THE GROWING PLANT 53 



(100), and escapes more or less rapidly by transpiration 

 (74), it is clear that in leafy plants a current of water, 

 the transpiration current, must pass from the roots 

 through the stem and branches into the leaves, and that 

 the rate of this current will depend much upon the rate 

 of transpiration from the foliage. When the soil moisture 

 is reduced and transpiration is excessive, this upward 

 current of water is not always sufficient to maintain the 

 normal pressure within the cells (62), hence the foliage 

 wilts, or the leaves roll up, as in Indian corn and some 

 other plants of the grass family. This current passes 

 chiefly through the younger vascular bundles (67), which 

 in trees constitute the so-called sap-wood, since the cells 

 of these are less obstructed by woody deposits than those 

 of other tissues. 



The physical forces that cause the soil water to rise 

 to the tops of the tallest trees are not fully understood. 

 Apparently, the pull resulting from the transpiration of 

 water is the chief thing. This is supplemented apparently 

 by the process known as osmosis, which is the tendency 

 that causes two liquids of different densities to mix with 

 each other when separated by a permeable membrane. 

 The less dense liquid tends to flow into a denser one with 

 a force corresponding to the difference in their densities. 

 Cell contents are denser than soil water, hence the latter 

 tends to flow into the cells, and thus to rise in the plant. 



78. The flow of sap in spring. — In the temperate 

 zones, evaporation from the leafless stems of deciduous 

 trees and shrubs nearly ceases during winter. The por- 

 tion of the roots of these plants, however, that lies be- 

 low the frost line continues to absorb water, which grad- 

 ually accumulates in the stems and branches. Ordinarily 

 this water is absorbed and held within the cells so that if 



