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 

 so 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. 



If 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 



2 * 



