38 
THE ROOT 
that is continually renewed from within as it wears away 
outside, like the skin of the hand. It protects the delicate 
growing point from injury as it is pushed through the soil. 
Only rarely (cf. Pandanaceae, Lemna) is the cap distinguish- 
able by the naked eye. It is absent in Azolla, Neottia, &c. 
(adventitious roots). 
From the main root of a gourd or bean there sprout 
lateral roots arranged in four vertical ranks. This is due to 
the fact that they arise deep down in the main root, at points 
opposite to the xylem bundles, of which there are four (in 
these plants). The young root burrows its way out into the 
soil through the outer tissue of the parent root ; such 
development is termed endogenous , in contrast to exogenous 
development, from superficial tissue. The lateral roots are 
arranged in a definite order of age, the youngest being 
nearest to the apex of the main root; this is termed arrange- 
ment in acropetal succession. They are but little sensitive 
to gravity and grow more or less horizontally, but are 
governed in their growth by definite though ill-understood 
laws. If, for instance, the northward growth of one of them 
be interrupted by a stone, the root, as soon as it has reached 
the edge of the obstacle, tends to resume that direction of 
growth. This tendency of lateral roots to grow away from 
the main root is termed exotropism and is of importance to 
the root by enabling it to spread in all directions through the 
soil in which it is growing. The lateral roots may be again 
branched and so on. 
If, as in the bean, the main root grows to be much 
longer than the branches, it is termed the tap-root. 
The functions most characteristic of roots are the absorp- 
tion of food-materials from the substratum, and the anchorage 
of the plant therein. The former is generally effected by 
the agency of the fine unicellular root-hairs which clothe 
the tips of the roots a little way behind the actual apex 
(easily seen in mustard seedlings or in many water roots). 
The branching and mode of growth of the root admirably 
fit it for the absorption of all the food-materials obtainable 
in the soil in which it grows. In saprophytic plants (see 
Chap. III.) the root absorbs the products of decaying organic 
matter (humus) from the soil, as well as mineral salts. In 
most of our forest trees and in many other plants, the root 
