42 MOVEMENTS OF PLANTS 



the root away from, the light — and this is equally 

 inexplicable on a mechanical basis. 



But it may be urged that it is not fair to com- 

 pare a root and a stem which are structurally 

 unlike. Let us, therefore, keep to roots. When 

 the root of a bean has grown vertically down into 

 the soil for some distance it begins to bud forth 

 into side roots. These are exactly like the primary 

 root from which they spring ; there is no difference 

 in structure or in machinery of growth. Yet the 

 secondary roots do not grow vertically down, but 

 obliquely, or in some cases horizontally. There is 

 one more striking fact about the roots of the bean. 

 The secondary, like the primary roots, give off 

 branches, and these — the tertiaries — behave differ- 

 ently from both the elder generations of roots. 

 For instead of directing themselves vertically or 

 horizontally, they simply treat the force of gravity 

 with contempt and grow where fancy leads them. 

 The point on which I wish to insist is, that it is 

 impossible to explain on any theory of the direct 

 action of gravity why the three orders of roots have 

 three distinct modes of growth. They may remind 

 us of three generations, grandfather, father, and 

 son, all of one blood and yet behaving towards the 

 universe in three distinct ways — o. fact not unknown 

 in human society. 



On the other hand, it would not be difficult to 

 show that the behaviour of the three orders of 

 roots is well suited to the plant's needs, and there- 

 fore we can understand how the power of reacting 

 in three different ways to the same signal has been 

 evolved. The main root takes the shortest course 



