172 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



result of a degeneration of the generative nucleus and three divisions 

 of the vegetative nucleus. 



De Mol (1923) observed this so-called "Nemec-phenomenon" in 

 the anthers of other varieties of Hyacinthus orientalis which had 

 been subjected to certain special conditions in order to obtain early 

 flowering. He attributed the origin of the abnormality to a dupli- 

 cation of the generative nuclei. 



Stow (1930, 1934) found similar embryo-sac-like pollen grains or 

 "pollen-embryo sacs" in the anthers of a variety called "La Victor" 

 whose bulbs had been subjected to a temperature of 20°C. at the 

 time of meiosis and were further "forced" in a greenhouse. He 

 traced their development more fully than either Nemec or De Mol. 

 At first the microspores increase in size to form large sac-like bodies 

 (Fig. 105 A, B), after which the nucleus undergoes three successive 

 divisions (Fig. 105C-F) to form 8 daughter nuclei. Of these, 3 lie 

 at the end where the exine is still intact, 3 at the opposite end, and 

 2 in the middle. The 6 nuclei at the two poles organize into cells, 

 while the remaining two fuse in the center (Fig. 105(7). Since the 

 three cells at the exine end were found to remain healthy for a much 

 longer time than those at the opposite end, Stow regards the former 

 as corresponding to the egg and synergids, and the latter to the 

 antipodals. In addition certain abnormal pollen-embryo sacs were 

 also seen, showing the following types of organization: (1) 8 nuclei 

 forming an egg, two polars, and five antipodal cells; (2) 4 nuclei 

 forming an egg, two polars, and one antipodal cell; (3) 4 nuclei 

 forming a polar and three antipodal cells but no egg; (4) 16 nuclei 

 forming a 5- to 10-celled egg apparatus, one or two polars, and a 

 few antipodal cells; and (5) more than 16 nuclei without any definite 

 arrangement. 



According to Stow, it is not the divisions of the vegetative or 

 generative nucleus which give rise to the pollen-embryo sacs but 

 those of the microspore nucleus itself. Once the vegetative and 



Fig. 105. Development of pollen-embryo sacs in Hyacinthus orientalis. A, 

 microspore in metaphase of first division. B, microspore showing tendency toward 

 formation of pollen-embryo sac; nucleus is in metaphase. C, second nuclear divi- 

 sion in pollen-embryo sac; on right, young pollen-embryo sac with undivided 

 nucleus. D, four- and two-nucleate pollen-embryo sacs. E, division of four 

 nuclei; metaphase. F, same; anaphase. G, well-developed pollen-embryo sac. 

 (After Stow, 1930.) 



