OF THE GROWTH OF ROOTS. 161 



\vith moisture than others, and these will be first affected by drought : 

 their points will in consequence become rigid and inexpansible, and they 

 will thence generally cease to elongate at an early period of the summer. 

 The descending current of sap will be then employed in promoting the 

 growth and elongation of those roots only, which are more favourably 

 situated, and which, comparatively with other parts of the tree, will grow 

 rapidly. Gravitation will direct these roots perpendicularly downwards, 

 and the tree will appear to have adopted the wisest and best plan of con- 

 necting itself with the ground : and it will really have employed the readiest 

 means of doing so, as effectively as it could have 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 natural consequence of 

 an improved and more extensive pasture. 



When the seeds of the carrot and parsnip, in the experiments 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 gravitation ; whilst 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 of plants 

 were placed in a, rich superficial soil, their situation was analogous to that 

 of a tree fully exposed, on every side, to the light, whose branches would 

 be extended, in every direction, immediately above the surface of the 

 ground : and as the fibrous 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 ; and they conse- 

 quently 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 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 requires but little moisture 



