)o6 MOLECULAR FORCES IN THE PLANT. 



may be answered without hesitation in the affirmative, since we have to do here with 

 molecuhir forces in opposition to which the action of gravity altogether disappears. But 

 it is another question whether the rapidity of the molecular movements of water of this 

 nature is sufficient to cover the requirements of the foliage of a tree which amounts on 

 a hot day to hundreds of pounds \ 



The hypothesis finally that the w^ater is forced up into the stem and even into the 

 leaves by root-pressure must be abandoned, since this could only operate in the cavities 

 of the wood ; and these are always empty in energetically transpiring plants. In the 

 case of tall trees the pressure would also not be sufficient ; and if 1 at one time assumed 

 that this might be a cooperative cause at least in shrubs and annual plants, I must 

 retract this after my observations made in the year 1870; since these show that the 

 root-stock of such plants as the sun-flower, gourd, &c., is even subject to a negative 

 pressure when they are transpiring strongly ; i. e. does not press water up, but greedily 

 sucks it in at a cut surface above the ground {'vide infra). 



The insufficiency of all attempts hitherto made to explain the movement of water 

 in the wood due to transpiration is especally noticeable from the fact that it is only 

 under certain internal conditions which cannot be more accurately ascertained that 

 wood is capable of conducting w^ater with the force and rapidity required by the eva- 

 poration from the leaves. \¥oody but air-dry branches wath a lower cut surface 

 placed in water are never able to raise up as much water as is necessary to' replace 

 the evaporation even from an upper cut surface ; while the same branch in a fresh 

 state conducts water fast enough to replace the much greater amount of evaporation 

 from the numerous leaves. A change is thus caused in wood simply by drying up which 

 deprives it of the power of conducting water rapidly. The natural alteration which 

 takes place in w^ood, by which it is transformed as it increases in age into ' duramen ' — 

 the cell-walls becoming harder and of a deeper colour — also deprives it of this power. 

 If a tree is deprived not only of the bark but also of the ' alburnum ' (the light-coloured 

 younger wood on the outside), in an annular zone, the foliage of the tree, according to 

 the statement of different writers, dries up, because the water is not conducted suffi- 

 ciently rapidly through the duramen. 



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 w^hen cut off in air. If the cut leafy end of the stem of He I i ant bus 

 anniuis, 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. 439. 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 {a), 

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

 This show^s that the forcing in of w^ater is only necessary at first, but that the revived 

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

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

 leaves. Thus much \vas known about the phenomenon of the withering of cut shoots 

 placed in 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 



' See Nilgeli u. Schwendener. Das Mikroskop. vol. IT. p. 364 ef seq. 



