318 CONDUCTING SYSTEM 



for the purpose of raising large quantities of water to considerable 

 heights. 



Where the vessels and tracheides are associated in bundles in such a 

 manner that their readily permeable walls are laterally contiguous, 

 water can pass in a continuous stream past the relatively stationary 

 air-bubbles, provided the latter are not too long. Here the bubbles are 

 comparable as Schwendener has pointed out to islands which break 

 up a river into a network of channels. The conditions are different, if 

 the transpiration current flows through vessels which are surrounded on 

 every side by non-conducting tissues ; this arrangement is exemplified, 

 according to Strasburger, by the genera Salix and Ficus, and also by 

 certain Leguminosae. In these circumstances, there are several ways in 

 which the ascent of sap might theoretically take place, and the 

 same statement doubtless applies to all cases in which continuous 

 water-tubes are present. One may, in the first place, suppose the 

 alternating columns of water and air which constitute a so-called 

 Jamin's chain to move bodily under the influence of appropriate forces. 

 It must not be forgotten, however, that it is very exceptional for vessels 

 to be as long as the stem or trunk in which they are located. Hence, 

 in estimating the force which opposes the movement of the Jamin's 

 chain as a whole, one must take into account not only the resistance 

 opposed to displacement of the chain itself, but also the resistance 

 which the air-bubbles would have to overcome in passing through the 

 successive transverse septa ; since the walls are highly impervious to 

 air, this latter resistance must be very considerable. This mode of 

 ascent of sap would, of course, be even more difficult of execution in a 

 system of tracheides, on account of the far greater number of transverse 

 walls. In the second place, the ascent of sap might consist as 

 Vesque maintained and Strasburger has reasserted not in a bodily 

 displacement of the Jamin's chain, but in a movement of water past the 

 stationary bubbles ; if this is actually the case, there must be a thin 

 film of water between the air-bubbles and the wall of the tube, linking 

 up the apparently isolated water-columns. Finally, it is conceivable 

 as Westermaier first asserted that the living xylem-parenchyma cells 

 which are closely associated with all vessels and tracheides, and perhaps 

 also the elements of the medullary rays, may act like so many minute 

 suction- and compression-pumps, withdrawing water, by virtue of their 

 osmotic activity, from the several liquid columns of the Jamin's chain, 

 only to discharge it at a higher level, either into another liquid column 

 in the same chain, or into a different section of the conducting tube. 

 These different theoretical possibilities might, of course, also be combined 

 in various ways. The whole question, however, has not as yet progressed 

 beyond the stage of more or less well-founded hypotheses and suggestions. 



