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Choline For Growth: It is possible to induce the pneumococcus, which requires choline for growth, to substitute certain analogous compounds for this structural unit. These substitutions markedly impair the ability of the cells to take up transforming DNA, to resist lysis (dissolution) by bile salts, and to separate on cell division. This approach seemed likely to advance our understanding of the functional significance of various aspects of the molecular structure of the cell wall.
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.
Growth Cycles.—Longitudinal and lateral (diametral) growth do not proceed continuously, but rather are cyclical phenomena with periods of growth activity alternating with periods of relative inactivity. One of the best illustrations of the periodic nature of growth in large woody stems is to be found in the annual ring pattern which characterizes the wood of trees of temperate regions. It has been suggested recently that even in the apparently ever-growing trees of the tropical rain forest, cambial growth is not a continuous process.
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