in the trees of all but tropical climates,
which enables the woodman to name with
unerring accuracy the age of the forest tree.

Until the discovery of the cell as the basis
of all vegetation, and the investigation of the
physical laws by which it is governed, the
circulation of the sap was formerly quite
inexplicable. Botanists conceal their ignorance,
by talking learnedly of a mysterious vital
action —words without meaning; —and by
speaking of the ascent of the sap through
certain vessels, and its circulation through the
plant, and descent by other vessels, just
as the blood is circulated in the body of
animals. In plants, this involved a
contradiction of the laws of gravitation, which was
got over by calling it a vital action. We no
longer acknowledge the possibility of any
operation in nature which contravenes the
laws established by nature's great Master.
The life of the Plant-cell is but a fact of the
life of the entire material world, and is
subjected to the same organic laws. The discovery
of the manner in which the cell absorbs its
food, and its relations to heat and light, have
harmonised what had been observed of the
ascent of the sap during spring with the
action of the great physical laws. Look out
from the window this wintry day, and observe
both plants and trees stripped of their leaves,
with nothing but the stems and branches
covered by bark or rind remaining. No
evaporation is taking place, and, consequently,
no absorption; or these processes are carried
on to so very slight an extent, as only to suffice
to preserve the vitality of the last-formed
cells. The plant is hybernating. Its life is
dormant. With spring come light and heat
— the two great agents in the chemical actions
of the cell. Evaporation commences, and with
it the absorption of nutritive fluid; fresh cells
are rapidly formed, to carry on actively the
processes of primary cell-life. Buds sprout
forth, leaves are unfolded and exposed to the
influence of the sun's rays. These act chemically
upon the raw fluid as it passes through
them, and thus the interior cells receive a
more highly elaborated juice —the sap. It is
the passage of this sap through the walls of
the Plant-cells that constitutes the ascent of
the sap, which takes place in spring, for
reasons we can now easily appreciate. The
descent of the sap was a clumsy fiction
intended to complete the old theory of its
circulation. If, after the water has risen,
in the experiment described, to the top
of the sponge, and saturated its walls, and
filled its interstices, we cut off the upper part
and suspend it, the fluid will trickle away—
dropping from the cut end of the sponge. And
if we cut of the part of a branch, of which
the cells are filled with sap, and allow the cut
end to depend, the sap will exude. But is
this a "vital" process either in the sponge or
the twig, or is it not merely an instance of
the ordinary gravitation of fluids?

For the alteration of the raw materials of
the plant into the sap, and their further
conversion, by chemical changes, into the
secretions of the plant, not only heat, but light is
necessary. Heat appears only to act in driving
off the water, depositing the dissolved
substances. Light seems to give rise to the
chemical processes by which these substances
are made to undergo changes which fit them
for the immediate purposes of vegetable and
animal life. If a plant be placed in a dark
cellar, although it may be surrounded with
an atmosphere well supplied with all the
materials of nutrition, it will not be nourished;
for the processes of cell-life will not be carried
on. Carbonic acid will not be decomposed, nor
oxygen given off. The plant will not grow.
But admit the light, and it will grow.
Deprived of a due supply of light, the plant
languishes, and the cell carries on but feebly
all its vital functions; it becomes pale and
colourless, neither developing its colouring
matter, nor any of its special secretions. The
gardener has availed himself of this fact;
and by moderating the supply of light to the
growing parsley or celery, checks the development
of otherwise poisonous secretions. Light
is the great agent by which is effected the
chemical change of the materials of the Plant-cell
into starch, and sugar, and albumen, and
fibrine.

Science has divided the rays of the sun into
blue, red, and yellow, to each of which
different actions are ascribed. To these
influences the term Actinism has been given.
The relations which they have been shown to
hold to the Plant-cell are very simple and
very beautiful. Experimental research has
proved that the blue rays are those most
favourable to germination, the yellow rays
to the production of leaves, and the red
rays to the perfection of the fruits. Further
experiments have shown that, in accordance
with these requirements of the plant, it is
in spring, when germination is taking place,
that the blue rays abound; it is in summer,
when the plant is clothing itself with leaves,
that the yellow rays are most abundant;
and it is in autumn, when the fruit is ripening,
that the red rays predominate.

We must guard ourselves from the absurdity
of supposing that this is ordained with a
special view to the well-being of the plant only.
We see here only one of the innumerable
instances which nature affords of the marvellous
harmony of all the great operations of the
world's forces, unanimously bearing witness
to the omniscience of the Mighty Designer.

Tracing the history of cell-life, we have
seen that the first function of the cell is to
absorb the raw nutritive fluid; the second is
to form out of the sap the peculiar secretions
of the plant. At this stage man enters the
field; he converts the plant to his uses; feeds
on the materials it prepares for him, and thus
builds up the structure of his body; and not
only man, but all the graminivorous division
of the animal world. The number of vegetable