THE STEM AND BRANCHES. 169 



jreely from below upward, but does not allow it 

 to pass back again from above downward. Thus 

 we get a constant state of what is called turgidity 

 in the lower cells; they are as full as they can 

 hold, and they keep on contracting elastically, so 

 as to expel the water they contain into other cell's 

 next in order above them. By means of such 

 root-pressure, as it is called, raw sap is being for 

 ever forced up from the soil beneath into the 

 stem and branches, to supply the leaves with water 

 and food-salts, especially in early spring, when 

 the processes of growth are most active and 

 vigorous. 



It is owing to this peculiar property of root- 

 pressure that cut stems " bleed " or exude sap, 

 especially in spring-time. The root-pressure con- 

 tinues of itself in spite of the fact 'that the stem 

 has been divided ; and the sap absorbed by the 

 roots is thus forced out at the other end by the 

 continuous elasticity of the cells and vessels. 

 The fact that severed stems will thus " bleed " or 

 exude raw sap shows in itself the reality of root- 

 pressure. 



But root-pressure alone would not fully suffice 

 to raise so large a body of water as the plant re- 

 quires to so great a height above the earth's sur- 

 face. It is therefore largely supplemented and 

 assisted by the second or subsidiary power of 

 evaporation. This evaporation, or " transpira- 

 tion " as it is generally called, is just as necessary 

 and essential to plants as breathing is to men and 

 animals. 



We must therefore enter a little more fully 

 here into the nature of so important and universal 

 a plant function. You will remember that when 

 we were discussing the nature of leaves, I gave 



