the Radicle and Germen of Seeds. 107 
less affected by gravitation. Their points, however, generally 
shew an inclination to turn upwards ; but the operation of 
light, in this case, has been proved by Bonnet* to be very 
considerable. 
The radicle tapers rapidly, as it descends into the earth, 
and its lower part is much compressed by the greater solidity 
of the mould into which it penetrates. The true sap also con- 
tinues to descend from the cotyledons and leaves, and occa- 
sions 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 is 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 with most rapidity, and producing the 
greatest number of ramifications, wherever they find most 
warmth, and a soil best adapted to nourish the tree. As these 
horizontal, or lateral, roots surround the base of the tree on 
every side, the true sap descending 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 of proper nutriment, 
and, ceasing almost wholly to grow, becomes of no import- 
ance to the tree. The tap root of the oak, about which so 
* Recherches sur I’Usage des Feuilles dans les Plantes. 
P 2 
