C 30 j 



When the centrifugal force was made merely ' to 

 modify the force of gravitation in the horizontal wheel 

 when thegreatest volocity of revolution was given, 

 the radicles pointed downwards about ten degrees be- 

 low, and the germens as many degrees above the 

 horizontal line of the wheel's motion ; and the devia- 

 tion from the perpendicular was less in proportion, 

 as the motion was less rapid.* 



These facts afford a rational solution of this cu- 

 HOUS problem, respecting which different philosophers 

 have given such different opinions ; some referring it 

 to *he nature of the sap, as De la Hire, others, as 

 Darwin, to the living powers of the plant, and the 

 stimulus of air upon the leaves, and of moisture upon 

 the roots. The effect is now shewn to be connected 

 with mechanical causes ; and there seems no other 

 power in nature to which it can with propriety be 

 referred but gravity, which acts universally, and 

 which must tend to dispose the parts to take a uni- 

 form direction. 



If plants in general owe their perpendicular di- 

 rection to gravity, it is evident that the number of 

 plants upon a given part of the earth's circumference, 

 cannot be increased by making the surface irregular, 

 as some persons have supposed. Nor can more stalks 

 rise on a hill than on a spot equal to its base ; for the 

 slight effect of the attraction of the hill, would be only 



* Fig. 1 represents the form of the experiment when the vertical wheel was 

 made to perform 150 revolutions in a minute. 



Fig. 2 represents the case in which the horizental wheel performed 250 revo- 

 lutions. 



