1907I PACE— FERTILIZATION IN CYPRIPEDIUM 



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spirem is formed, and tubes less than the length of the pollen grain 

 have the three nuclei in them ; so that the generative nucleus probably 

 divides into the male nuclei as the tube develops. 



It will be remembered that the embryo sac ready for fertilization 

 contains only four nuclei — the egg apparatus in the usual position, 

 and the remaining nucleus near the center of the sac or toward the 

 antipodal end {figs. 29, jo). As the pollen tube penetrates the sac, one 

 of the synergids may be in front of it and a little lower than the other, 

 as if it were being pushed from its usual position by the inrush of the 

 contents of the tube^ or moving for some other reason toward the 

 antipodal end of the sac (fig. 44), The egg nucleus may be already in 

 the spirem stage. The pollen tube here shows the tube nucleus 

 in advance^ with the two male nuclei elongated and near it. A stage 

 just later than this shows two nuclei below the egg, and from the 

 lines of cytoplasm one seems to be the synergid which has moved to 

 that position [fig. 42). The two male nuclei are still somewhat 

 elongated and are in advance of the tube nucleus. Fig. 43 shows 

 the polar nucleus and the synergid near the antipodal end of the sac, 

 and the two male nuclei still elongated. The primary endosperm 

 nucleus is formed in this case by the fusion of a male cell, the solitary 

 polar nucleus, and the migrating synergid. In this fusion, as well 

 as in the fertihzation of the egg, the fusing nuclei often come into 

 contact in the resting condition {f^g. 45), the figure showing the male 

 nucleus in contact with the egg still curved and elongated. Many 

 \aews of this stage might be given with slight variation in the position 

 of the nuclei; and while fusion in the resting condition was apparently 

 the more common in my material, it was not at all uncommon to find 

 spirems already well formed and in different stages of development. 

 The pollen tube was often traced from the ovary wall through the 

 micropyle into the embrj^o sac. The male nuclei are usually elongated 

 and sometimes in advance of the tube nucleus, but the tube nucleus 

 usually precedes them {fig. 46), In no case does the pollen tube pass 

 directly to the micropyle, but bends in various directions* This may 

 be due to the attraction of other o\niles, several of which will be 

 almost as near as the one entered. It may be that after leaving the 

 ovary wall the tube simply wanders in this space until it comes v^iihin 

 the influence of the ovule it enters. . . 



