Fertilization in Cypripedium

Abstract

1. In Cypripedium two cells are formed by the mother cell, a wall not appearing in the second division, even in the rare instances in which the nuclei of both daughter cells divide. Two megaspore nuclei are used in forming the embryo sac, the Lilium type using four, and most angiosperms only one. The two megaspore nuclei in the sac of Cypripedium and the four in that of Lilium may be related to double fertilization. 2. Cypripedium has only four nuclei in the completed embryo sac. The Lilium type and Cypripedium have the fewest divisions from mother cell to egg reported among angiosperms. A comparison with the animal egg shows only one more division from the mother cell to the egg in Lilium and in Cypripedium than in the maturation of the animal egg. 3. So-called double fertilization is probably constant in Cypripedium. The primary endosperm nucleus (triple-fusion nucleus) is formed by the fusion of the polar, one synergid, and one male nucleus. Endosperm of four nuclei was found and the probability of other divisions taking place was noted. 4. In the presynaptic nucleus there is an evident pairing of the threads, probably of paternal and maternal origin. The gametophyte number of chromosomes is eleven, the sporophyte number is twenty-two, and the endosperm number is probably thirty-three. Distinct spirems were often found in all fusing nuclei, forming in one instance before contact. From these examples, the possibility of two spirems always forming in the sporophyte nucleus is suggested; this would secure the independence of the chromatin, and the fusing in synapsis would permit whatever exchange and mingling of parts there may be.