424 On the Causes which ivfluevce 



tively with other parts of the tree, will ^row rapidly. Gra- 

 vitation will direct these roots perpendicularly downwards, 

 and the tree will appear to have adopted the wisest and 

 best plan of connecting itself with the ground : and it will 

 really have employed the readiest means of doing so, as 

 efiectivelv as it coidd liave done if it had possessed all the 

 feelings and instinctive passions and powers of animal life. 

 The subsequent vigorous growth of such a tree is the na- 

 tural consequence of an improved and moie extensive pas- 

 ture. 



When the seeds of the carrot and parsnip, in the experi- 

 ments I have stated, were placed in a poor superficial soil, 

 but which permitted the roots of the plants to pass readily 

 through it, these were conducted downwards by iiravitationj 

 whilst the plants grew feebly, because they received but lit- 

 tle nutriment. The roots were in a situation analogous to 

 that of the stems of tree in a crowded forest ; and whtn the 

 leading fibres of the roots came into contact with the rich 

 mould, they acquired a situation correspondent to that of 

 the leading branches of such trees which are alone exposed 

 to the lieht. 'I he form of the roots of the plants was con- 

 sequently long, slender, and cylindrical, like the stems of 

 such irees. I'he roots of the one required the actual con- 

 tact ot proper soil and nutriment ; and the branches of the 

 other required the actual contact of light, to promote their 

 growth. 



When on the contrary the seeds of the preceding species 

 of plant? were placed in a rich superficial soil, their situa- 

 tion was analogous to that of a tree fuily exposed, on every 

 side, to the light, whose branches would be extended, '\h 

 every direction, iirmiedialelv above the surface of the 

 ground : ami as the hbrous roots of the plants came into 

 contact with the subsoil, which was not well calculated to 

 promote their growth, their situation became analogous to 

 that of shaded branches j and they consequently ceased to 

 extend downwards. The fibrous roots of a tree, under si- 

 milar circumstances, would have extended along the lower 

 surface of the favourable soil ; but after these roots had 

 much increased in bulk, they would be found partly com- 

 pressed into the subsoil, however poor and unfavourable, 

 provided it contained no ingredients actually noxious. In 

 obedience to similar laws, the roots of an aquatic tree will 

 not extend freely in dry soil, nor those of a tree which re- 

 quires but little moisture in a wet soil; and on this account 

 the roots of the one will appear to have sought, and those 

 of the other to have avoided, the contiguous water j though 



bo.lh, 



