Page:The Oak.djvu/37

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19
THE ACORN AND ITS GERMINATION.

spicuous change; and in any case it requires a period of rest before the presence of the oxygen of the air and the moisture of the soil are effective in making it germinate—a fact which suggests that some profound molecular or chemical changes have to be completed in the living substance of the cells before further activity is possible. We have other reasons for believing that this is so, and that, until certain ferments have been prepared in the cells, their protoplasm is unable to make use of the food materials, and consequently unable to initiate the changes necessary for growth.

Sooner or later, however, and usually as the temperature rises in spring, the embryo in the acorn absorbs water and oxygen, and swells, and the little radicle elongates and drives its tip through the ruptured investments at the thin end of the acorn, and at once turns downward, and plunges slowly into the soil (Fig. 3). This peculiarity of turning downward is so marked that it manifests itself no matter in what position the acorn lies, and it is obviously of advantage to the plant that the radicle should thus emerge first, and turn away from the light, and grow as quickly as possible towards the center of the earth, because it thus establishes a first hold on the soil, in readiness to absorb water and dissolve mineral substances by the time the leaves open and require them.

The two cotyledons remain inclosed in the coats of the acorn, and are not lifted up into the air; the developing root obtains its food materials from the stores