as large at the top as at the base. As dicotyledonous plants grow, the stems become thicker each year, for the delicate active cambium layer forms new cells from early spring until midsummer or autumn, adding to the wood within and to the bark without. As the growth in spring is very rapid, the first wood-cells formed are much larger than the last wood-cells formed by the slow growth of the late season, and the spring wood is less dense and lighter colored than the summer wood; hence the time between two years' growth is readily made out (Figs. 77 and 78). Because of the rapid growth of the cambium in spring and its consequent soft walls and fluid contents, the bark of trees "peels" readily at that season.
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Fig. 77.—White Pine Stem, 5 years old. The outermost layer is bark.
Medullary Rays.—The first year's growth in dicotyledons forms a woody ring which almost incloses the pith, and this is left as a small cylinder which does not grow
