190 HISTORY OF BOTANY 



considered here is the facile mobility of the water thus 

 held fast in the cell walls. The ascending current of water 

 depends upon the motion of the relatively small number 

 of water molecules which are contained between the 

 micellae of the wood cell walls. This much is estabUshed, 

 that this movement can only occur when the wood cell 

 walls at the upper end of this system lose a portion of 

 their water molecules. By this loss its state of saturation 

 with water becomes disturbed, and the equihbrium 

 altered ; the parts of the wood cell walls which have 

 become poorer in water will tend to restore the equilibrium 

 by attracting water from the nearest wood cells, which, 

 in their turn and for the same reason, take it up again 

 from parts of the wood situated lower, until finally this 

 movement, extending backwards, proceeds from the 

 foUage of a land plant down through the stem into the 

 young roots, which absorb the water out of the earth." 

 You will see from this extract that Sachs attributes the 

 whole ascent to the suctional effect of transpiration 

 acting on molecules of water travelhng in the walls of 

 the xylem elements. 



Apart from the fact that the anatomical features pre- 

 sented by the xylem vessels and fibres all tend to throw 

 grave doubts on the probabiHty of Sachs's imbibition 

 theory, the obvious method of directly testing its truth 

 was by occluding the lumina of the vessels and observing 

 whether or not the leaves wilted, and it seems strange 

 that Sachs did not apply that test himself. This was 

 done by Elfving and Vesque in 1882 and 1883 respectively, 

 and also by Kohl in 1885 and by Strasburger in 1891, 

 using such occluding materials as cacao butter, melted 

 paraffin, or gelatine. These experiments proved con- 

 clusively that Sachs's theory was incompatible with the 

 facts, for the results showed that although some small 

 quantity of water might ascend by the walls of the vessels, 

 on the occlusion of the lumina, the amount so raised 

 was quite negligible. 



