704 PART IV. THE PHYSIOLOGY OF PLANTS. 



fact which has been proved beyond question by special experi- 

 ments : second, that the only force remaining to which the move- 

 ment of the transpiration-current in the stem can be attributed, is 

 that of the osmotic absorption of water from the tracheal tissue 

 by the adjacent cells of the leaves when transpiration is going 

 on. In accordance with these facts it has been suggested that 

 the water travels molecularly through the substance of the 

 lignified cell-walls, the water taken up from the wood by the 

 transpiring leaves being at once replaced by fresh molecules 

 coming up from below. The merit of this view, known as the 

 " imbibition- theory," is that it overcomes the difficulty of account- 

 ing for the raising of water through the cavities of the trachea! 

 tissue; for the molecules of the water thus conceived of as being 

 held in the substance of the cell-walls will travel, like the mole- 

 cules of a gas, in the direction of least resistance, that is, towards 

 the transpiring organs. Moreover it is true that the transpiration- 

 current will only travel through wood the walls of which are 

 saturated with water. But it is objected to this theory that it 

 does not accord with the fact that any diminution or interruption 

 of the lumina of the vessels, by compression for instance, or by 

 section, diminishes or arrests the transpiration-current ; clearly 

 this ought not to be the case if the current travels exclusively in 

 the walls. Moreover it has been ascertained that the actually 

 conducting-tissue of the wood always contains water in its cavities 

 even when transpiration is most active, though bubbles of gas are 

 also present, at any rate in the wider vessels or tracheids. 



The conclusions to be drawn as to the mechanism of the trans- 

 piration-current in lofty trees, may be stated as follows. In the 

 spring the wood is full of water forced into it by root-pressure. 

 When the leaves unfold, and begin to transpire, water is gradually 

 withdrawn from the conducting tracheal tissue, and the tissue is, at 

 any rate for the most part, occupied by a system of short columns 

 of water with intervening gas-bubbles, the columns of water being 

 in communication by delicate films along the cell-walls. If the 

 whole of the tracheal tissue be in this state, it is suggested that 

 as water is withdrawn from the upper part of the wood by the 

 transpiration of the leaves, a current is set up, the water travelling 

 along the cell-walls, between them and the gas-bubbles. But it 

 may be that a continuous system of tracheids completely filled 

 with water is maintained, and that it is through this that the 

 current travels. The conducting-tissue is supplied with water, 



