MOVEMENT OF WATER IN PLANTS. 



687 



I observed in this manner in the summer of 1870 for five days two equally strong 

 root-stocks of the Sunflower^; and the result was that the difference of the outflow was 

 but small, although the amount of pressure in one case was zero, in the other case 

 17 cm. of mercury. In the first thirty-three hours the outflow where there was no 

 pressure at the cut surface amounted to 26*45 cubic cm.; when the pressure was 17 cm. 

 of mercury it was 20*9 cubic cm. A sudden change in the pressure of the mercury 

 of I or 2 cm. also caused no considerable alteration in the rapidity of the outflow. 



Our object now is to form some idea as to the cause of this powerful ascent of water 

 in the wood of the root-stock, and to explain how it happens that the water absorbed 

 at the surfaces of the roots not only passes into the cavities of the wood, but is pressed 

 upwards with so great a force as to be able to overcome a considerable resistance at the 



Fig. 469.— Apparatus for measuring the outflow under a considerable and constant pressure. The cork /has a lateral incision 

 in order to allow of the escape of the air when the mercury is dropped in. 



cut surface ; for it is obvious that the water which flows out above must have been 

 absorbed below at the surfaces of the roots. This absorption can only be induced by the 

 endosmotic action of the parenchymatous cells of the cortex of the root. If we suppose 

 that this endosmotic force is very considerable, these cells will swell greatly ; and as 

 much water will filter through the cell-walls into the cavities of the wood as is ab- 

 sorbed from without by endosmose. The parenchymatous cells which are gorged by 

 endosmose drive into the vessels the water which presses into them in consequence of 

 the endosmose, with such force that in flowing out above from the vessels it is still able 

 to overcome a considerable pressure. It follows from this explanation that the pressure 

 which acts at the cut surface must, in accordance with the laws of hydrostatics, be 



I cannot here describe the whole series of minute observations. 



