EOOT-PEESSUEE 



427 



Fig. 1184. 



different observers ; an idea of its amount may be gathered 

 from the fact that a mediiun-sized Fuchsia in a pot has been 

 found able to send a cohimn of water up a tube of the same 

 diameter as its stem to a height of twentj-hve feet. 



This force is continually at work while the transmission of 

 water is going on, but it is not easily seen later in the year. If 

 the stem of the vine be cut in July instead of April, no bleeding 

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

 activity by the roots, hut to the fact that a copious evaporation 

 is taking jilace from the leaves. In the 

 experiment in April the conditions were 

 different, there were no expanded leaves, 

 and the water absorbed and sent upwards 

 by the root consequently accumulated in 

 the vessels of the stem, escaping at once 

 when the latter was cut ; in July the 

 vessels had been emptied b}' the transpi- 

 ration, and there was no accumulation of 

 water there to overflow. The apparatus 

 just described will show, however, that 

 the root-pressure is stiU at work if it be 

 used in July. 



The root-pressure, though always con- 

 siderable, is not constant in amount ; it 

 is lowest in the early morning, when it 

 begins to increase ; it continues to rise till 

 about midday or a little later, then gradu- 

 ally sinks. A second rise takes place 

 towards evening, and then it sinks con- 

 tinuously all night. The causes of this 

 rhythmic daily period are at present 

 unknown ; it does not appear to depend 

 upon variations of its surroundings, but to 

 arise from some cause inherent in the con- 

 stitution of the plant. 



Transpieatiox. — The modified evaporation taking place from 

 the surfaces of the succulent parts of plants, and regulated 

 in amount bj' the protoplasm of the cells, is known as tran- 

 spiration. It is easy to demonstrate the fact of its continuous 

 existence by enclosing a plant, or part of one, in a dry glass 

 vessel, which can be closed so as to admit no air. Very soon 

 the surface of the glass is cohered by a fine dew, which is the 

 condensed vapour escaping from the plant. The same thing 



Fi<l- 1184:. Apparatus for the 

 estimation of root-pres- 

 sure. (After Sachs.) 



