86 VEGETABLE PHYSIOLOGY 



of the process. If there is too great a preponderance of 

 neutral salts such as sodium chloride or potassium nitrate, 

 so that the liquid presented to the roots is practically a 

 saline solution, the exudation will cease ; indeed, under such 

 circumstances water may actually be withdrawn from the 

 plant. 



Eoot-pressure is continually at work while the trans- 

 mission of water is going on ; but it is not easily seen later 

 in the year when the development of the leaves has caused 

 an active transpiration to proceed. If the stem of the vine 

 be cut in July instead of in March, no bleeding follows the 

 wound. This is not, however, due to the absence of 

 activity in the roots, but to the fact that the copious 

 evaporation of transpiration prevents the necessary accu- 

 mulation of water in the cavities of the woody elements. 

 In the experiment in the early spring the conditions were 

 different ; there were no expanded leaves, and the water 

 .absorbed and sent upwards by the root consequently 

 remained in the vessels of the stem, escaping at once when 

 the latter was cut. In July the vessels have been emptied 

 by the transpiration, and there is no accumulation of 

 water to overflow. The apparatus described will show, 

 however, if the experiment with it is continued for some 

 time, that root-pressure is still at work, even though 

 transpiration is vigorous until the stem is severed. 



The force of root-pressure must therefore be regarded 

 always as a factor in maintaining the transpiration current. 

 It is continually forcing water into the vessels of the axis, 

 and the fact that transpiration prevents an accumulation 

 there does not show that the influence of root-pressure is 

 done away with as soon as it ceases to be easily demon- 

 strated. 



The root-pressure, though always considerable, is not 

 the same at all times of the day and night. It can be 

 measured by observing the output of water in the second 

 form of the apparatus described above, measurements 

 being taken every hour ; or in the first form of the 



