ACTION OF GRAVITATION ON VEGETATION. 
771 
Darwin's Theory of Descent ; viz. that, under the influence of long-continued natural 
selection, only those structures are finally able to maintain their existence which, 
while sufficient for the other requirements of life, are so arranged that the weight of 
the part is not injurious or is even useful. It must not be inferred from this, nor 
does observation render it probable, that gravitation takes any direct part in these 
phenomena. 
Gravitation however exerts a direct influence on the growth of young parts of 
plants as soon as the longitudinal axis of the growing organ is inclined obliquely 
to the perpendicular and therefore to the action of gravitation. In this case the 
growth in length of the oblique organ is different on the upper and under sides, 
and the more so the more nearly horizontal the axis of growth. According to 
the nature of the organ and its purpose in the economy of the plant, either the 
upper side grows more strongly than the under side, or the reverse. A curvature 
concave either downwards or upwards is thus caused by the influence of gravita- 
tion and growth, and this curvature increases until the free-growing end is directed 
Fig. 477. — A seedling of l-^ia'a Faba, the root and stem of which were straight, which was so fixed that the apex of the 
root lay nearly horizontally upon the surface of the mercury (black in the figure), by means of a pin to the cork k; 1111 is the 
layer of water on the mercury. The figure shows the seedling 24 hours after its fixture ; the root has bent sharply down- 
wards at its growing portion, so that its apex penetrates the mercury ; the resistance experienced by it in so doing is shown 
in the form of the part of the root lying just behind the downward curve. The stem has curved upward from its base. The 
position of the bud is independent of the action of gravitation ; it is a phenomenon of nutation. 
vertically either downwards or upwards ; the former, for example, in primary roots, 
the latter in many primary stems. In lateral branches, leaves, and secondary roots, 
similar phenomena occur, though not so markedly. Internal processes of vegetation, 
the weight of the upper parts, or the influence of light, act in opposition to that of 
gravitation, so that conditions of equilibrium arise which cause the organs to stand 
horizontally or obliquely to the perpendicular. 
Thus the vertical direction of primary roots and stems, and the oblique direction 
of their lateral branches, are determined solely by gravitation, or at any rate to 
some extent, so long as these parts are still growing; when they subsequently 
become lignified or cease to grow, they maintain the position once acquired. If 
therefore a growing plant rooting in the ground (inside a pot) is placed horizon- 
tally, the mature parts remain in this position ; but the apex of the primary root 
turns downwards, and the growing internodes of the end of the stem turn upwards, 
the leaves, branches, and secondary roots also bend until they make about the same 
angle with the horizon that they did before the change in their position. The parts 
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