58 Princrples of Plant Culture. 
that the rate of this current will depend much upon the 
rate of transpiration from the foliage. When the soil moist- 
ure is reduced, and evaporation is excessive, this upward 
current of water is not always sufficient to maintain the nor- 
mal pressure within the cells (63), 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 (68), which, in trees, constitute 
the so-called sap-wood, since the cells of these are less ob- 
structed 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 very imperfectly understood, 
but the pull produced by the evaportion of water from the 
leaves and osmosis* play important parts. 
79. 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 
portion of the roots of these plants, however, that lies 
below the frost line, continues to absorb water, which gradu- 
ally 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 happens when we tap a sugar maple tree in spring. 
Alternate rise and fall of temperature increases the flow of 
sap, because with each contraction, new supplies of water 
* Osmosis is the tendency that causes two liquids of different densities, when 
separated by a permeable membrane, to mix with each other. The less dense 
liquid tends to flow into the more dense one with a force corresponding to the dif- 
ference in their densities. Cell contents are usually denser than soil water, hence 
the latter tends to flow into the cells, and thus to rise in the p’ant. 
