2-20 STRUCTURAL BOTANY 



through the plant is a subject which is by no means 

 thoroughly understood at present. There are two 

 distinct processes which have to do with it: one is 

 root-pressure, the other is transpiration. The water 

 taken up by the root through its hairs is forcibly 

 pumped upwards, as can be easily proved by cutting 

 off the stem or branches of a plant, especially in 

 spring, when the ascent of the sap has begun, and 

 before there is any appreciable loss of water from the 

 surface of the leaves. From the cut stumps of the 

 stem or branches great quantities of water, amounting 

 in some cases to several quarts, are exuded. This has 

 long been observed in the case of the Vine in spring, 

 when it is said to Need. The same thing can be seen 

 at any time during active vegetation, but in the 

 summer the plant is losing water rapidly by its leaves, 

 and consequently the water-conducting tissues are 

 comparatively empty. Under these circumstances it 

 is necessary to wait for a time after cutting oS a 

 branch, before the welling-out of water from the 

 stump begins. 



This movement of the water is undoubtedly due to 

 the action of the living cells of the root. Through 

 the root-hairs they take up water from the outside, in 

 the manner explained above. The elastic cell-walls 

 become distended, while the sap within is at a high 

 pressure, which may amount to three or four times 

 the pressure of the atmosphere. The cell-wall is 

 -easily permeable by water, and such pressure in the 

 interior of the cell could not be maintained if it were 

 not for the protoplasmic lining by which the entrance 



