NATURAL 
hypothesis, (if applied to the subse- 
quent growth and forms of trees) 
arises frém the facts that few of their 
branches rise perpendicularly up- 
wards, and-that their roots always 
spread horizontally ; -but this objec- 
tion [ think may be readily answered. 
The luxuriant shoots of trees, 
which abound in sap, in whatever 
direction they are first protruded, 
almost uniformly turn upwards, and 
endeavour to acquire a perpendicu- 
lar direction ; and to this their points 
will immediately return, if they are 
bent downwards during any period 
of their growth; their curvature up- 
wards being occasioned by an in- 
creased extension of the fibres and 
vessels of their under sides, as in the 
elongated germens 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 am- 
ply supplied with sap, they are less 
affected by gravitation. Their 
points, however, generally shew an 
inclination to turn upwards; but 
the operation of light, in this case, 
had 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 
continues to descend from the co-~ 
tyledons and leaves, and occasions a 
continued increase of the growth, and 
is subsequently augmented by the 
effects of the motion, when the ger- 
men lies above the ground. The 
true sap is, therefore, necessarily ob- 
structed in its descent; numerous 
lateral roots are generated, into 
which a portion of the descending 
sap enters. The substance of these 
HISTORY. 931 
roots, like that of the slender horia« 
zontal branches,’is much less succu- 
leat 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 exterd hori. 
z0ntally 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 mois- 
ture to the plant, whilst young, is 
thus deprived of proper nutriment, 
and, ceasing almost wholly to grow, 
becomes of no importance to the 
tree. The tap root of the oak, about 
which so much has been written, 
will possibly be adduced as an exe 
ception ; but having attentively exa- 
mined at least 20,000 trees of this 
species, many of which had grown 
in some of the deepest and most fa- 
vourable soils of England, and never 
having found a single tree possessing 
a tap root, I must be allowéd to 
doubt that one ever existed.» | 
As trees possess the power to 
turn the upper surfaces of their 
leaves, and the points of their shoots 
to the light, and their tendrils ia any 
direction tu attach themselves to 
contiguous objects, it may be sus- 
pected that their lateral roots are by 
some means directed to any soil in 
their vicinity, which is best calcue 
lated 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 
302 has 
