Distribution of the Digested Food.—After being changed to the soluble form, this material is ready to be used in growth, either in the leaf, in the stem, or in the roots. With other more complex products it is then distributed throughout all of the growing parts of the plant; and when passing down to the root, it seems to pass more readily through the inner bark, in plants which have a definite bark. This gradual downward diffusion through the inner bark of materials suitable for growth is the process referred to when the "descent of sap" is mentioned. Starch and other products are often stored in one growing season to be used in the next season. If a tree is constricted or strangled by a wire around its trunk (Fig. 118), the digested food cannot readily pass down and it is stored above the girdle, causing an enlargement.
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Fig. 118.—Trunk Girdled by a Wire. See Fig. 85.
Assimilation.—The food from the air and that from the soil unite in the living tissues. The "sap" that passes upwards from the roots in the growing season is made up largely of the soil water and the salts which have been absorbed in the diluted solutions (p. 67). This upward-moving water is conducted largely through certain tubular canals of the young wood. These cells are never continuous tubes from root to leaf; but the water passes readily from one cell or canal to another in its upward course.
The upward-moving water gradually passes to the growing parts, and everywhere in the living tissues, it is of
