548 SUMMARY AND CHAP. XII. 



modified, and the part bends either to or from the 

 exciting cause ; or it may occupy a new position, as 

 in the so-called sleep of leaves. The influence which 

 modifies circuninutation may be transmitted from one 

 part to another. Innate or constitutional changes, 

 independently of any external agency, often modify 

 the circumnutating movements at particular periods 

 of the life of the plant. As circumnutation is uni- 

 versally present, we can understand how it is that 

 movements of the same kind have been developed in 

 the most distinct members of the vegetable series. 

 But it must not be supposed that all the movements 

 of plants arise from modified circumnutation ; for, as 

 we shall presently see, there is reason to believe that 

 this is not the case. 



Having made these few preliminary remarks, we 

 will in imagination take a germinating seed, and con- 

 sider the part which the various movements play in 

 the life-history of the plant. The first change is the 

 protrusion of the radicle, which begins at once to 

 circumnutate. This movement is immediately modi- 

 fied by the attraction of gravity and rendered geo- 

 tropic. The radicle, therefore, supposing the seed to 

 be lying on the surface, quickly bends downwards, fol- 

 lowing a more or less spiral course, as was seen on the 

 smoked glass-plates. Sensitiveness to gravitation re- 

 sides in the tip; and it is the tip which transmits 

 some influence to the adjoining parts, causing them 

 to bend. As soon as the tip, protected by the root- 

 cap, reaches the ground, it penetrates the surface, if 

 this be soft. or friable ; and the act of penetration is 

 apparently aided by the rocking or circumnutating 

 movement of the whole end of the radicle. If the sur- 

 face is compact, and cannot easily be penetrated, then 



