The mechanics of mitosis in the pollen-tube of Tulipa

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Abstract
Mitoses of the generative nucleus in germinating pollen tubes of the diploid Tulipa cretica, greigi, eichleri, montana, and the polyploid spp. korolkowi and clusiana are characterized by early breakdown of the nuclear membrane at prophase, a protracted "pro-metaphase" in which the nuclei elongate, so that the chromosomes are slow in coming to the metaphase plate. Plates were found in all spp., invariably oblique in the polyploids, oblique or transverse in the diploids. At anaphase, owing to restricted lateral movement, the nuclei may be separated by as much as 30 times their diameter. The amount of available space and the chromosome number determine the arrangement at metaphase in the pollen tube the spindle poles are widely separated, so that the repulsions due to the centromeres and chromosomes are predominant; in the pollen grain, where cell volume in relation to chromosome volume is great, the repulsion of the poles predominates, and the typical hollow spindle is formed; in root tips, and when the generative nucleus divides within the pollen grain, intermediate arrangements occur. From measurements of the chromosome threads and a count of the number of coils in the spiral at metaphase, the diameter of the threads is calculated to be 0.52 [mu], about 1/2 the diameter of the chromatid. Crossing-over was found in a few cases in T. cretica.
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