ROOT-PRESSURE. 203 



An excellent example of this, known to every one, is the wet-and- 

 dry-bulb thermometer : some cotton wrapped round the bulb has 

 the ends placed in a cup of water, whatever the rate of evaporation, 

 the bulb is thus kept wet. There is certainly no root-pressure and 

 no vital action here. Another experiment is to place a large very 

 dry sponge gently in a shallow vessel of water ; the water rises 

 suddenly about £ inch in the sponge ; if the sponge be left still, it 

 will yet be found wet to the top in six hours ; if it be given 

 any motion it will be wet to the top in six minutes. It is difficult to 

 imagine how fluid can rise in a tree by capillary action if the tree 

 were kept absolutely still; but it is almost equally difficult to 

 imagine a tree absolutely still, and the most minute vibratory 

 motion would be sufficient. 



The contradictory "facts" stated by physiologists have been 

 arrived at mainly from experiments with the manometer. I hold 

 that the manometer records the capillary action at the point (very 

 imperfectly), and nothing about the pressure at the root. We have, 

 in a cut stem, a bundle of tubes of various sizes, besides the wall of 

 cells. In a single inelastic capillary tube, whether fluid exudes from 

 the top or not, depends on several things — the viscosity of the fluid, 

 the roughness of the tube, the state of the atmosphere, &c. When 

 an elastic tube is cut across with an oblique ragged edge, it would 

 be still more difficult to say what the manometer measures. In the 

 case of a stem cut across we have additional complications from 

 the varying size of the tubes, the creeping down of air-bubbles, &c. 

 I cannot discuss all the recorded manometer experiments. The 

 following is selected by Dr. Vines as one of the most decisive : 

 " The exudation of drops from the leaves depends upon the forcing 

 of water into the cavities of the vessels by the root-pressure. For 

 if the stem be cut off and placed in water, no more drops will 

 appear on the leaves. Again, Mohl has shown that if the root- 

 pressure be replaced in the case of a cut-off branch by the pressure 

 of a column of mercury, an exudation of drops will take place." 

 This proves to me nothing as to what takes place in the uninjured 

 Plant. A piece of sponge might fill a glass tube, and you might 

 force water through it by a pressure of so many pounds to the 

 inch ; or the wate°r might get through (without any pressure) by 

 capillary action. In the case of the plant, it seems to me quite 

 possible that fluid might (before the stem is injured) be rising by 

 the cells and the smaller vessels, and at the same time be descend- 

 kg in the larger vessels. Herr Mohl cuts this stem and forces 

 water up the larger vessels by a pressure of so many pounds to tlie 

 inch. What is proved by this experiment ? 



But I do not pretend to explain all the facts of fluid-motion in 

 Plants. My point is, that the difficulties of the subject are in no 

 ;vay decreased but rather increased by the fiction of root-pressure. 

 1 point physiologists to capillary action (rather than to their 

 equation p = g,z), as the place where they should look for the 

 mechanical portion of their explanations. 



In the subsequent discussion, Mr. A. W. Bennett said he had for 



some time past regarded " root-pressure " as one of the class 01 



