The Water of Plants and Its Movements. 61 



foliage. When the soil moisture is reduced and trans- 

 piration 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 con- 

 stitute 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 tallest trees are not well understood, but 

 osmosis* (os-mo-sis) and the pull produced by the evap- 

 oration of water from the leaves, play important parts. 



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 

 gradually accumulates in the stems and branches. On 

 the return of spring weather, the rise in temperature 

 causes expansion of the tissues of the stem, as well as 

 of the air and water within it. This creates so much 

 pressure in some trees and shrubs that water flows 

 freely from wounds in the wood, bearing with it, of 

 course, the materials it holds in solution. This hap- 

 pens when we tap a sugar maple tree in spring. Alter- 



* Osmosis 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. 



