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INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



1928, 1932). n As in other members of the family, the nucellus is 

 reduced to a single layer of cells surrounding the megaspore mother 

 cell (Fig. 193 A). The latter gives rise to three or four daughter 

 cells of which the chalazal functions and proceeds to form the em- 

 bryo sac (Fig. 19SB-E). Its development becomes arrested at the 

 four-nucleate stage, but meanwhile one or two cells of the nucellar 

 epidermis show a considerable increase in size and begin dividing to 

 give rise to adventive embryos which are very close to the apex of 



E 

 Fig. 193. Adventive embryony in Nigritella nigra. A, l.s. young nucellus, shov- 

 ing megaspore mother cell. B, older stage, showing micropylar dyad cell in course 

 of degeneration. C, functioning megaspore with remains of degenerating mega- 

 spores; note enlargement of two cells of nucellar epidermis. D, two-nucleate 

 embryo sac with young adventive embryos arising from cells of nucellar epidermis. 

 E, large nucellar embryo lying at apex of four-nucleate embryo sac. (After Afzelius, 

 1928.) 



the embryo sac and are enclosed by the integuments (Fig. 193B-E). 

 Zeuxine sulcata (Swamy, 1946) is very similar except that here, 

 owing to disturbed meiosis, there is no regular megaspore formation 

 and if any embryo sacs are produced they abort at a very early 

 stage. As in Nigritella, the cells of the nucellar epidermis possess a 

 remarkable capacity for growth and differentiation. One or two 

 of them elongate considerably to give rise to filamentous proem- 

 bryos (often four-celled and therefore looking like megaspore 



12 It is interesting to note that those forms of this species which occur in the 

 Alps show normal sexual reproduction (see Gustafsson, 1947a). 



