INTRODUCTION ii 



plants to stimuli is all of the nature of irritability. 

 This has- reference to the necessity of self-preservation, 

 and it may be added that even a mineral, which has 

 never so far been accredited with any definite type of 

 sensibility, has the power of repairing injury. 



There is thus a kind of progressive development, 

 parallel for the same type of activity, in each of the 

 three realms or states of nature. 



There is little to distinguish the sense of contact 

 common to plants and animals from irritability. In 

 animals which have other senses such as sight, 

 hearing, smell, taste, and the power of speech, etc., 

 however, it is developed into the sense of touch. In 

 plants it is rare, and found mainly in insectivorous 

 plants and in climbing plants. 



The stimulus which causes plants to extend the 

 root downwards is gravity, a force which equally 

 affects all matter. By a mechanical device Knight (see 

 Fig. 4) overcame this force and caused roots and 

 stems to develop at an angle of forty-five degrees to 

 the vertical, whereas under gravity plants place them- 

 selves parallel with the direction of its force, or 

 normally vertically. The effect of gravity, however, 

 does not extend in direction to the lateral root or 

 stem branches, and these grow out at right angles or 

 obliquely, and are diageotropic. This power of move- 

 ment then is distinctly connected with growth. But 

 the power of the root-tip to respond to gravity is 

 only found in the cells at the tip, and if the growing 

 point is destroyed there is no response to the 



