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Root Growth: root growth System.—The root growth system of a tree functions as an anchoring and absorbing complex. In some species (shortleaf pine) the root growth system comprises a large, vertically oriented unit with lateral members. This arrangement is termed a taproot growth. Alternatively, species such as yellow birch and sugar maple are characterized by diffusely spreading root growth systems, shallowly disposed, and with no single main root growth. The internal structure of large root growths is similar to that of the bole.
Tree Growth.—A tree grows longitudinally through the activity of a permanently embryonic (meristematic) Tissue situated at the apex of each root growth and shoot. This embryonic activity comprises three overlapping phases: cell division, cell elongation, and cell maturation. The combined effect of this activity is an increase in the length of root growth and shoot.
When all of the coats of seeds of some rosaceous forms, such as peach, apple, and hawthorn, are removed, a certain percentage of the embryos, which require cold stratification for normal development, will grow at greenhouse temperatures to form dwarf plants. In such physiological dwarfs, normal growth is readily initiated after exposure of the seedling to a cold period.
Finally, an extreme type of dormancy is shown by seeds of such forms as Convallaria, Smilacina, and Trillium. These require pretreatment at low temperature to break the dormancy of the root growth, a period at high temperature to permit the root growth to grow, another period at low temperature to break epicotyl dormancy, and a second period at high temperature to permit the growth of the afterripened epicotyl to form a green shoot.
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