. Elementary botany. Botany. 80 PHYSIOLOGY. elongated slowly at first. The rapidity of the elongation of these cells in- creases until it reaches the maximum. Then the rapidity of elongation les- sens as the cells come to lie farther from the tip. The period of maximum elongation here is the zone of maximum growth of these cells. 192. Just as the cells exhibit a zone of maximum growth, so the members of the plant exhioit a similar zone of maximum growth. In the case of leaves, when they are young the rapidity of growth is comparatively slow, then it increases, and finally diminishes in rapidit
. Elementary botany. Botany. 80 PHYSIOLOGY. elongated slowly at first. The rapidity of the elongation of these cells in- creases until it reaches the maximum. Then the rapidity of elongation les- sens as the cells come to lie farther from the tip. The period of maximum elongation here is the zone of maximum growth of these cells. 192. Just as the cells exhibit a zone of maximum growth, so the members of the plant exhioit a similar zone of maximum growth. In the case of leaves, when they are young the rapidity of growth is comparatively slow, then it increases, and finally diminishes in rapidity again. So it is with the stem. When the plant is young the growth is not so rapid; as it approaches middle age the rapidity of growth increases; then it declines in rapidity at the close of the season. 193. Energy of growth.—Closely related to the zone of maximum growth is what is termed the energy of growth. This is manifested in the compara- tive size of the members of a given plant. To take the sunflower for example, the lower and first leaves are comparatively small. As the plant grows larger the leaves are larger, and this increase in size of the leaves increases up to a maxi- mum period, when the size decreases until we reach the small leaves at the top of the stem. The zone of maximum growth of the leaves corresponds with the maxi- mum size of the leaves on the stem. The rapidity and energy of growth of the stem i- also correlated with that of the leaves, and the zone of maximum growth is coincident with that of the leaves. It would be instructive to note it in the case of other plants and also in the case of fruits. 194. Nutation.—During the growth of the stem all of the cells of a given section of the stem do not elongate simultaneously. For example the cells at a given moment on the south side are elongating more rapidly than the cells on the other side. This will cause the stem to bend slightly to the north. In a few moments later the cells on the west side are
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