184 THE STORY OF THE PLANTS.. 



root-cells ; it allows water to pass into it freely 

 /from below upward, but does not allow it to 

 v pass ; back again from above downward. Thus 

 we get a constant state of whiat is called 

 tiirgidiiy 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 cells 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 

 continues 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 r-aise so large ft bo4y of water as the plant 

 requires \o> SQ great a hpigh^ above the earth's 

 surface^ Jt fs therefore largely supplemented 

 an4 assisted by the second or subsidiary power 

 pf evaporation. This evaporation, QJC " transpi- 

 ration " 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 



