The temporary occurrence of the callose plate between the generative and vegetative cells in pollen grains was observed in 82 angiosperm species and 4 gymnosperm species. The period of isolation of the generative cell by callose, determined as the #x201C;callose stage#x201D;, includes the early interphase of the generative nucleus. The significance of the #x201C;callose stage#x201D; in the differentiation process of generative cells is discussed. The opinion is expressed that the temporary occurrence of a callose plate may explain why some authors only observe a double plasma membrane separating the generative and vegetative cells of the pollen grain, while others observe a wall-like structure between these membranes.
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