CHAP. VII. EPIRRHEOLOGY. 293 



parts which were alternately uppermost and lower- 

 most in each revolution. The only cause which 

 could have produced the effects described must be the 

 centrifugal force, which has here replaced the force of 

 gravity, compelling the root to grow outwards and the 

 stem inwards, instead of downwards and upwards. The 

 effect produced upon the horizontal wheel is evidently 

 the result of the combined action of the two forces 

 gravity inclining the root downwards, and the centri- 

 fugal force propelling it outwards ; and the reverse 

 with regard to the stem. Although it is plain that 

 gravity is the efficient cause in establishing the direc- 

 tions of the stems and roots of plants, it is not so easy 

 to understand the manner in which it produces opposite 

 effects on these two organs. Various theories have been 

 formed to account for this, and the most plausible is 

 that which ascribes it to the different manners in 

 which the newly developed tissues are added to the 

 root and stem. In the root the addition is almost 

 entirely confined to the very extremity, whilst the stem 

 continues to increase for some time throughout its whole 

 length. Hence it is supposed that the soft materials 

 continually deposited at the extremity of the root must 

 ever be tending downwards from the mere effect of 

 gravity alone. In the stem, gravity would cause a sub- 

 sidence of the denser and more nutritious materials to 

 the lower side, and this side would consequently be 

 more nourished thart the upper, supposing the stem to 

 be somewhat inclined from the perpendicular. The 

 consequence of one side being better nourished than the 

 other, whilst the whole was in a growing state, would 

 be a greater extension of that side; and thus a slight 

 curvature upwards would be given to the stem, which, 

 being continually repeated as it develops, would always 

 tend to keep it more or less in a vertical position. Per- 

 haps we want sufficient data to allow us to lay any 

 great stress upon this explanation. 



(301.) Effect of Light on Vegetation. Light is 

 another cause which produces a great effect in modify- 

 u 3 



