22 THE BIOLOGY OF FLOWERING PLANTS 



often transported to great distances by wind, water, or ice. 

 Through the action of earthworms humus is incorporated 

 with this basic material, and produces a medium of a 

 pecuHar crumby structure largely dependent on the proper- 

 ties of the colloid constituents. The im.portance of this 

 structure is threefold. It provides a medium which is 

 well aerated, well watered — retaining moisture, yet allowing 

 it to move freely — and easily pervious to plant roots. The 

 soil water is a dilute solution which contains the salts 

 necessary to plant growth. It supports an extensive fauna 

 and flora, largely microscopic, playing an essential part in 

 the maintenance of fertility. The soil is penetrated by 

 the roots of the higher land plants of which it is the 

 characteristic medium of growth. 



§ 3. The Root System 



Penetration and Direction. — It is a familiar fact that the 

 primary root of the seedling grows into the soil ; if the seed 

 happens to be inverted, a cui"vature of the radicle brings the 

 point vertically downwards. The most important directive 

 influence acting on the radicle is the force of gravity ; if 

 the radicle lies vertically it is in a position of equilibrium ; 

 if not, then the unilateral stimulus of gravity produces an 

 excitation which results in a growth curvature and brings 

 the tip into the vertical position. This reaction is called 

 geotropism, a tropism being a movement of a plant organ 

 induced and directed by an external stimulus. It has been 

 shown that the stimulus is perceived solely, or at least 

 preponderatingly, in the apical i| mm. of the root tip. 

 The excitation is conducted to the growing zone just behind, 

 where a differentiation of the growth rate on the upper 

 and lower sides produces the reaction. Many roots are 

 also phototropic, curving away from a source of light, but the 

 geotropic reaction is the one chiefly responsible for the 

 vertical direction of growth. The side roots do not grow 

 vertically ; they extend at a definite angle from the main 

 root. This position is a resultant of gravitational excitation 



