62 BOTANY 



essentially purposeful. The only mode of securing this 

 adjustment with the environment which is possible is 

 that of regulating its growth. 



During the early development and growth the plant 

 exhibits in its shoots as in its roots powers of purposeful 

 response to certain features of the environment which it 

 is capable of appreciating. If we examine the plumule 

 or young bud of the seedling as soon as it begins to grow, 

 we shall notice the same perception of direction as we 

 observed in the root. As the latter would persist in 

 growing downwards, curving itself if its apex pointed 

 in any other direction, so the shoot persists in growing 

 upwards. The sensitive part is not so easy to localise 

 as in the root, but careful experiments made on various 

 plants have proved that the perceptive part of the 

 shoot is the tip and that the sensitive zone does not 

 extend far downwards. The response to the stimulus is 

 brought about in the same way in the two cases, viz., by 

 a modification of the growth, and it is clearly purposeful, 

 to plant the root in the earth and the shoot in the air. 

 There is a close resemblance again in their behaviour 

 between the primary branches of the stem and root. 

 None of them grows in the same direction as the axis 

 from which it springs, but usually they stretch out nearly 

 at right angles to it. This is a response to the stimulation 

 of gravitation in both. 



Another factor which is of much greater importance 

 in the case of the shoot than in that of the root is the 

 incidence of lateral light, which helps to determine the 

 position of the branches as well as of the leaves. If the 

 light falls on a shoot more intensely on one side than 

 another, the rate of growth very speedily changes so as 

 to cause the growing region of the stem to bend or curve 

 till its apex is directed towards the point from which the 

 strongest light is coming. 1 The plant exhibits a power 



1 A figure illustrating this is given in the Biology primer, p. 71. 



