18 THE VEGETABLE WORLD. 
rootlets follow an intermediate direction between a vertical one, , 
which would be determined by the power of gravitation, and a 
horizontal one, resulting from centrifugal force. As the move- 
ment communicated to the wheel is increased in rapidity, the 
angle made by the root with the plane of the wheel becomes more 
acute also. When in a line with the wheel, the root is hori- 
zontal, and its direction outside the wheel. 
The influence of gravitation in directing the course of the root 
is put beyond doubt by these curious experiments. 
It must, however, be acknowledged that all is not mechanical 
in this tendency of roots to bury themselves in the earth. There 
exists beyond any doubt a real organic faculty belonging to the 
living plant. 
If we compare a transverse section of the stem, with one cut 
from the root of one of our forest trees, the difference between 
the two parts of the vegetable amounts to very little. The exte- 
rior of the root is covered with a bark, very similar to that on 
the trunk of the tree, only the parenchyma, or cellular tissue, is 
never green in roots. The interior is a woody cylinder, composed 
of fibres, vessels, and medullary rays. Wood, therefore, forms 
the central portion of the root, which is almost always unprovided 
with the kind of vessel known by the name of duct, or trache@.* 
It is chiefly in this last particular that the root varies from the 
stem as regards its structure. When considering the stem in the 
next chapter, we shall get to understand these woody fibres and 
medullary rays. 
Roots ‘increase their growth at their extremities only. These 
extremities are always fresh, and always furnished with porous 
and soft cellular tissues, the fidrilla, and their terminal point the 
spongiala, whose function is the absorption of the liquids or gases 
which are destined to penetrate into the interior of the vegetable. 
This absorption is facilitated and increased by means of the fine 
elongated hair-like fibres attached to all roots. Fig. 20 represents 
the terminal part of a root, as seen in the microscope. The true 
seat of absorption is not situate, as one might suppose, at the 
* One of the many striking analogies which exist in the structure of animals and 
vegetables. The trachex of insects very closely resembles the vegetable spiral duct. 
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