202 ROOT-PRESSUKE. 



"The hypothesis finally that the water is forced up into the 

 stem, and even into the leaves, by root-pressure must be aban- 

 doned.' ' 



Dr. Vines, in his ■ Lectures on Physiology, 1 1886, accepts this 

 doctrine. He says (p. 91), " The root-pressure causes a flow of 

 sap from the cut surfaces of plants ; it also causes in many plants 

 the exudation of drops of sap at the free surface." 



The way I propose to reconcile the manifest difficulties in these 

 statements is by denying that any root-pressure exists in any case. 

 The foregoing extracts represent that the vital action of the cells of 

 the root exert a pressure supporting a column of fluid. If the tree 

 were 140 feet high, and the fluid about the density of water, the 

 root-pressure required would be about 60 lbs. to the inch. Simi- 

 larly, if a stem is cut 10 feet above the ground, and the manometer 

 showed that the fluid exuded with a force of 7 lbs. to the inch, the 

 root-pressure required would be 7 lb., plus the weight of a column 

 of fluid 10 feet high, in all 11 or 12 lbs. to the inch. 



It is to be noted that on this hypothesis the pressure would be 

 the same in all the cells at the same altitude, and would (in the tree 

 140 feet high) diminish gradually from 60 lbs. at the base to zero 

 at the summit. All this would be so if the cells and vessels in 

 vegetables were 6 inches in diam., and it is only the conceiving a 

 "vital force" in the cells that can account for their not being 

 blown to atoms. The believers in root-pressure attempt to dimi- 

 nish this difficulty ; they say (relying on manometer experiments) 

 that the root-pressure will not support a column of water more 

 than 30 feet high or thereabout, and that the forcing of water 

 higher than this must be effected by some other force. This seems 

 to me to increase the difficulty ; a column 30 feet high would give 

 a pressure of 12 lb. to the inch or more, which would burst all the 

 tender cells (unless they are supposed endowed by vital force) ; and 

 on the other hand, if in a tree 100 feet high, some " other force" 

 sustains the pressure due to 70 feet of height, why should we feign 

 the existence of this troublesome and inadequate root-pressure? 

 The advocates of root-pressure refer also to the variety in the form 

 and complex arrangement of the cells to account partially for 

 these difficulties. But if the pressure is transmitted from the root 

 in the way they imagine the equation p = gf>z would hold (as they 

 suppose). This is their hypothesis. 



My hypothesis is, that the cells being all " capillary" (though 

 varying considerably in size and form), the equation p = gpz does 

 not hold at all ; that the whole mechanical fluid action in plants 

 must be considered in accordance with the laws of capillarity ; and 

 that the fluid-pressure in every plant cell is very nearly zero. I do 

 not say that all the fluid motion observed in plants can be 

 explained by capillarity ; but capillary action can do a great deal 

 more than Sachs imagines. It can only raise water in one inelastic 

 tube a small height, but in a bundle of tubes capable of motion 



can 



£ 



laterally (by pressure or other cause) pass into another, and there rise 

 by capillary action another i inch, and so on nearly ad infinitum. 



