Cell Division and Expansion in the Growth of the Shoot Apex

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Abstract
The number and the total volume of cells produced in small portions of the apical region of the shoot have been measured at different stages of development. In non-vernalized seedlings of winter rye, grown in July, the cells in the portions examined divided about once every 1.8 days during the first week of growth, and there was about a 2.2-fold increase in the volume of each cell generation. Four weeks later the apices were still vegetative, but the rate of division in the portions then examined had fallen to once every 5.8 days and there was slightly less than a 2.2-fold increase in the volume of each generation. The average cell volume decreased as the apices developed. In vernalized seedlings more cells were produced in unit time during the first 4 days of growth than in non-vernalized seedlings, but, as in the latter, the rate of production fell during vegetative development. The rate of cell expansion in the vernalized seedlings was probably slightly lower than in the non-vernalized seedlings during the first 4 days of growth and tended to increase during vegetative development. At the time of transition to reproductive growth the cells were dividing about once every 2.0 days and there was about a 2.2-fold increase in the volume of each succeeding generation. After transition the rates increased. From 4 to 8 days after transition the cells divided once every 1.4 days and there was then a 2.4-fold increase in the volume of each succeeding generation. Subsequently, the rate of division changed little but the rate of expansion decreased. Higher rates of division and expansion were found in the apices of lupin seedlings grown in July than in November. In the portions of the meristem examined during the first 5 days of growth the cells divided about once every 1.3 days in July, but only once every 1.8 days in November. The increase in volume of each generation in the same time was slightly higher in July than in November. Both the rates of division and expansion decreased until transition to reproductive growth occurred, and then increased. About 5 days after transition the cells divided once every 1.4 days in July and once every 2.0 days in November. In both experiments there was a 2.4-fold increase in the volume of each generation. Both the rates of division and expansion decreased subsequently. The data are discussed in relation to the changes in size which accompany the development of the shoot apex.

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