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tion they are first protruded, almost uniformly turn upwards, and endeavour 
to acquire a perpendicular direction; and to this their points will imme¬ 
diately return, if they are bent downwards during any period of their 
growth; their curvature upwards being occasioned by an increased extension 
of the fibres and vessels of their under sides, as in the elongated plumes of 
seeds. The more feeble and slender shoots of the same trees will, on the 
contrary, grow in almost every direction, probably because their fibres, being 
more dry, and their vessels less amply supplied with sap, they are less 
affected by gravitation. Their points, however, generally shew an inclina¬ 
tion 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 continues to descend from the cotyledons 
and leaves, and occasions 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 plume 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 conse¬ 
quence 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 it is thus deprived 
of proper nutriment, and, ceasing almost wholly to grow, becomes of no 
importance to the tree. 
Thus is this dark matter attempted to be unfolded: but I do not think 
there is much difference between these kind of explanations and that of those 
writers who have ascribed the ascent of the plumule and the descent of the 
* Recherches sur l’Usage des Feuilles dans les Plantes. 
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