DIRECTION OF THE GROWTH OP ROOTS. 375 



stibsequent vigorous growth of such a tree is the natural 



consequence of an improved and more extensive pasture. 



When the seeds of the carrot and parsnep, in the expe. Growth of the 



riments I have stated, were placed in a poor superficial soil, ^^""^^"'^ , 



. ' * ' ' ' paisn.'^p seeds 



but which permitted the roots of the plants to pass readily explained m 



through it, these were conducted downward by gravitation ; ^'^^ ^''^^ ^^^^* 



■while the plants grew feebly, because they received but 



little nutriment. The roots were in a situation analogous 



to that of the stems of trees in a crowded forest; and 



when 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 light. The form of the roots of the plants 



was consequently long, slender, and cylindrical, like the 



stems of such trees. The roots of the one required the 



actual contact of 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 i"^' in the 

 of plants were placed in a rich superficial soil, their situa-^^'^"" " 

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

 side, to the light; the branches of which would be extend- 

 ed, in every direction, immediately above the surface of 

 the ground : and as the fibrous roots of the plants t-ame 

 into contact with the subsoil, which was not well calculated 

 to promote their growth, their situation became analogous 

 to that of shaded branches; and they consequently ceased 

 to extend downwards. The fibrous roots of a tree, under 

 similar 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 

 compressed into the subsoil, however poor and unfavour- 

 able, provided it contained no ingredients actually noxious. 

 In obedience to similar laws, the roots of an aquatic tree Growth of 



will not extend freely in dry soil, nor those of a tree which I'^eesjequirinj 

 •' • ' , much or liVtl* 



requires but little moisture in a wet soil; and on this ac- moisture. 



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



those of the other to have avoided, the contiguous water; 



though both, in the first period of their growth, pointed 



their roots alike in every direction, 



Whea 



