86 A GLIMPSE OF THE LIFE OF CHLOROPHYLLOUS PLANTS. 
through which it penetrates, and it is only—-a few milli- 
metres—behind this cap that the root grows. The branches 
of roots grow from the inside, or their growth is endogenous, 
and the hairs which roots develop are protuberances of 
their epidermal cells. These hairs consist of very narrow 
tubes with exceedingly thin walls through which water can 
easily penetrate. As these are very short, though numerous, 
they cannot reach the particles of water in the soil to 
deprive them of their moisture to the extent which we 
know they do without some other factor coming into play. 
The earth in large glass vessels in which plants are 
grown dries up not merely in the immediate neighbourhood 
of the absorbing roots, but, so far as the colour of the 
earth allows us to recognize it, the desiccation increases 
equally in all parts even far from the roots. This move. 
ment of water on the surface of the particles of soil, easily 
deduced from molecular forces, is confined therefore not to 
microscopic distances. Every root-hair is itself the centre 
of a current directed from all sides towards it, and at the 
surface of a small piece of root covered with thousands of 
root-hairs there results a similar movement which carries 
the aqueous particles in the soil from all sides, but 
especially radically towards the axis of the root. 
This gives an explanation of how plants obtain water 
through their root-hairs, and, it is the only way in which 
they do obtain water, and, with it, salts and also nitrogen 
in some shape or other. It does not however account for 
the ascent of water through the stem to the leaves. 
Plants, besides being constructed of innumerable cells, 
have, as a consequence of the mode of deposition of these 
cells, numerous capillary tubes. 
The causes of movement of water in plants are: capillarity, 
diffusion or osmose, root-pressure, and the natural conduct- 
ibility of wood for water, which is possessed by no other 
substance. In conjunction with these there is root-pressure, 
