MOVEMENT OF WATER IN PLANTS. 6ll 



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. 



It is necessary then to make some conjecture as to the cause of this powerful ascent 

 of water in the wood of the root-stock ; how it happens that the water sucked up 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 cut 

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

 in below at the surfaces of the roots. This suction can only be induced by the en- 

 dosmotic 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 to the cavities of the wood as is sucked 

 up from without by endosmose. The parenchymatous cells which are gorged by endos- 

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

 endosmose, and 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 

 exerted also against the inside of the vessels which receive the water from the turgid 

 parenchymatous cells. But the water which enters them has also to overcome the re- 

 sistance to filtration exercised by the cell-walls. The endosmose of the cortical cells 

 of the root must overcome these resistances. Although we do not know the magnitude 

 of the endosmotic force, yet we have ground for supposing that it is much greater than 

 that given by Dutrochet's experiments on animal membranes; and this explanation 

 would therefore be very probable. But a difficulty occurs in answering the question 

 why the turgescent cortical cells of the root expel their water only inwards into the 

 woody tissues and not also through their outer walls. We may however here be helped 

 by the supposition that the molecular structure of the cells is different on their outer 

 and inner sides, and that those facing the exterior of the root are best adapted to allow 

 endosmose, while those facing the interior of the root are best adapted for permitting 

 filtration under high endosmotic pressure. It must however be observed that this 

 supposition is at present only a hypothesis for the purpose of explaining to a certain 

 extent the processes which take place in the root. The exudation of drops of water 

 from the upper cell of the Fungus Pilobolus crystallinus, from the root-hairs of a Mar- 

 chant ia grown in damp air, &c., shows moreover that cells distended by endosmotic 

 tension can in fact exude water at certain spots. It is difficult to give any other ex- 

 planation of the exudation of nectar in flowers ; the excreting cells must evidently absorb 

 the water or the sap with great force on one side, and then exude it on the other side. 

 That in this case pressure from the root does not directly cooperate is shown by the 

 fact that this exudation of nectar, which is often very copious, as in the flowers of Fritil- 

 laria imperkdis, takes place even when cut flowers are simply placed in water. In this 

 respect these exudations of fluid diflfer from the exudation of drops on the leaves of 

 many plants, which only takes place when they are still in connection with the root, 

 and which is clearly caused by the forcing power of the root (as in Aroidece, &c.). It 

 also happens however sometimes that drops of water are exuded from cut surfoces 

 of the tissue, while another cut surface of the organ sucks up water. This I found, 

 for instance, to be the case with pieces of the young stems of diff"erent Grasses, cut off" 

 from 6 to 10 cm. in length, which were placed with the lower end in damp soil; the 

 free upper end then repeatedly and continuously exuded drops of water in darkness and 

 in an atmosphere saturated with moisture. Here the parenchymatous cells of the 

 lower cut surface clearly acted as the cortical cells of the root, sucked up by endosmotic 

 action, and probably pressed the water thus sucked up into the vessels, from which it 

 then escaped to the upper cut surface. 



(/) The combined action of transpiration, conduction, and absorption of nvater through 

 the roots takes place under ordinary and favourable conditions in such a manner that 

 nearly as much water is absorbed through the roots and conducted upwards through the 



R r 2 



