[Chap. XXV TRANSPIRATION AFFECTS PLANT DEVELOPMENT 235 



Table 8. Rates of Water Loss per Day in Midsummer: 



A single plant of corn may lose 3 to 4 quarts 



A single plant of giant ragweed 6 to 7 quarts 



A single young 10-foot apple tree 10 to 20 quarts 



A 12-foot columnar cactus 0.02 quart 



A coconut palm in the moist tropics 70 to 80 quarts 



A date palm in a Sahara desert oasis 400-500 quarts 



lose the equivalent of 100 per cent of the water contained within it. On 

 the same basis a date palm may lose 90 per cent and a columnar cactus 

 .04 per cent. 



Table 9. The Water Losses from Single Plants during a Growing Season 

 Estimated from Experimental Data 



Effects of excessive water loss. When rains are frequent and well spaced 

 throughout the growing season and the soil contains a favorable supply 

 of water, the enormous losses of water vapor by individual plants, or by 

 plant communities, are of little importance in the survival of the plants. 

 However, if transpiration in excess of water absorption leads to a 

 reduced water content in tissues, photosynthesis is decreased and growth 

 and reproduction are retarded. If this retardation of processes takes 

 place during the early life of many species of annual plants, the plants 

 may not fully recover from these effects when subsequently supplied 

 with an abundance of water. 



Transpiration also affects the rate of loss of water from the soil. Water 

 may evaporate directly from the soil or it may pass from the soil into 

 the roots and then through the plant to the atmosphere. Every farm 

 boy knows that the soil under grass loses water more rapidly in the 



