420 On the Causes ivhich ivfluence 



springing to the vertex; so that here the strength decreases 

 as the weight increases. 



When the theories are extended to domes, results of the 

 most extraordinary character are nroduced : we find a ver- 

 tical line passing through the vertex, an asymptote; the 

 extrados being terminated at the vertex by diflech.e injinie ; 

 but these phaenomena were not discovered by Bouguer, 

 Bossutj and Robison. Soufflot'b knowledge of geometry is 

 even questioned, because he difl not terminate the dome ot 

 the Pantheon FrauQois en Jieclie injinie. It has been as- 

 serted, not very humbly, by the followers of th» Emerson 

 theory, that " the wedge theory of La Hire is now com- 

 pletely exploded," and~that the theory which they follow 

 is the " only true one." In the preceding pages it has been 

 attempted to preserve a sceptic's indifference. 



LXXVI. On the Causes which injiuence the Direction of 

 the Growth of Roots. By T. A. Knight, Esq. F.R.S. 

 Jn a Letter to the Right Hon. Sir Joseph Banks, Bart, 

 K.B.P.R.S* 



jL have shown, in a former communication, the effects of 

 centrifugal force upon germinating seeds; from which I 

 have inferred that the radicles are tnade to descend towards 

 the earth, and the germs, or elongated plumules, to lake 

 the opposite direction, by the influence ol" gravitation ; and 

 I believe the facts I have stated to be sufncient to support 

 the inferences I have drawn f. But the fibrous roots of 

 plants, being much less succulent, though not uninfluenced 

 m the directions they take by gravitation, are, to a great 

 extent, obedient to other laws, and are generally found to 

 extend themselves most rapidly, and (o the greatest length, 

 in whatever direction the soil is most favourable : whence 

 many naturalists have been disposed to believe that these 

 are guided by some degrees of feeling and perception, ana- 

 logous to those of animal life. 



I shall proceed to state some of the facts upon which this 

 hypothesis has been founded, and others which have occur- 

 red in the course of my own experience, and which are fa- 

 vourable to it ; after which I shall endeavour to trace the 

 effects observed to the operation of different causes. 



When a tree which requires much moisture has sprung 

 lip, or been planted, in a dry soil, in the vicinity of water, 



* From Philosophical Traniactions for 181,1, part ii. 

 f Phil. Tran». 1800, part i. page 5. 



