246
Book IV.
Plants re
mnine lond
Water ne
cessary.
VEGETATION,
we examine this increase more minutely, we find that a
new layer of wood, or rather of alburnum, has been
added to the tree in every part, and this addition has
been made just under the bark. We find, too, that a
layer of alburnum has assumed the appearance of per
fect wood. Besides this addition of vegetable fibre, a
great number of leaves have been produced ; and the
tree puts forth flowers, and forms seeds.
It is evident from all this, that a great deal of new
matter is continually making its appearance in plants.
Hence, since it would be absurd to suppose that they
create new matter, it must follow that they receive it
by some channel or other. Plants, then, require food
as well as animals. Now, what is this food, and whence
do they derive it? These questions can only be an
swered by an attentive survey of the substances which
are contained in vegetables, and an examination of those
substances which are necessary for their vegetation. If
we could succeed cómpletely, it would throw a great
deal of light upon the nature of soils and of manures,
and on some of the most important questions in agricul
ture. But we are far indeed at present from being able
to examine the subject to tie bottom.
1. In the first place, it is certain that plants will not
vegetate without water; for whenever they are deprived
of it, they wither and die. Hence the well-known use
of rains and dews, and the artificial watering of ground.
Water, then, is at least an essential part of the food of
plants. But many plants grow in pure water; and
therefore it may be questioned whether water is not the
only food of plants. This opinion was adopted very
long ago, and numerous experiments have been made in
order to demonstrate it. Indeed, it was the general opi¬