214 GRAVITY, LIGHT, AND CALORIC. L.ECT. XL 



in order to place themselves between the horizontal position 

 which tends to make them acquire the centrifugal force, 

 and the vertical, which is natural to them, and which they 

 take in their ordinary conditions. 



It is evident that, in order to find an explanation of the 

 facts discovered by Hunter and Knight, we must admit: 



1st. A more or less liquid condition of the new parts of 

 the young plant; 



2dly. A different density in the different parts of the 

 latter; and, 



3dly, That the denser parts of the new plant are directed, 

 at least in the first stage of germination, towards the roots. 



From these conclusions it follows that, in the case of the 

 vertical wheel, the parts of the young plant, being submitted 

 to the action of the centrifugal force only, developed them- 

 selves by having their densest parts, namely, their roots, 

 at the circumference ; while, in the case of the horizontal 

 wheel, they took an intermediate position, between that 

 which the centrifugal force impressed on them, and that 

 which they would have acquired if they had been under the 

 influence of gravity only. 



Dutrochet, without denying the influence of gravity upon 

 the ordinary direction of roots and stems, admits, neverthe- 

 less, a second cause for this phenomenon. It may depend 

 on the unequal development of the cellular system of the 

 roots and stems, and on the different turgescence which en- 

 dosmose produces in the cells of this system. 



Action of Light on Jlnimals.-^I^et us now speak of 



light. 



We know nothing, or scarcely any thing, respecting the 

 action which this agent exercises upon animals. Edwards 

 has proved, that the ova of the frog are more rapidly deve- 

 loped in the sun than in darkness ; and also, that tadpoles 



