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VEGETABLE PHYSIOLOGY. 
external cause, and not by any power inherent 
in vegetable life; and I see little reason to doubt 
that gravitation is the principal, if not the only, 
agent employed in this case by nature. I shall, 
therefore, endeavour to point out the means by 
which I conceive the same agent may produce 
effects so diametrically opposite to each other.— 
The radicle of a germinating seed (as many 
naturalists have observed) is increased in length 
only by new parts successively added to its apex 
or point, and not at all by any general extension 
of parts already formed; and the new matter 
which is thus successively added, unquestionably 
descends in a fluid state from the cotyledons. 
On this fluid, and on the vegetable fibres and 
vessels while soft and flexible, and whilst the 
matter which composes them is changing from a 
fluid to a solid state, gravitation would, I con- 
ceive, operate sufficiently to give an inclination 
downwards to the point of the radicle; and as 
the radicle has been proved to be obedient to 
centrifugal force, it can scarcely be contended 
that its direction would remain uninfluenced by 
gravitation. I have stated that the radicle is 
increased in length by parts successively added 
to its point; the germen, on the contrary, elon- 
gates by a general extension of its parts pre- 
viously organised, and its vessels and fibres ap- 
pear to extend themselves in proportion to the 
quantity of nutriment they receive. If the mo- 
tion and consequent distribution of the true sap 
be influenced by gravitation, it follows that 
when the germen, at its first emission or subse- 
quently, deviates from a perpendicular direction, 
the sap must accumulate on its under side; and 
I have found, in a great variety of experiments 
on the seeds of the horse-chestnut, the bean, and 
other plants, when vegetating at rest, that the 
vessels and fibres on the under side of the germen 
invariably elongate much more rapidly than 
those on the upper side; and hence it follows 
that the points of the germen must always turn 
upwards. And it has been proved that a similar 
increase of growth takes place on the external 
side of the germen, when the sap is impelled there 
by centrifugal force, as it is attracted by gravi- 
tation to its under side when the seed germinates 
at rest. This increased elongation of the fibres 
and vessels of the under side is not confined to 
the germens, nor even to the annual shoots of 
trees, but occurs and produces the most extensive 
effects in the subsequent growth of their trunks 
and branches. The immediate effect of gravita- 
tion is certainly to occasion the further depression 
of every branch which extends horizontally from 
the trunk of the tree, and when a young tree in- 
clines to either side, to increase that inclination ; 
but it at the same time attracts the sap to the 
under side, and this occasions an increased longi- 
tudinal extension of the substance of the new 
wood on that side. The depression of the lateral 
branch is thus prevented, and it is even enabled 
to raise itself above its natural level when the 
branches above it are removed; and the young 
tree, by the same means, becomes more upright, 
in direct opposition to the immediate action 
of gravitation, nature, as usual, executing the 
most important operations by the most simple 
means.—It has, however, been objected by Du- 
hamel (and the greatest deference is always 
due to his opinions) that gravitation could 
have little influence on the direction of the 
germen were it, in the first instance, protruded, 
or were it subsequently inverted, and made to 
point perpendicularly downwards. ‘To enable 
myself to answer this objection I made many 
experiments on trees of the horse-chestnut and 
of the bean, in the box I have already described ; 
and as the seeds there were suspended out of the 
earth, I could regularly watch the progress of 
every effort made by the radicle and germen 
to change their positions, The extremity of the 
radicle of the bean, when made to point perpen- 
dicularly upwards, generally formed a consider- 
able curvature within three or four hours when 
the weather was warm, The germen was more 
sluggish; but it rarely or never failed to change 
its direction in the course of twenty-four hours ; 
and all my efforts to make it grow downwards 
by slightly changing its direction were invariably 
abortive. — Another, and apparently a more 
weighty objection to the preceding hypothesis 
(if applied to the subsequent growth and form of 
trees) arises from the facts that few of their 
branches rise perpendicularly upwards, and that 
their roots always spread horizontally; but this 
objection may, I think, be readily answered. 
The luxuriant shoots of trees, in whatever di- 
rection they are first protruded, almost uniform- 
ly turn upwards, and endeavour to acquire a 
perpendicular direction ; and to this their points 
will immediately return if they are bent down- 
wards during any period of their growth, their 
curvature upwards 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 amply supplied 
with sap, they are less affected by gravitation. 
Their points, however, generally show an incli- 
nation 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 cotyle- 
dons and leaves, and occasions a continued in- 
crease of the growth of the upper parts of the 
radicle; and this growth is subsequently aug- 
mented by the effects of motion, when the ger- 
men has risen above the ground. The true sap 
is, therefore, necessarily obstructed in its descent, 
numerous lateral roots are generated, into which 
