2IO 



FACTORS AFFECTING TRANSPIRATION 



temperatures except when exposed to direct sunlight, and atmospheric vapor 

 pressures are usually higher during cloudy days than on clear days at the same 

 season of the year. The magnitude of transpiration under such conditions 

 is usually greatly reduced, and transpiration periodicity curves plotted for 

 plants exposed to such conditions usually present a somewhat foreshortened 

 and greatly flattened appearance. 



A deficient soil water supply is most commonly the factor which causes 

 marked departures from the type of transpiration periodicity already con- 

 sidered, especially during the summer months. The effect of a gradual 

 diminution in soil water content upon transpiration periodicity is illustrated 

 graphically in Fig. 51. A reduction in soil water content has two pro- 

 nounced effects upon the daily march of transpiration. The total daily 



Fig. 51. Daily periodicity of transpiration of bean (P/iaseolus lulgaris) for three 

 successive days during a period when the soil was gradually becoming dryer. Data 

 of Chung (1935). 



magnitude of water loss is decreased, and the peak of the transpiration curve 

 often occurs somewhat earlier in the day than under conditions of abundant 

 soil water supply. Since, even in temperate regions, periods of decreased 

 soil water supply are of common occurrence during the summer months, and 

 in many habitats are the rule rather than the exception, transpiration period- 

 icity curves such as those shown in Fig. 51 are, for many species of plants, 

 much of the time, more nearly representative than the curves shown in Fig. 50. 

 Internal factors may also be responsible for transpiration periodicities of 

 a different trend from those which have already been discussed. In some 

 species of plants, as has already been described, the stomates remain open to 

 a greater or lesser extent during the hovirs of darkness. Such plants have 

 higher transpiration rates at night than those in which the stomates are closed. 

 In some species of cacti there is a complete inversion of the usual transpiration 



