\?m. 



SECT. II. PHYSICAL PHENOMENA. 25 



air was just as mysterious as that of the sun, and 

 the subject as much in want of elucidation as 

 before, 



Dr. Darwin has, however, endeavoured to account By Dar* 

 for the phenomenon chiefly upon the principle now 

 specified. Supposing the radicle to be naturally 

 stimulated by moisture, and the cotyledons and 

 plumelet by air, the difficulty is, as he thinks, easily 

 solved ; each being thus elongated in the direction 

 in which it is most excited.* This hypothesis is 

 no doubt sufficiently ingenious ; but is by no means 

 to be regarded as a satisfactory solution of the dif- 

 ficulty. For at this rate all cotyledons ought to rise 

 above ground, which all cotyledons do not. And 

 all seeds ought to germinate either in the earth or 

 water ; though many of them will germinate in 

 neither; but on trunks or stumps of trees, and 

 even on the surface of the bare and flinty rock. 

 The radicle ought also to elongate itself in a ver* 

 tical direction, if it could be but lodged in the 

 lower surface of an insulated mass of mould, so as 

 to have the moisture of the mass and grand ex- 

 citing cause of its elongation placed above it. Now 

 this must inevitably have happened in one or other 

 of Du Hamel's repeated inversions, and yet the 

 result was always the same ; the radicle having 

 uniformly bent itself downwards in the direction of 

 the surface of the earth. 



* fhytologia, $ect. ix, 



