As we have seen, root and foliage are the main life organs of the tree.
The trunk and branches serve to carry the crown upward and expose it to
the light, which is necessary in order to prepare the food and increase
the volume of the tree, and also as conductors of food materials up
and down between root and foliage. A large part of the roots, too,
aside from giving stability to the tree, serve only as conductors of
water and food material; only the youngest parts, the fibrous roots,
beset with innumerable fine hairs, serve to take up the water and
minerals from the soil. These fine roots, root hairs, and young parts
are therefore the essential portion of the root system. A tree may
have a fine, vigorous-looking root system, yet if the young parts and
fibrous roots are cut off or allowed to dry out, which they readily
do--some kinds more so than others--thereby losing their power to take
up water, such a tree is apt to die. Under very favorable moisture
and temperature conditions, however, the old roots may throw out now
sprouts and replace the fibrous roots. Some species, like the willows,
poplars, locusts, and others, are especially capable of doing so.
All trees that "transplant easily" probably possess this capacity of
renewing the fibrous roots readily, or else are less subject to drying
out. But it may be stated as a probable fact that most transplanted
trees which die soon after the planting do so because the fibrous roots
have been curtailed too much in taking up, or else have been allowed to
dry out on the way from the nursery or forest to the place of planting;
they were really dead before being set. Conifers--pines, spruces,
etc.--are especially sensitive; maples, oaks, catalpas, and apples
will, in this respect, stand a good deal of abuse.
Hence, in transplanting, the first and foremost care of the forest,
grower, besides taking the sapling up with least injury, is the proper
protection of its root fibers against drying out.
The water, with the minerals in solution, is taken up by the roots
when the soil is warm enough, but to enable the roots to act they must
be closely packed with the soil. It is conveyed mostly through the
outer, which are the younger, layers of the wood of root, trunk, and
branches to the leaves. Here, as we have seen, under the influence of
light and heat it is in large part transpired and in part combined
with the carbon into organic compounds, sugar, etc., which serve as
food materials. These travel from the leaf into the branchlet, and
down through the outer layers of the trunk to the very tips of the
root, forming new wood all the way, new buds, which lengthen into
shoots, leaves, and flowers, and also new rootlets. To live and grow,
therefore, the roots need the food elaborated in the leaves, just as
the leaves need the water sent up from the roots.
Hence the interdependence of root system and crown, which must be kept
in proportion when transplanting. At least, the root system must be
sufficient to supply the needs of the crown.
"SAP UP AND SAP DOWN."
The growing tree, in all its parts, is more or less saturated with
water, and as the leaves, under the influence of sun and wind and
atmospheric conditions generally transpire, new supplies are taken in
through the roots and conveyed to the crown. This movement takes place
even in winter, in a slight degree, to supply the loss of water by
evaporation from the branches. In the growing season it is so active
as to become noticeable; hence the saying that the sap is "up," or
"rising," and when, toward the end of the season, the movement becomes
less, the sap is said to be "down." But this movement of water is
always upward; hence the notion that there is a stream upward at one
season and in one part of the tree, and a stream downward at another
season and perhaps in another part of the tree, is erroneous. The
downward movement is of food materials, and the two movements of water
upward and food downward take place simultaneously, and depend, in
part at least, one upon the other, the food being carried to the young
parts, wherever required, by a process of diffusion from cell to cell
known as "osmosis."
[Illustration: Fig. 1.--Physiological importance of different parts of
the tree; pathways of water and food materials. (Schematic.)]
These food materials are, by the life processes of the active cells,
changed in chemical composition as need be, from sugar, which is
soluble, into starch, which is insoluble, and back into sugar, and
combined with nitrogenous substances to make the cell-forming material,
protoplasm (fig. 1).
In the fall, when the leaves cease to elaborate food, both the upward
and the downward movement, more or less simultaneously, come to rest
(the surplus of food materials, as starch, and sometimes as sugar,
being stored for the winter in certain cell tissues), to begin again
simultaneously when in spring the temperature is high enough to
reawaken activity, when the stored food of last year is dissolved
and started on its voyage. The exact manner in which this movement of
water upward and food materials downward takes place, and the forces
at work, are not yet fully understood, nor is there absolute certainty
as to the parts of the tree in which the movement takes place. It
appears, however, that while all the so-called "sapwood" is capable
of conducting water (the heartwood is probably not), the most active
movement of both water and food materials takes place in the cambium
(the growing cells immediately beneath the bark) and youngest parts of
the bark.
The deductions from these processes important to the planter are:
That injury to the living bark or bast means injury to growth, if not
destruction to life; that during the period of vegetation transplanting
can be done only with great caution; that the best time to move trees
is in the fall, when the leaves have dropped and the movement of
water and food materials has mostly ceased, or in spring, before the
movement begins again, the winter being objectionable only because of
the difficulty of working the soil and of keeping the roots protected
against frost. All things considered, spring planting, before activity
in the tree has begun, is the best, although it is not impossible to
plant at other times.