212 THE MOVEMENTS OF WATER 



agency of the cortex and by means of the root-hairs. No one-sided 

 pumping action of the cortical cells is necessary for such absorption, though 

 if any does actually occur, it naturally aids and accelerates the latter. 

 Water is absorbed almost solely by the younger portions of the root, but 

 nevertheless the total absorbent surface is so great as to render only a slow 

 transference through the cortical cells necessary (Sect. 26). 



That the consumption and removal of water by an organ should 

 directly cause a fresh supply to be drawn in is a highly advantageous 

 arrangement, and indeed is necessary for the essential self- regulation 

 exhibited by every functional activity. Thus according to their respective 

 transpiratory activities, one leaf will obtain a large supply of water, another 

 little or none, while if necessary, the cortex may withdraw water from the 

 vascular system at any point of its course. The conducting channels are 

 moreover capable of conveying \vater in the opposite direction, as is shown 

 by the fact that twigs remain fresh when placed upside down with their 

 distal ends in water, and that a small tree, the stem of which has been 

 cut through, may be provided with water by means of a branch which has 

 coalesced with another tree l . As a matter of fact, the water-current seems 

 to pass with equal rapidity in either direction, for the slight delay which is 

 generally observed is satisfactorily explained by the fact that the current in 

 the downward direction has frequently a longer course to travel through the 

 vascular bundles in order to reach a given point, than was the case with 

 the ascending stream 2 . 



From what has been said above, it is evident that a sufficiently dry 

 soil will withdraw water from a turgid root, and if water is supplied through 

 the leaves or twigs, a current may pass from these to the dry soil (cf. Sect. 28). 



Without any interchange of water with the surrounding medium, 

 a transport of fluid may take place within the plant, corresponding to the 

 attracting forces at work at a given point. Thus water may be withdrawn 

 from a potato suspended in air by the shoots which it may develop 3 , while 

 a plant of Scinpervivnm hanging freely in a glass cylinder has been observed 

 to grow in length apically for a long time by means of water extracted from 

 its older parts 4 . 



Transpiration removes water in the first instance from the cell-wall, but in each 

 cell equilibrium is rapidly restored between the imbibitory and osmotic attractions for 



1 Hales, Statics, 1748, p. 77; Duhamel, Naturgesch. d. Baume, 1765, Bd. n, p. 240; Cotta, 

 Naturbeobachtungen iiber d. Bewegung d. Saftes, 1806, p. 22; Unger, Sitzungsb. d. Wien. Akad., 

 1868, Bd. LVIII, Abth. i, p. 7 (Sep.) ; Strasburger, Leitungsbahnen, 1891, p. 583. [In large banyan 

 trees with numerous secondary stems, this phenomenon may occur naturally, for as the older central 

 trunks die away, it may occasionally happen that in certain of the horizontal branches the original 

 direction of the current of sap becomes reversed over a certain distance.] 



2 Cf. Strasburger, I.e., p. 583. 



s Nageli, Sitzungsb. d. Bairischen Akad., 1861, I, p. 249. 



" Cf. de Candolle, Pflanzenphysiol., 1833. Bd. I, p. 176, and Treviranus, Physiol., 1835, Bd. I, 

 p. 511. 



