98 THE VEGETATIVE ORGANS 



when the water evaporates ; this occurs in Wheat seedlings, 

 and especially in some Saxifrages, where the salts form 

 chalky incrustations on the ends of the teeth (see Fig. 59). 

 Water is usually given off, however, in the form of vapour, 

 especially during the day, when the stomata are open ; 

 loss of water may be so great in warm sunny weather 

 that the plants droop. At night the stomata close, but 

 absorption of water by the root goes on ; the plants 

 become turgid, and as the amount absorbed exceeds the 

 amount of water- vapour transpired, water in a liquid 

 state is forced out of the leaves. Some leaves, e. g. the 

 Garden Nasturtium, have special stomata which are 

 permanently open for this purpose and are called water 

 stomata. 



Obtain an actively growing plant of Sunflower, Fuchsia, 

 or Dahlia, cut off the shoot about three inches above the 

 soil, dry the cut end and examine the surface with a pocket 

 lens. Soon water exudes from the cut surface. Arrange 

 an experiment as shown in Fig. 60. By means of rubber 

 tubing (c), attach to the stump (s) a bent tube (g). Water 

 (W) now collects in this tube, and if mercury (Q) is 

 placed in the bend of the tube, the column will be forced 

 upwards. By this means the pressure of exudation may 

 be measured. The pressure which exists in the tissues of 

 the root and aids the upward flow of sap in the stem is 

 called root-pressure. 



The amount of sap which ascends in the stems of plants 

 in spring is often very great. If, for example, the stem 

 of the Vine is cut as the leaves are unfolding, so great 

 is the flow of sap, that it can only with difficulty be 

 stopped. This exudation of sap is known to gardeners 

 as 'bleeding'. 



Force of transpiration. It is obvious that considerable 

 force must be exerted in drawing water up a stem to the 

 leaves to replace that given off as water-vapour through 



