CHAP, i Processes of Absorption of Water 271 



' The wood consists of a framework of lignified lamellae, 

 which enclose cavities (cell cavities). According to cir- 

 cumstances, the cavities may contain water, or air (with 

 aqueous vapour), or both. The walls themselves may be 

 dry, or contain water of imbibition, and their volume or 

 condition of swelling alters with the amount of water 

 contained. The cell cavities of the wood are capillary 

 spaces ; the cell walls on the contrary contain no capillaries 

 into which liquid or air could directly penetrate. In order 

 to be able to judge of the movement of water in the wood 

 produced by transpiration and other causes, it is necessary 

 to distinguish sharply between the capillarity of the cavities 

 and the imbibition of the cell walls.' 



In his Lectures on the Physiology of Plants, published 

 three years later, he says again : 



' The wood owes its significance as the organ for con- 

 ducting water to a series of most remarkable properties, 

 which are found in no other natural body ... it depends 

 not upon a phenomenon of capillarity, but upon imbibition 

 and swelling. . . . The one point of special importance to 

 be considered here is the facile mobility of the water thus 

 held fast by 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 estab- 

 lished, 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 equilibrium 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 foliage, of a land-plant down 

 through the stem into the young roots, which absorb the 

 water out of the earth.' 



Sachs' hypothesis thus attributed to the evaporation of 

 transpiration the whole of the force which causes the current 



