60 Frhiciples of Plant Culture. 



78. The Evaporation Current. Since the water of plants 

 is taken in from the soil throngh the root-hairs (101), 

 and escapes more or less rapidly by transpiration (75), 

 it is clear that in leafy plants a current of water 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 (63), hence the foliage wilts, or the leaves roll up, 

 as in Indian corn and some other plants of the grass 

 family. This cui'i-ent passes chiefly through the younger 

 vascular bundles (68), 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 well understood, but 

 osmosis* and the pull produced by the evaporation of 

 water from the leases, 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 por- 

 tion 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 



* Osmosis is the tendency tbat causes two liquids of different densities 

 to mix witli eacti other when separated by a permeable membrane. The 

 less dense liquid tends to flow into the denser one with a force correspond- 

 ing 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. 



