VEGETATIOX OF SEEDS. 415 



CwlitinuGS to descend from the cotyledons and leaves, and oc- 

 casions a continued increase of the growth of the upper parts 

 of the radicle ; and this growth is subsequently, augmented by 

 the effects of motion, when the germen has risen above the 

 ground. The true sap, therefore, necessarily obstructed in its 

 descent, numerous lateral roots are generated, into which a 

 portion of the descending sap enters. The substance of these 

 roots, like that of the slender horizontal branches, is much less 

 succulent than that of the radicle first emitted, and they are in 

 consequence less obedient to gravitation ; and therefore meeting 

 less resistance from the superficial soil, than from that beneath 

 it, they extend horizontally in every direction, growing witli 

 most rapidity, and producing the greatest nu.mber of ramifica- 

 tions, wherever they find most warath and a soil best adapted 

 to nourish the tree. As these Jiorizontal or lateral roots 

 surrouful the base of the tree on every side, the true sap de- 

 sceading down its bark, enters almost exclusively into them ; 

 and the first perpendicular root having executed its office of 

 securing moisture to the plant whilst young, is thus deprived o( 

 proper nutriment, and ceasing almost wholly to grow, becomes 

 of no importance to the tree. The tap root of the oak, about The notion of 

 which so much has been written, will possibly be adduced as ^ *^^**\°^^* 

 an exception, but having attentively examined at least 20,000 

 trees of this species, many of which had grown in some of the 

 deepest and most favouiable soils of England, and never having 

 fyund a single tree possessing a tap root, I must be allowed to 

 doubt that one ever existed. 



As trees possess the power to turn the upper surfaces of their Trees have a 



leaves and the points of their shoots to the light, and their P'^'^^'' of shoat- 



11. T • , , , " . ing towards 



tendrjls in any direction to attach themselves to contiguous moist or dry 



objects, it may be suspected that their lateral roots are by some Peaces accord- 

 v , .,.,.... , ingtotheirne- 



raeans directed to any soil in their vicinity which is best calcu- cessities. 



laled to nourish the plant to which they belong ; and it is well 



known, that much the greater part of the roots of an aquatic 



plant which has grown in a dry soil, on the margin of a lake or 



river, have been found to point to the water ; whilst those of 



another species of tree which thrives best in a dry soil, have 



been ascertained to take an opposite direction. But the result 



of some experiments I have made is not favourable to this 



h y^-yothesis ; s^nd I am rather inclined to believe that the roots 



disperse 



