IV.] VARIOUS THEORIES, &c. 115 



on the walls of the tracheide containing it. The 

 diameter of the tracheides (in Pinus = 0*015 to 0'02 

 mm.) shows they could support columns much longer 

 than they do. 



Thus the water in the stem is in long columns, broken 

 at short intervals by valves, reminding us of a hint 

 thrown out long ago by Do Candolle and Mongolfier. 



To get over the difficulty as regards vessels, Scheit 

 raises doubt as to the continuity of their lumina : in 

 any case he regards the capillary water in them as of 

 the nature of a reserve. 



The causes of the water-movement are as follows : — 

 Transpiration tends to exhaust the reservoirs of water ; 

 the osmotic pressure in the root drives in the closing 

 membranes of the bordered pits, rendering them per- 

 meable. The water thus pressed into the vessels, &c. 

 is at once removed from the action of gravity by 

 means of capillarity ; and thus the root-pressure has 

 nothing further to do than press the valves and drive 

 water in. Th. Hartig's experiment with the vertical 

 cut branch shows how little pressure is required to do 

 this, and hence the slightest swaying by the wind may 

 be a co-operating cause of movement. 



Scheit then quotes experiments in which he injected 

 branches, &c., cut off under the surface of a liquid, 

 and which completely confirm the negative pressure 



I 2 



