THE DIFFERENTIATION OF THE PLANT-BODY 19 
or too wet; it may be slimy like clay, or very stony like 
gravel. The amount of water the soil contains and the 
degree in which air is present are also factors which must 
be taken into account in considering the growth. After 
a plant has once established itself and secured firm anchorage 
it still has to deal with varying conditions of a similar 
nature, for the character of the soil is very liable to changes, 
depending on conditions of temperature, weather, and 
go on. 
Besides the advantage of a firm anchorage, the root 
depends upon the soil for the supply of certain materials 
which ultimately take part in some way in its nutritive 
processes. Certain minerals are necessary to every green 
plant, many others are advantageous, some are deleterious. 
The copious branching which the root system exhibits 
goes a long way to secure good anchorage, and at the same 
time to draw supplies from a large bulk of soil. The 
branching is supplemented by the development of the 
root hairs, long unicellular outgrowths of the surface of 
each young branch of the root, which come into very close 
contact with the ultimate particles of the soil. 
Ii we turn to inquire what dangers beset the part of 
the plant we have called the shoot, which grows up into 
the air and forms a head that is frequently of large size, 
we find them associated with the various atmospheric 
changes incident to every climate. First of these we may 
place wind or tempest. As the shoot-body grows it must 
offer more and more resistance to air currents, a resistance 
which may easily result in a violent uprooting of the plant. 
This involves such a subdivision of the plant-body as will 
allow the wind to penetrate through it without serious 
disturbance. We see consequently a continuous tapering 
off of the branches and twigs, which become more and 
more flexible as they are increasingly slender. In the 
central part of the shoot system they are rigid and can 
resist a storm ; where by their dimensions resistance becomes 
impracticable we find flexibility, enabling them to bow 
9 * 
