CHAPTER XXV 



TRANSPIRATION AFFECTS PLANT DEVELOPMENT 

 AND DISTRIBUTION 



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Environmental factors that affect the rates of transpiration, photosyn- 

 thesis, or respiration often hmit the development and distribution of 

 plants. In green plants both photosynthesis and respiration are essential 

 processes. Without them these plants neither grow nor survive very 

 long. When respiration continuously exceeds photosynthesis green plants 

 die of starvation. 



Although transpiration, unlike respiration and photosynthesis, is not 

 known to be essential to plants, its harmful effects become apparent 

 when water loss exceeds absorption and results in wilting, desiccation, 

 and even death. Excessive transpiration may profoundh' affect food 

 manufacture, growth, and the production of flowers, fruits, and seeds. It 

 is often an effective factor in limiting the geographic distribution of 

 individual plants and of plant communities. Because of its effects on the 

 qualities and yields of crop plants, transpiration has been an ever-present 

 influence in the regional allocation of crops in the United States. 

 Throughout human history it has been one of the primary factors that 

 limited the population of various geographic regions. 



Tissue development in the growing regions of plants depends upon 

 an adequate water content in the cells. The enlargement of cells and 

 tissues ceases when transpiration exceeds the water supply. During hot 

 dry weather in summer a plant exposed to full sunlight may be actually 

 smaller in the evening that it was in the early morning. Tree trunks may 

 be measurably smaller in diameter in late afternoon than they were in 

 early morning. 



Each spring we may observe the opening of buds and the grovvi:h of 

 new leaves on trees and shrubs. There is little cell division after the 

 leaves are one-fourth grown, and further expansion is the result of the 

 growth of cell walls and the osmotic absorption of water in the cells 

 of the leaf. During wann moist weather the volume of a young growing 

 leaf may become doubled during one night. When transpiration exceeds 



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