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"The range of percent composition of unimbibed seed weight is given to include values reported in the literature. Modified from Trelease and Domatl (1984).

"The range of percent composition of unimbibed seed weight is given to include values reported in the literature. Modified from Trelease and Domatl (1984).

cell division preceded cell elongation in embryo growth in Japanese black pine seeds (Goo, 1952). In cherry laurel, however, cell division and cell elongation began more or less simultaneously in embryonic organs (Pollock and Olney, 1959). Reserve foods in the seed sustain the growing embryo until the cotyledons and/or leaves expand to provide a photo-synthetic system and roots develop to absorb water and minerals, thereby making the young plant physiologically self-sufficient. Seedlings of English walnut depended on reserve carbohydrates for respiratory substrates and growth for the first 21 days after the seeds were sown (Maillard et al., 1994). Photosynthesis was confirmed by day 22. At day 29 current photosynthetic products were used for 25 and 30% of the respiration of the root and shoot, respectively. Current photosynthate was incorporated into the shoot beginning on day 32 and into the taproot after 40 days. After 43 days the contribution of reserve carbohydrates to plant growth were negligible.

As the embryo resumes growth during seed germination, the radicle elongates and penetrates the soil. In some woody plants—including most gymnosperms, beech, dogwood, black locust, ash, and most species of maple—the cotyledons are pushed out of the ground by the elongating hypocotyl (epigeous germination). In other species—including oak, walnut, buckeye, and rubber—the cotyledons remain underground while the epicotyl grows upward and develops foliage leaves (hypogeous germination) (Figs. 2.3 and 2.4).

Whereas all embryo cells divide during early seed germination, as seedlings develop the division of cells becomes localized in shoot and root apices. Important events following seed germination include sequential formation of leaves, nodes, and internodes from apical meristems. Shoots may originate from apical meristems in leaf axils, providing the young plant with a system of branches. The root apical meristem forms a taproot or primary root. Often branch roots or secondary roots originate at new apical meristems in the pericycle of the taproot.

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