COMPOSITION OF SOIL 105 
reducing, and catalytic actions, etc., are now rapidly 
extending our knowledge of the soil as a medium for 
plant growth.”* 
Such, then, is the nature of the soil in which plants 
grow, and from which, by means of innumerable 
elongated cells (the root-hairs) proceeding from near 
the tips of the roots, food materials dissolved in water 
are absorbed; these food materials being produced 
partly by solution of mineral constituents contained 
in the soil, partly by the action of bacteria in breaking 
up organic matter. Soil suitable for plant growth may 
be looked on as consisting of a mineral framework, 
carrying in its meshes water (about three-tenths of its 
volume) and air (about one-tenth of its volume); 
mixed with the mineral particles is humus of varying 
amount; and supported largely by the humus is a vast 
population of organisms, both animal and vegetable, 
from earthworms to bacteria, whose activities are 
often essential, generally beneficial, and occasionally 
prejudicial to plant growth. 
The root of a young plant grows downward into 
the soil under the influence of gravity. Its tip, which 
has to force its way through the rough material of 
sand and clay, is beautifully protected by a special 
root-cap, which covers the growing point as with a 
cushion. The surface of the root-cap is slimy, to aid 
it in slipping forward, and its cells, which are being 
worn away constantly, are replaced by the growth of 
the interior. Should an obstacle such as a pebble be 
encountered, a root will bend round it and then return 
to its former direction. Branch roots are given off on 
* W. B. Bortromtey in ‘‘ The Exploitation of Plants,” edited by 
F, W. Oliver, 1917, p. 12. 
