Explanation

In both gymnosperms and angiosperms, the megaspore mother cell undergoes meiosis and produces four haploid megaspore. Out of four megaspore three will degenerate. Therefore, for formation of 64 zygotes in gymnosperm and angiosperm 64 meiosis in megaspore mother cell will required. Whereas the microspore mother cell in both gymnosperm and angiosperm undergoes meiosis and produced four haploid microspore. All the four will be functional therefore, for formation of 64 zygotes, 16 meiotic division in microspore mother cell will be required.