Pollen Development in the Apple, with Special Reference to Chromosome Behavior

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Abstract
Pollen development is studied in Delicious and Stayman Winesap varieties. The haploid number of chromosomes in Delicious is 14. Pollen development in Delicious is normal, and as pointed out in the literature review this variety is characterized by a high percentage of pollen germination in artificial media, and is very successful in cross fertilization with other cultivated apple varieties. On the other hand, abnormal pollen development is found in Stayman Winesap, a variety noted for self-, inter-, and cross-sterility, and for its low percentage of pollen germination. The chromosomes in the pollen mother cells of Stayman Winesap occur as bivalents and univalents, and are paired and distributed irregularly. During meiotic divisions some of the chromosomes lag on the spindle and are extruded into the cytoplasm, where they may become organized as small supernumerary nuclei. This irregular behavior leads to polyspory and polycary, and is reflected in the abnormal size and shape of the pollen grains, resulting also in a high degree of pollen sterility. Measurements and grouping into size classes of pollen grains mounted in lactic acid indicate that Paragon and Stayman Winesap differ from Delicious in the greater frequency with which small and large sized grains occur, and in the smaller number of uniform medium sized grains.

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