LECTURE VI. 



THE MOVEMENT OF WATER IN PLANTS. 



WE have now to consider the distribution throughout the 

 plant of the water which, as we saw in the last lecture, is ab- 

 sorbed by the roots. 



There is no doubt that the distribution of water takes 

 place, to some extent at least, in the same manner and by 

 virtue of the same forces as its absorption, that is, it passes 

 by osmosis from one cell to another just as it passed ori- 

 ginally from without into the superficial cells of the plant. 

 Further, inasmuch as the movement is the expression of a 

 tendency towards fluid equilibrium in the plant, that is, that 

 the proportion of water in each cell should be the same 

 throughout, the direction of this movement is not necessarily 

 constant: it proceeds from those parts which are relatively 

 rich in water towards those which are relatively poor, and 

 naturally if water is given off at any point of the free surface 

 of the plant, a current will be set up in the tissues towards 

 that point. 



These statements apply not only to water, but also to the 

 gases and the substances which the water holds in solution. 

 These travel, as we have seen (p. 44), by osmosis from cell to 

 cell, the direction of their movement being determined simply 

 by the relative quantities of them in different parts of the 

 plant. The final cause of the movement is the removal of 

 the various substances from the sphere of osmotic activity, 

 in consequence of the chemical or physical changes which 

 they undergo in the living cells of different parts of the plant. 



