VEGETABLE PHYSIOLOGY : COMPOUND ORGANS. 
33 
and the bark of all plants., yet the greater part of their food is 
as certainly taken up by the roots. They do not, however, 
absorb nutriment by their whole surface, but it is to the spong- 
ioles almost exclusively that this office is confided; and hence 
arises the importance of preserving them in the process of 
transplantation. Roots serve also as an anchor, to fix plants in 
their respective situations. 
It is generally believed that roots increase only by their 
extremities, and that, once formed, they do not undergo any 
subsequent elongation. Many experiments have been made on 
this point by Duhamel, Knight, and others, which tend to 
prove this; and we hence gain a beautiful view of the manner 
in which they are enabled to penetrate the crevices of rocks, by 
the continual formation of new matter at the advancing point: 
when once this is insinuated the force of horizontal expansion 
speedily enlarges the cavity, and, if any obstacle arrest their 
progress, they simply stop, cease to elongate in that particular 
direction, and follow the surface of the opposing matter until 
they reach a suitable medium. 
Besides taking up food and transmitting it to the leaves, it is 
commonly believed that the roots also perform the office of 
returning a certain portion from the system, which are termed 
the excrementitious rejections of vegetables. It appears to 
consist of certain peculiar secretions of the plants, and varies 
greatly in different plants, and in accordance also with the 
quality and nature of the food with which they have been sup¬ 
plied. Although a disputed doctrine, yet the experiments 
which have been instituted to prove the existence of these 
secretions appear to be somewhat satisfactory. I may mention 
that of Macaire, who introduced part of the roots of Mercurialis 
annua into a bottle containing pure water, and the remainder 
into another containing a solution of acetate of lead : in a short 
time the bottle in which the pure water was placed was found 
to have received a notable proportion of the acetate, which 
could have been received from no other source than as being 
given out by the roots placed in that bottle, whilst it was taken 
into the vegetable system by the root in the other bottle. 
The necessity and advantages of rotation of crops is probably 
explained by this doctrine, without the admission of which 
both this and many other important matters would be found to 
be difficult to solve. 
