=745. Annual rings in woody stems.=—The wood produced by the cambium each season is not homogeneous throughout, but is usually much denser toward the outer part of the yearly cylinder, wood fibers here predominating. In the inner portion vessels predominate, giving a much more porous effect. The transition from one year’s growth to another is very abrupt, giving rise to the appearance of rings in cross-section. Since ordinarily in temperate climates but one cylinder of wood is added each year, the number of rings will indicate the age of the trunk or branch. This is not absolutely accurate, since in some trees under certain conditions more than one ring may be produced in a summer. The porous part of the ring is often termed “spring wood,” and the denser portion “fall wood,” but since growth from the cambium ceases in most trees by the middle of July, “summer wood” would be more appropriate for the latter. It is mainly the alternation of the cylinders of the spring and summer wood that gives the characteristic grain to lumber. Pith-rays play an important part in wood graining only in a few woods, as, for instance, in quartered oak. The reason for the production of porous spring wood and dense summer wood is still one of the unsolved problems of botany. FOOTNOTES: [40] Besides these specialized shoots for the storage of food, food substances are stored in ordinary shoots. For example, in the trunks of many trees starch is stored. With the approach of cold weather the starch is converted into oil, in the spring it is converted into starch again, and later as the buds begin to grow the starch is converted into glucose to be used for food. In many other trees, on the other hand, the starch changes to sugar on the approach of winter. [41] This topic was prepared by Dr. K. M. Wiegand. [42] See discussion of Tropophytes in Chapter XLVI. [43] Chapter V, and Organization of Tissues in Chapter XXXVIII. CHAPTER XL. FOLIAGE LEAVES. I. General Form and Arrangement of Leaves.