THE CONDUCTION OF WATER. I 49 



thus obtained. Plants treated in this way, though once more placed in water, 

 wilted with great rapidity ; they were no longer capable of absorbing water, 

 and the conducting power of the stem was destroyed. At the low tempera- 

 tures at which gelatine and cacao-butter can be used, injury to the living cells 

 is out of the question, so that it may be considered as certain that the vessels 

 are essential to the transport of water and that that transport takes place by 

 the lumen of the vessel, not as Sachs (1879) believed, by its wall. We must 

 draw attention to the fact, however, that the walls of the vessels as well as the 

 neighbouring parenchyma cells may also be concerned in the process ; for our 

 experiment only teaches us that the lumina of the vessels are essential, 

 but gives us no information as to the participation of other elements in 

 the conduction. By far the most convincing experiment is that which was 

 made, first by Vesque (1883), and later by Kohl (1886) and Strasburger 

 (1891). It is possible, by means of a screw clamp, to compress the lumina of 

 the vessels and so occlude them, the parenchyma being at the same time almost 

 entirely crushed. So long as the vessels are squeezed in this way, the stem is 

 unable to conduct water, but its capacity for doing so returns at once when, by 

 removal of the pressure, the vessels regain their original shape by elastic recoil 

 and once more exhibit distinct lumina. This experiment is most effective 

 if water-cultures be employed, or if it be carried out on cuttings, with the aid of 

 a potometer. Constriction of the lumina of the vessels at once makes itself 

 felt, for the absorption of water rapidly sinks to nil ; on the other hand, re- 

 laxation of pressure is immediately followed by the inrush of water into the 

 vessels with increased rapidity. This tightening and slackening of the screw- 

 clamp may be repeated many times, but always with the same result. 



Before we make any attempt to examine more closely the ascent of water, 

 we must inquiire how the water succeeds in entering the vessels in the 

 first instance. In the familiar experiments with amputated branches the 

 water enters the cut ends of the vessels, very much in the same way as it 

 enters a capillary glass tube whose open end has been submerged. In the 

 normal plant, however, the vessels are both at their lower ends, and 

 laterally as well, completely enclosed by living cellular tissue, and abut upon 

 other vessels higher up, so that any water that succeeds in entering them must 

 first of all have penetrated the living tissue by which they are surrounded. 

 Now we have already seen in Lecture III how the cells of the root epidermis 

 absorb water osmotically from the soil, and at the commencement of the present 

 lecture we have determined under what circumstances water travels from cell 

 to cell. The water absorbed by the epidermis is transferred to the centre of the 

 root since the cell-sap is in a state of greater concentration there than it is in the 

 epidermis, and it will continue to be so transferred until a similar osmotic pressure 

 prevails throughout all the cells of the transverse section. Water in the same 

 way will pass osmotically into segments of young vessels while these are still 

 in an embryonic state and possessed of normal cell contents. When, however, 

 a segment fuses with the next older segment an immediate dilution of its 

 osmotically active cell-sap must take place since it is essentially water that is 

 found in adult vessels. The question then comes to be how can water be 

 abstracted from the cell-sap of a parenchymatous cell and transferred to the 

 lumen of a vessel ; one would expect the precisely converse process to take 

 place. Before we attempt to answer this question it will be advisable to make 

 ourselves thoroughly acquainted with the details of the process. 



It is by no means difficult to demonstrate that water is given up to the 

 vessels by the parenchyma cells. In many cases all that need be done is to cut 

 off the aerial shoots of a herbaceous plant, when at once, or after a short time, 

 a quantity of sap may be seen escaping from the wound. Owing to the 

 turgidity of surrounding parenchyma, sap may also be squeezed out of the 



JOST E 



