 conservation of all available supplies keynote the current developments in 

 agriculture. 



Summary: — A favorable water balance is essential to growth and development 

 of plants. Considerable energy may be expended in the absorption of water from the 

 soil ; however, there are definite limits of DPD of water in the plant above which the 

 plant cannot operate normally. 



Transpiration may be essential in the transport and distribution of mineral nutrients 

 throughout the plant ; it usually exceeds by far the proportions required for their func- 

 tion; it has been termed a necessary evil for it is an inevitable consequence of the 

 gaseous exchange necessary for photosynthesis. Water conservation is important in 

 the economy of most plants and many of the mechanisms considered in studies on 

 transpiration serve a vital role in water retention. 



Transpiration may be stomatal or cuticular. Much of the regulation of water loss 

 is effected by stomatal movements. 



Studies on evaporation and factors controlling it have aided in an understanding of 

 transpiration. Evaporation follows definite physical laws and is affected by osmotic 

 concentration of the solution, temperature, dryness of the air, atmospheric pressure, 

 and wind velocity. 



Many methods have been devised for measuring transpiration such as loss of 

 weight, water absorption, and various measurements on water vapor given off. All 

 of these involve restriction of some type upon the natural activity of the plant or the 

 creation of artificial conditions. The greatest problem in such studies is interpretation 

 of the results. 



The cuticle of plants is a wax-like coating that may vary widely in composition 

 and thickness. Cuticular transpiration may make up an appreciable portion of the total 

 water loss of some shade plants. It may be negligible in desert succulents. 



Fluctuations in transpiration may be related to wall structure, physical resistance 

 of the root system to absorption, osmotic properties of cells, internal surface, stomatal 

 number and distribution, and other internal factors as well as the many external factors 

 of the environment that govern evaporation. 



Opening and closing of stomata have been studied by many methods. Such studies 

 show that the stomatal mechanism plays an essential role in regulating transpiration 

 but that the laws are complex and difficult to interpret. Mineral nutrients may affect 

 transpiration through their osmotic effects upon the DPD of the soil solution, their 

 tendency to increase the concentration within plant cells, and through secondary effects 

 such as prevention of deficiency diseases, water permeability, stomatal movement, leaf 

 abscission, and the like. 



