—Sap is the life blood of the tree. In the winter when most of the trees are bare of leaves there is but very little circulation of the sap. The coming of spring with its increase of heat and light, causes the tree to begin to take on new life; that is, the sap begins to circulate. This movement of sap causes the roots to absorb from the soil certain elements such as hydrogen, oxygen, nitrogen and carbon, also mineral salts in solution. The liquid thus absorbed works its way upward, mainly by way of the sapwood and medullary cells. Upon reaching the cambium layer, the nourishment which it provides causes the cells to expand, divide and generate new cells. It also causes the buds to take the form of leaves.
When the sap reaches the leaves a chemical change takes place. This change takes place only in the presence of heat and light, and is caused by the action of a substance called chlorophyll. The importance of the work performed by chlorophyll cannot be overestimated. Nearly all plant life depends upon it to change mineral substance into food. Animals find food in plant life because of this change.
Assimilation is the process of taking up and breaking up, by the leaves, of carbonic acid gas with which the cells containing chlorophyll come in contact. Carbon, one of the elements, is retained, but oxygen, the other element, is returned to the air. Carbon is combined with the oxygen and hydrogen of the water, which came up from the roots, to form new chemical compounds. Nitrogen and earthy parts, which came with the water, are also present.
Chlorophyll gives to leaves and young bark their green color.
The roots of the trees are constantly drinking plant food in the daytime of spring and early summer. From midsummer until the end of summer the amount of moisture taken in is very small so that the flow of sap almost ceases.
The leaves, however, are full of sap which, not being further thinned by the upward flow, becomes thickened thru the addition of carbonic acid gas and the loss of oxygen.
Toward the end of summer this thickened sap sinks to the under side of the leaf and gradually flows out of the leaf and down thru the bast of the branch and trunk, where another process of digestion takes place. One part of this descending sap which has been partly digested in the leaves and partly in the living tissues of root, trunk and branch, spreads over the wood formed in the spring and forms the summer wood. The second part is changed to bark. What is not used at once is stored until needed.
The leaves upon losing their sap change color, wither and drop off. By the end of autumn the downward flow of changed sap from the leaves is completed and the tree has prepared itself for the coming winter.
It must be remembered that the foregoing changes are made gradually. After the first movement of the sap in the early spring has nourished the buds into leaves of a size sufficient to perform work, there begins a downward movement of food materials—slight at first, to be sure, but ever increasing in volume until the leaves are doing full duty. We may say, therefore, that the upward movement of the sap thru the sapwood and the downward flow of food materials thru the bast takes place at the same time, their changes being of relative volumes rather than of time.