. A textbook of botany for colleges and universities ... Botany. 690 ECOLOGY greatest in spring, because the ascent of sap is more active then than later. The new cells take part in this movement, and from the ascend- ing materials derive the substances used in their development. Here, then, as in primary hadrome, maximum growth is correlated with a large movement of materials. Variations in the width of annual rings. — The width of the annual ring is subject to considerable variation, which is dependent in part upon the age of the tree and in part upon seasonal conditions. In early life there
. A textbook of botany for colleges and universities ... Botany. 690 ECOLOGY greatest in spring, because the ascent of sap is more active then than later. The new cells take part in this movement, and from the ascend- ing materials derive the substances used in their development. Here, then, as in primary hadrome, maximum growth is correlated with a large movement of materials. Variations in the width of annual rings. — The width of the annual ring is subject to considerable variation, which is dependent in part upon the age of the tree and in part upon seasonal conditions. In early life there is a / _ period of acceleration, ^i^^^, ^ during which the width of the rings usually in- creases from year to year, probably because of the increased absorptionand food supply which are made possible by a more extensive root system and by a greater ex- panse of foliage. Even if the rings are of equal width year by year, there is an increasing increment of tissue, owing to the enlarging circumference. After some years, which may be few or many accord- ing to the species, there comes a period of maturity, characterized by approximate constancy in the annual increment. Finally, there is a period of retardation, which is marked by an actual decrease in the amount of tissue laid down year by year, the annual increment approaching zero in extreme old age. While new roots and branches develop yearly throughout life, the loss of old branches by death finally the gain, and it is possible also that in an aging tree a given amount of leaf or root surface becomes less effective. In long- lived trees (such as the oak, chestnut, or yew) the final period may not begin for 150 or 200 years, and may continue some hundreds of years Fig. 1019. — A cross section through an annual ring in the secondary wood of the Austrian pine {Pmus Laricio), showing the large-calibered vessels of the spring wood (5) and the small-calibered vessels of the preceding autumn wood (ii); not
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
