MOVEMENTS OF PLANTS 43 



to the deeper layers of earth ; the four or five ranks 

 of secondary roots divide the world between them 

 and push forth all round, keeping sKghtly below 

 the horizontal ; the tertiaries take it for granted 

 that their predecessors have done the usual thing, 

 and that they can satisfactorily occupy the spaces 

 left among their elders by random growth. The 

 fact that the tertiary roots have no specialised 

 sensitiveness of gravitation shows that their unre- 

 gulated growth is good enough for the necessities 

 of the case. For among organised beings necessity 

 is the mother of development, and what their 

 brethren of second rank have developed they too 

 could assuredly have gained. To this point of view 

 I shall return, but first I should like to give a few 

 more instances of actions carried out in response to 

 the signal of gravity ; and these examples shall be 

 from stem-structures. 



The ripe flower-heads of a clover {T. subter- 

 raneum) bury themselves in the ground, thus 

 effectually sowing their own seeds, and they are 

 guided to the ground by their unusual capacity of 

 curving down and directing themselves like a 

 primary root towards the centre of the earth. 



Other flower-stalks are guided by gravitation 

 for quite different purposes. Take, for instance, 

 a common narcissus. In the young condition 

 there is a straight shaft ending in a pointed flower- 

 bud ; but as the flower opens the stalk bends close 

 to the top and brings the flower-tube into a roughly 

 horizontal position, where it shows off its brightly 

 coloured crown to the insects that visit it. The 

 flowers are guided to the right position by the 



