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Summary 1. This paper deals with the effect of pyrazone (10 −1 to 10 −4 mol· l −1 ) on growth and development of shoot apices of three-day-old, seven-day-old, and twenty-one-day-old Beta -seedlings, respectively. 2. A description of the structure and the development of the shoot apices of untreated seedlings is given. The apices have a slightly convex shape, their breadth ranging from 145 to 160 μ m with maximum surface area of the apices, and from 50 to 65 μ m with minimum surface area. The apices consist of no more than two independent layers (L 1 and L 1 ). In the initial zone layer 2 produces cells through periclinal division. Older apices may have a third or even a fourth layer. There are no peculiarities in the development of the leaf primordia. The first three layers have been found to participate in the formation of the primordia. The axillar buds always develop through periclinal division of layer 2. 3. Pyrazone at concentrations of 10 −1 and 10 −2 mol· l −1 inhibited the growth and caused the death of a number of seedlings. The losses were found to be greater during the experiments with younger seedlings. There were no damages at lower concentrations (10 −3 and 10 −4 mol· l −1 ). At the latter concentrations there was even a slight stimulation in growth. 4. After the treatment of seedlings with 10 −1 molar pyrazone the following reactions were observed in the shoot apices: a) an inhibition of cell division and the occurrence of particularly large cells and nuclei, b) a shrinkage of the meristem while the number of the cells remained constant, c) a shift of the zones of growth from the shoot apices to the axillar buds, d) increased periclinal division in the layer 2 of the shoot apex accompanied by a change of its shape. 5. 10 −2 molar pyrazone brought about the following reactions in the shoot apices: a) an increase in the mitotic activity b) an increased periclinal division in the layer 2 c) inhibition in growth and shrinkage d) cell enlargement due to the inhibition of cell division. |
