INTRODUCTION 37 



6. Transpiration and the Ascent of the Sap. 



When a tree has been felled especially in spring or 

 summer the surface of the section of the trunk is 

 seen not long afterwards to be covered with a watery 

 solution, often sticky from the formation of sugar or 

 starch. This is the cell-sap, which has been pumped 

 upwards from below, and escapes where the tissues 

 end. Such stems are said to bleed. A beetroot 

 bleeds in the same way, the sap being in this 

 case red. 



The necessity for the cells to be filled with sap, 

 and maintained in a state of turgescence, as seen 

 above, correlated with the permeable character of 

 the stretched cell-walls, naturally causes the water to 

 rise. If a tree is ringed (Fig. 3), the leaves on the 

 branches above the ring will become limp, owing to 

 lack of turgor, or cutting off of the transpiration 

 current ; those below will remain turgid. There is, 

 in fact, a pressure in the region of the root of the 

 inflowing liquid upwards, and this root-pressure 

 causes the ascent of the sap. Moreover, the liquid 

 does not permeate all the cells, but entering at the 

 borders is radially forced into the central long-celled 

 vessels or the tracheal tubes and tracheids of the 

 inner part of the stem. The outer cells and cortex 

 serve to protect the plant from injury and loss of 

 water. It is the young vessels that serve as water 

 conductors. 



