20 THE PHYSIOLOGY OF PLANTS 



We see, therefore, within the vascular system of the whole 

 plant a very active process of suction taking place. This 

 reaches right back into the absorptive regions of the root. 

 There a very active interchange takes place. The parenchy- 

 matous cells which adjoin the vessels are able, owing to their 

 rich cell-contents, to absorb large quantities of water, or rather 

 of a solution of nutritive salts. Filled with water in this way^ 

 they become turgid, and their turgidity causes a pressure to 

 be exerted on the cell-wall. When such a turgid cell borders 

 on a vessel from which water is being withdrawn at one end, 

 as is the case here, it readily forces water into the vessel. 

 But as hundreds of thousands of such cells surround the 

 vessels of the root, and press their superfluous water into 

 the vascular cylinder, a considerable pressure arises, which 

 forces up the columns of water through the vessels into the 

 stem structures. Such a pressure (root pressure) actually 

 exists in all roots, and is often of considerable strength. In 

 the case of the vine, for example, a root pressure has been 

 observed strong enough to force a column of mercury to the 

 height of 39 inches. 



The root therefore acts continuously like a force-pump. 

 The effect of this pumping action is of course not always so 

 marked as in the case of the vine, in which considerable quan- 

 tities of water will for days escape from the cut end of a stem 

 (bleeding of plants). A strong vine yields during the first day 

 after it has been cut about a quart of sap. If the soil is very 

 damp and the transpiration of the plant is reduced, the root 

 pressure will be increased ; the reverse conditions diminish the 

 root-pressure. At times it is so great that it will force drops 

 of water out of the apex or teeth of the leaves. This occurs 

 frequently in the autumn in hothouses which have been cooled 

 down considerably during the night, and also in cool houses 

 in which the heating has not yet been started. At such times 

 drops of water will be found to make their appearance at the 

 tips and on the teeth of leaves of many Aroideae {Calla 

 oethiopica, for instance), of grasses, Fuchsias, Pelargoniums, &c. 



The suction due to the transpiration which takes place from 

 the surface of the leaves and the forcing action of the root 

 pressure are compensating factors in the passage of water 



