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occurs and produces the most extensive effects in the subsequent growth of 
their trunks and branches. The immediate effect of gravitation is certainly 
to occasion the further depression of every branch which extends horizon¬ 
tally from the trunk of the tree; and, when a young tree inclines to either 
side, to increase that inclination: but it at the same time attracts the sap to 
the under side, and thus occasions an increased longitudinal 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 im¬ 
mediate action of gravitation: Nature, as usual, executing the most impor¬ 
tant operations by the most simple means. 
I could adduce many more facts in support of the preceding deductions, 
but those I have stated 1 conceive to be sufficiently conclusive. 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 direc¬ 
tion of the plume, were it in the first instance protruded, or were it sub¬ 
sequently inverted, and made to point perpendicularly downwards. To 
enable myself to answer this objection, I made many experiments on seeds 
of the horse chesnut and of the bean, in the box 1 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 plume to change 
their positions. The extremity of the radicle of the bean, when made to 
point perpendicularly upwards, generally formed a considerable curvature 
within three or four hours, when the weather was warm. The plume 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 forms of trees,) arises 
from the facts that few of their branches rise perpendicularly upwards, and 
that their roots always spread horizontally; but this objection I think may 
be readily answered. 
The luxuriant shoots of trees, which abound in sap, in whatever direc- 
This effect does not appear to be produced in what are called weeping trees; the cause of 
which 1 have endeavoured to point out in a former Memoir. Phil. Trans. 1804. 
