its hidden processes, but it certainly is no less so.
We do not think of the function of the leaves, nor of the bark, nor of
the roots and rootlets, when we gaze upon a noble oak or an elm; we
admire it for its form, its sturdiness, or its grace; it is akin to
ourselves; it is the work of a vast community of cells like those that
build up our own bodies; it is a fountain of living matter rising up out
of the earth and splitting up and spreading out at its top in a spray of
leaves and flowers; and if we could see its hidden processes we should
realize how truly like a fountain it is. While in full leaf a current of
water is constantly flowing through it, and flowing upward against
gravity. This stream of water is truly its life current; it enters at
the rootlets under the ground and escapes at the top through the leaves
by a process called transpiration. All the mineral salts with which the
tree builds up its woody tissues,--its osseous system, so to speak,--the
instruments with which it imprisons and consolidates the carbon which it
obtains from the air, are borne in solution in this stream of water. Its
function is analogous to that of the rivers which bring the produce and
other material to the great cities situated upon their banks. A cloud of
invisible vapor rises from the top of every tree and a thousand
invisible rills enter it through its myriad hairlike rootlets. The trees
are thus conduits in the circuit of the waters from the earth to the
clouds. Our own bodies and the bodies of all living things perform a
similar function. Life cannot go on without water, but water is not a
food; it makes the processes of metabolism possible; assimilation and
elimination go on through its agency. Water and air are the two ties
between the organic and the inorganic. The function of the one is mainly
mechanical, that of the other is mainly chemical.

As the water is drawn in at the roots, it flows out at the top, to which
point it rises by capillary attraction and a process called osmosis.
Neither of them is a strictly vital process, since both are found in the
inorganic world; but they are in the service of what we call a vital
principle. Some physicists and biochemists laugh at the idea of a vital
principle. Huxley thought we might as well talk about the principle of
aqueosity in water. We are the victims of words. The sun does not shoot
out beams or rays, though the eye reports such; but it certainly sends
forth energy; and i