290 THE INTERNAL WATER RELATIONS OF PLANTS 



tions (Chap. XIII). As shown by his data (Fig. 75) the leaf water content 

 decreased during the morning and early afternoon hours, reaching a minimum 

 at about 5 :CK) p.m. Thereafter the leaf water content increased, culminating 

 in a maximum which was attained at about i :oo a.m. During the early 

 morning hours the leaf water content again decreased. Similar observations 

 have been made by Kramer (1937) on leaves of several species. In the 

 sunflower, for example, the minimum leaf water content was attained at 

 about 4:00 P.M., and the maximum at about midnight. The most probable 

 explanation of the occurrence of the maximum leaf water content during 

 the middle hours of the night is that during the early morning hours the leaves 



II \2 

 MIDNIGHT 



Fig. 75. Daily variation in the water content of leaves of Boston Ivy {Ampelopsis 



tricuspidata). Data of Stanescu (1936). 



lose water by translocation to other organs of the plant. The mechanism of 

 such internal redistributions of water in the body of a plant is discussed later 

 in this chapter. Under such conditions the water content of the plant as a 

 whole might be increasing as a result of continued absorption of water, while 

 that of certain organs, such as the leaves, might be diminishing. 



The magnitude of the frequently recurrent diurnal reduction in the water 

 content of the leaves and other organs of a plant varies not only with the 

 species, but also with the environmental conditions and their influence on the 

 relative rates of transpiration and absorption. On cool, cloudy or rainy days 

 when the soil is well provided with water often little or no water deficit 

 develops within the plant during the daylight hours. On bright, sunny, but 

 not extremely hot days, while the soil water supply is abundant, an internal 

 water deficit will develop, but it seldom is severe enough to induce more than 

 incipient wilting. On clear, hot days, especially when the soil water supply 

 is not entirely adequate, a more marked shrinkage in the volume of water 



