NUTRITION. 



157 



and forming the finer veins, the water may be distributed to 



every part of the plant body. Within 



the wood it travels chiefly in the cavities 



of the large ducts or vessels, when these 



are present, though the walls, also, are 



saturated with it, and permit a slower 



movement. These ducts, although of 



great relative length (some up to i m.), 



are not continuous tubes like the veins 



of an animal, nor are they filled with 



water. The water is broken into short 



columns by numerous gas-bubbles, and 



in ascending to any considerable height 



must traverse many cell -walls. 



207. Motive power.- -The force by 

 which water is raised in the larger plants 

 remains yet to be ascertained. The 

 water does not flow along the ducts in a 

 continuous current, as the blood in the 

 veins, propelled by a force behind, for 

 root pressure is not adequate to push it to 

 the height attained. On the contrary, 

 during the times of most active evapo- 

 ration from the leaves, i.e., when the 

 greatest supply is needed, root pres- 

 sure becomes almost or quite negative. 

 Capillarity is also inadequate. The 

 diameter of the largest ducts is too small 

 and the friction of the water against 

 their sides consequently too great to 

 permit the movement, by this means, of 

 a sufficient amount of water to supply 

 Moreover, the interruption of the water 



FIG. 172. Apparatus for 

 measuring root pressure. 

 To the stump of a plant 

 a T-tube, R, is attached by 

 a piece of rubber tubing, 

 ?'. The other openings are 

 closed by rubber corks, /r, 

 through one of which 

 passes a small glass tube, 

 r, bent into two unequal 

 legs, containing mercury. 

 Through the upper should 

 pass a short piece of glass 

 tubing drawn out to a fine 

 point, to be sealed off in a 

 flame after fi is filled with 

 water. At the beginning of 

 the experiment the inert ury 

 is about at the same level 

 in both legs. As water is 

 forced from the stump into 

 A by root pressure the mer- 

 cury rises in the arm </' 

 and falls correspondingly 

 in q, After Sachs. 



the evaporation, 

 columns by gas- 

 bubbles produces surface tensions which quite overcome that 



