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easily seen when a section is cut, but it is carried upwards at 

 once in the walls of the vessels to the leaves, to compensate 

 there for the loss of liquid due to transpiration. On the 

 contrary, if we pour water on the cut surface, it will be 

 rapidly absorbed; for the vessels, as we have seen, contain no 

 water, but only air at a negative pressure. 



We see, then, that there are two kinds of pressure manifested 

 by the roots : First, the endosmotic activity of the cells of the 

 root, absorbing fluid from the exterior and passing it through 

 their cell- walls and protoplasm, thence forcing it under pressure 

 into the lignified walls of the vessels, whence it is conducted at 

 once to the leaves in obedience to the demand created in them 

 by transpiration. This pressure is always constant, and we 

 cannot measure it. Secondly, the activity obtained by the 

 presence of a condition of turgidity in the cells of the root, 

 for taking up fluids to a greater extent than they can contain, 

 and, consequently, for exercising a hydrostatic pressure, oc- 

 casioning a process of rapid filtration into the cavities of the 

 vessels : this is the externally evident root pressure of which I 

 have been speaking, and its amount can be measured. The 

 latter, as I stated, never coexists with transpiration. When we 

 have root pressure evident there is either no transpiration, or its 

 amount is exceedingly small, and in consequence of this the 

 liquid has time to collect in the cavities of the vessels ; but 

 when transpiration has once begun, it is impossible to detect 

 any evident root pressure, for the cells of the root never become 

 turgid to the excessive degree required to force liquid through 

 the walls of the vessels into their cavities, but remain tolerably 

 flaccid ; and, as we have seen, the evident root pressure depends 

 on the turgid condition of the cells. When transpiration is 

 going on we never have any water in the vessels : hence, when 

 the leaves are actively transpiring there is no evident root 

 pressure, and consequently, as we saw, the fluid transpired 

 could not be supplied to the leaves from the root pressure, as 

 was at one time supposed. 



In the experiment of measuring the amount of the root 

 pressure by means of the height of the column of mercury 



