224 THE MOVEMENTS OF WATER 



sideration of the movements of water in longitudinal rows of tracheae and 

 tracheides, and have neglected the possibility of any current flowing 

 obliquely or transversely. The same general principles which regulate 

 the former are, however, at work here also, and hence no special account 

 need be given of the details concerning the transverse convection of water. 

 It is also impossible at present to say whether the water-columns in 

 neighbouring tracheae correspond to one another, and by a transference of 

 water through the separating walls establish a continuous network of "water- 

 channels throughout the xylem as a whole. We do not know, moreover, 

 whether movements or alterations of size of the air-bubbles play any part 

 in the transference of water. 



A satisfactory explanation of the means by which the transpiration- 

 current is maintained has not yet been brought forward. If no vital actions 

 take part in it, then it is obvious that we have only an incomplete know- 

 ledge of the causes at work, and of the relationships of the different factors 

 concerned. A column of water in a suitable capillary tube, in the form 

 of a Jamin's chain, may attain a height of 142 metres, which is as much 

 as is necessary for the highest tree. The necessary capillary and other 

 conditions are not, however, fulfilled in a column of tracheae, and taking 

 into account all the conditions actually existing, no greater height than 

 50 metres could be attained in this manner 1 . 



Dixon and Joly, as well as Askenasy 2 , have lately attempted a purely 

 physical explanation of this problem. They point out that, owing to the 

 high cohesion of water, the breaking strain for a continuous column may 

 be more than 50 atmospheres when no lateral twisting or displacement 

 is possible. The suction exerted by the leaves is supposed to be 

 transmitted backwards as far as the roots through cohering columns of 

 water. More attention must, however, be paid to the conditions actually 

 existing in the plant, before any such explanation can be accepted. 

 Without attempting a critical discussion of the views of these authors, it 

 may be pointed out that the walls of the tracheae can absorb or give 

 off water with equal readiness, and that the negative pressure exerted 

 by a continuous water-column would tend to cause an inward and down- 

 ward stream of water in the upper parts at least. Besides, the presence 

 of air-bubbles forms one of the conditions which render the rupture of 

 a water-column readily possible, and the valuable experiments of Dixon 

 and Joly 3 do not afford conclusive proof that the water-columns in 



1 Schwendener, Sitzungsb. d. Berl. Akad., 1892, Bd. XLIV, p. 916; 1893, Bd. XL, p. 835; 

 Steinbrinck, Ber. d. Bot. Ges., 1894, p. 127. 



2 Dixon and Joly, Phil. Trans., 1895, Vol. CLXXXVI, p. 563; Proc. of the Irish Acad., 1896, 

 Vol. in, p. 767; also Ann. of Bot., 1896, Vol. X, p. 630; Askenasy, Ober d. Saftsteigen, 1895 

 (Sep.-abdr. a. d. Verhandlungen d. Naturh.-med. Vereins in Heidelberg) ; also 1896. 



3 Dixon and Joly, Ann. of Bot., 1895, Vol. IX, p. 403. 



