THE FEMALE GAMETOPHYTE 85 



illustrated account of the development of a monosporic 8-nucleate 

 embryo sac. 



In certain other monosporic embryo sacs, the megaspore nucleus 

 undergoes only two divisions and a micropylar quartet alone is 

 formed. This quartet gives rise to a normal egg apparatus and a 

 single polar nucleus. The lower polar nucleus and antipodal nuclei 

 are absent. This type of development is known as the "Oenothera 

 type" and has so far been reported only in the family Onagraceae. 



The bisporic embryo sacs are typically 8-nucleate ("Allium type") 

 and arise from one of the two dyad cells formed after Meiosis I. 

 Since no wall is laid down after Meiosis II and both the megaspore 

 nuclei formed in the functional dyad cell take part in the develop- 

 ment of the embryo sac, only two further divisions are necessary to 

 give rise to the 8-nucleate stage. A doubtful 4-nucleate type ("Po- 

 dostemon type") has been reported in a few members of the Podo- 

 stemonaceae but this is questionable and will not receive detailed 

 consideration. 



The tctrasporic embryo sacs present a great deal of variation. In 

 several cases 16 nuclei are formed as the result of two divisions fol- 

 lowing megasporogenesis. These are classified under the following 

 types, depending on the polarity and organization of the nuclei in 

 the sac: "Peperomia type," "Penaea type," "Drusa type." 



In some plants, owing to a crowding of 3 of the megaspore nuclei 

 into the chalazal end of the cell (1 + 3 arrangement), there is a 

 fusion of their spindles in the next division, resulting in a secondary 

 4-nucleate stage with 2 haploicl nuclei at the micropylar end and 

 2 triploid ones at the chalazal. The next division results in 8 nuclei, 

 4 of which are haploid and 4 triploid. This mode of development 

 is known as the "Fritillaria type." 



The "Plumbagella type," reported only in Plumbagella micrantha, 

 is similar to the Fritillaria type, except that here the development 

 stops at the secondary 4-nucleate stage, which is at once followed 

 by the organization of the embryo sac. 



Finally, there are the "Adoxa" and "Plumbago" types, in both 

 of which the 4 megaspore nuclei divide once to give rise to 8 nuclei. 

 In Adoxa, however, the organization is bipolar and in Plumbago it is 

 tetrapolar. 



All the variations of embryo sac development described above 

 are shown diagrammatically in Fig. 55. 



