THE WATER-RELATION 



109 



(Root Pressure) may be developed in exudation from a root system, 

 amounting in some cases to 1 atmosphere or more : it is possible 

 that in the intact, uninjured plant Root Pressure may reach consider 

 ably higher magnitudes. 



A phenomenon that is probably due to Root Pressure is to be seen 

 in plants that are provided with water glands. Drops of water issue 

 from the leaves of such plants when conditions are unfavourable to 

 transpiration, though still favouring absorption, as during a warm 

 night (Fig. 74 a). The water exuded in this way from the leaves of 

 Grasses at night is often mis- 

 taken for dew. It frequently 

 contains salts in solution, and 

 these may form an obvious 

 incrustation round the water 

 gland, as for example in var- 

 ious Saxifrages. The deposit 

 here is largely calcium car- 

 bonate. Exudation from leaves 

 is very common in tropical 

 rain-forests, where the humid 

 atmosphere depresses trans- 

 piration. 



It appears then that a mechanism is present in the roots of 

 plants which forces water up through the plant when transpiration 

 is not operating, as is the case at night or in a decapitated plant. 

 One suggestion is that special osmotic arrangements cause the inner- 

 most living cells to pump water into the xylem (see Figs. 68 and 72). 

 The endodermis, with its radial walls made impermeable to water 

 by the strip of corky material previously mentioned, may play an 

 important part in preventing the water which is accumulated under 

 pressure in the xylem vessels from leaking out of the stele along the 

 cell-walls, and perhaps eventually finding its way back to the soil. 

 The Root Pressure mechanism may co-operate with that contemplated 

 by the Cohesion Theory in raising the transpiration stream when trans- 

 piration is in progress. On the other hand, in many experiments only 

 feeble Root Pressure has been detected, while it is known that during 

 active transpiration the contents of the xylem are in a state of tension 

 rather than of pressure. Hence the opinion has for some time been 

 prevalent that Root Pressure plays only a minor part in promoting 

 the upward stream. Probably the significance of Root Pressure in 

 the plant has not yet been properly evaluated. Lastly, there has all 



Fig. 74 a. 



Exudation of water from the margin of the leaf of 

 Tropaeolum, through water stomata which are perma- 

 nently open. (After Strasburger.) 



