94 LECTURE VI. 



way through the layers of parenchymatous cells. It is ob- 

 vious, however, that water cannot pass by osmosis from one 

 of the cells of the innermost parenchymatous layer into an 

 adjoining vessel, for the conditions of osmosis are not fulfilled, 

 inasmuch as the vessel at first contains no liquid. If water 

 is to pass from a parenchymatous cell into the vessel it can 

 only do so by filtration. For this, a certain pressure is neces- 

 sary, and this is set up by the absorbent activity of the root- 

 hairs and of the parenchymatous cells; the system of cells 

 absorbs large quantities of water, more indeed than the cells 

 can contain, so that at length the resistance of the cell-walls 

 is overcome at what is presumably the weakest point, and 

 water filters into the cavities of the vessels of the wood. 

 There it collects, and it may in certain cases fill the whole 

 vascular system, and then, since absorption is still going on 

 at the roots, sufficient pressure is set up to cause that exuda- 

 tion of drops on the leaves with which we have already become 

 acquainted : if, however, the stem be cut across, the liquid in 

 the vessels will escape, and the phenomenon of " bleeding" 

 will take place. 



Since, as we have seen, the amount of liquid in the vessels 

 necessarily depends upon the absorbent activity of the roots, 

 the amount of liquid which escapes from the cut surface of a 

 root-stock in a given time may be taken as a measure of that 

 activity, and the force with which it escapes may be taken as a 

 measure of the osmotic forces which are in operation in the root. 



The following figures will give some idea of the rate of flow from the 

 cut surfaces, and of the pressure under which it escapes. Hofmeister 

 obtained the following quantities of liquid : 



Urtica urens gave in 99 hours, 3025 cub. mm. 



Phaseolus multiflorus 49 3630 

 Helianthus annuus 133 5830 



He also made the following determinations of the pressure by means of a 

 mercurial manometer ; the numbers given are maxima. 



Pisum sativum 12 mm. 



Phaseolus nanus ... ... ... $& 



Phaseolus multiflorus 179 



Urtica urens 354 



Vitis vinifera 804 



