84 MOVEMENTS OF WATER, ETC. [CH. 



passes from the pipes to the cells at these points of 

 contact, diffusing through the membrane separating the 

 cavity of the cell from that of the pipe. 



If, on the other hand, the mesophyll-cells contain a 

 surplus of water and such water will have dissolved in 

 it traces of bodies like sugars and other nutritious sub- 

 stances which, as we shall see later on, are produced in 

 the cells while the vascular pipe ends are lacking in such 

 water, then passage will occur through the separating 

 membranes from the cells to the vascular system. 



But it is a significant fact that the vascular pipes 

 which deliver the water to the mesophyll-cells are not 

 the same as those which collect from the latter ; though 

 both sets of pipes lie close together at the blind ends of 

 the veins. Whence we see that the terminals deliver 

 to the mesophyll-cells, by way of one of their double sets 

 of pipes, water containing traces of dissolved salts that 

 have been brought up the stem from the roots, which 

 absorbed them from the soil ; while they collect from 

 such mesophyll-cells water containing food-materials such 

 as sugars, &c., dissolved in it, and pass this on by the 

 second of their double sets of pipes, into the larger veins 

 and ribs of the venation. Thence the solutions pass down 

 through the petiole into other parts of the plant, where 

 the sugars, &c., are used as food-materials to build up the 

 structures of new buds, flowers, roots, &c., as required. 



It will now be understood why we cannot speak of the 

 above as a " circulation of sap " ; for it is not the same 

 liquid which was brought up by the afferent pipes of 

 the venation that returns by the efferent flow in the 

 other pipes. The former is water with traces of mineral 

 salts dissolved in it : the latter is water with organic 

 substances, such as sugar, and other bodies containing 

 carbon in the molecules, dissolved in it. We know of 



