MOVEMENT OF WATER IN PLANTS. 



683 



Among the most remarkable of the phenomena related to this is the fact that the 

 younger terminal portions of the stems of large-leaved plants partially lose the power 

 of conducting water when cut off in air. If the cut leafy end of the stem of Helianthus 

 annuus, H. tuberosus, Aristolochia Sipho, &c. be placed with the cut section in water, the 

 suction is not sufficient to compensate the evaporation from the leaves, which therefore 

 wither after a shorter or longer time ^. As I have already shown in the second edition 

 of this book, the withered shoot may in a short time be revived by forcing in water by 

 means of the contrivance represented in Fig. 468. I did not discover till afterwards that 

 the shoot remains turgid even when the pressure is reduced to zero, and even when 

 the mercury is raised up by the suction of the shoot in the same arm of the tube {q), 

 when therefore a force acts on the section of the shoot in the opposite direction. 

 This shows that the forcing in of water is only necessary at first, but that the revived 

 shoot has itself sufficient power of suction to raise up a column of mercury several 

 centimetres in height, and thus to replace the loss by transpiration from the leaves. 

 Thus much was known about the phenomenon of the withering of cut shoots placed in 



Fig. 468.— Apparatus for showinjj the revival of withered shoots by forcing water into them. The U-shaped glass 

 tube is first filled with water, and the perforated stopper of caoutchouc k in which the stalk of the plant is inserted 

 is then fixed in. When the shoot is withered, as represented by a, mercury is poured into the other arm of the tube, 

 so as to stand at q' some 8 or locm. above q, and the shoot then revives, as represented by b, even when the level q 

 becomes subsequently higher than q'. 



water, when Dr. Hugo de Vries^ took up the further investigation of it in the laboratory 

 of the Wiirzburg Institute. The results obtained by him I will now quote: — 



* If rapidly-growing shoots of large-leaved plants are cut off at their lower part 

 which has become completely lignified, and are placed with the cut surface in water, 

 they remain for some time perfectly fresh. But if they are cut through at the younger 

 parts of their stem and are then placed in water, they soon begin to wither, and the 

 more rapidly and completely the younger and less lignified the part where the section is 

 made. This withering can be easily prevented by making the section under water, and 

 taking care that the cut surface does not come into contact with the air, the conduction 



^ [Von Hohnel has shown (Bot. Zeitg. 1879) that the rapid loss of conductivity for water shown 

 by branches which have been cut off and placed in water is due to the fact that the contents of the 

 injured cells escape and form a layer on the cut surface ; this becomes infested with Bacteria, and 

 these form a membrane (zoogloea) over the surface which prevents the absorption of water.] 



^ [Arb. d. bot. Inst, in Wiirzburg, 1. 3, 1873; Ueb. das Welken abgeschnittener Sprosse.] 



