AERIAL ROOTS, DEVELOPMENT OP ROOTS. 



85 



force of gravity. He placed some germinating beans on the circum- 

 ference of a vertical wheel ; and made this re- 

 volve rapidly and continuously for several days. 

 From the constantly varying position of the 

 seed, the force of gravity was here neutralised ; 

 but a new force was substituted for it, the 

 centrifugal force, or tendency to fly from the 

 centre, which a stone (for example) let loose 

 from the circumference of such a wheel whilst 

 revolving would exhibit.* The root, influ- 

 enced by this force as ordinarily by that of 

 gravity, grew, in every instance, away from 

 the centre of the wheel, whilst the stem grew 

 towards it. 



112. In another experiment, the wheel 

 was made to revolve horizontally, so that the 

 force of gravity continued to operate, but in 

 combination with the centrifugal force; in 

 F '' 37 '"s"^D RWmATING this instance, the direction of the radicle 

 a, piumuia ; 6, radicle, showed the evident influence of both; for 

 it pointed downwards and outwards. Why the radicle should be 

 thus influenced by the force of gravity, when the plumula grows 

 in opposition to it, is a question that has not yet been answered ; 

 but it is interesting to know the fact. We are as completely 

 ignorant of the ultimate causes of the most common occurrences. 

 We do not know, for example, why the earth should attract the 

 bodies on its surface, or why it should itself be attracted by the 

 sun. We only know the general fact, that all masses of matter 

 attract one another. 



113. Roots are ordinarily distinguished from stems, not only 

 by their direction, but also by the presence of the absorbing fibres, 

 and by the absence of buds, which last are so characteristic of 

 the Stem, that any part on which they appear may be ordinarily 

 considered such, even though it is growing underground. In 

 general, however, the stem possesses the power of sending out 



See MECHAN. PHILOS. .211. 



