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Permit The Growth: Cells require a basic supply of raw materials for growth; when any of them is used up, growth stops, regardless of how much of the other nutrients remain. Second, the environment becomes too toxic to permit further growth. Cells pour out waste materials as they respire and synthesize needed components. At low population densities the level of these materials remains insignificant, but as the density increases, the environment becomes so toxic that further growth is impossible, regardless of the food supply.
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, a period at high temperature to permit the root 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.
Lateral growth in trees frequently begins later than growth in height, but the period of lateral growth is longer. Rates of lateral growth likewise are low at the outset, increasing after a few weeks and then diminishing. In view of the long annual period of lateral growth in trees, deleterious environmental conditions may exercise a pronounced effect on growth. In the wood of trees of temperate regions, narrow and wide growth rings may be observed, marking years characterized by unfavorable and favorable conditions for growth. |
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