298 MORPHOLOGY OF THE ANGIOSPERMS 



The one bisporic type is called the Alliutn type. Here, as in the 

 Oenothera type, the egg is formed at the fourth division. After the first 

 division, one of the dyads degenerates; the other dyad, by successive 

 divisions, forms eight nuclei. The mature sac has eight nuclei, which 

 are arranged as in the Polygonum type. Morphologically, the sac is 

 formed by two spores. 



Seven tetrasporic types are now known. As the name implies, in these 

 seven types, the protoplasts of all four spores formed by the megaspore 

 mother cell build together the embryo sac. The gametophyte is, morpho- 

 logically, a quadruple structure. 



In three of the tetrasporic types, 16 nuclei are formed at the second 

 division from the spore (the fourdi division from the mother cell). 

 Differences in the arrangement of the 16 nuclei and in the function of 

 some of them distinguish the three types. In the Feperomia type, the 

 egg and one synergid lie at the micropylar end of the sac; eight nuclei 

 form a central cluster; six nuclei lie at the periphery of the lower part 

 of the sac. In the Fenaea type, clusters of three nuclei lie at each end 

 and medianly on opposite sides of the sac, and the remaining four 

 form a central cluster. In the Drusa type, three nuclei, those of the egg 

 and two synergids, lie at the micropylar end of tlie sac, two in the 

 center, and the other eleven at the base. 



The fourth of the tetrasporic types in which the egg is formed at the 

 fourth division, the FritiUaria type, has an 8-nucleate sac, in contrast 

 with the 16-nucleate of the other three types. The formation of the 

 mature sac is complicated by the union of three of the four spore 

 nuclei in the base of the young sac in a multiple-spindle division, which 

 forms two triploid nuclei. The four nuclei now present in the sac divide, 

 and the eight nuclei become arranged as in the Polygonum type, but 

 the antipodal nuclei and one of the polar nuclei are triploid. 



In the fifth of the tetrasporic types — the Flumbagella type — the 

 embryo sac is 4-nucleate; the four nuclei, formed at the second division 

 of the megaspore mother cell, represent the nuclei of the four spores. 

 Three of the four spore nuclei unite in a triple-fusion division in 

 the base of the sac, as in the FritiUaria type, forming two basal triploid 

 nuclei. The mature sac, however, differs from that of the FritiUaria type, 

 in that the egg is alone at the micropylar end — there are no synergids — 

 and one of the two central polar nuclei and the solitary antipodal are 

 triploid. The last two tetrasporic, 8-nucleate types differ in the arrange- 

 ment of the nuclei in the sac. In the Fhimbago type, the nuclei of the 

 four spores become isolated around the periphery of the sac — one at each 

 end, the other two near the center. Division of the spore nuclei gives an 

 8-nucleate sac — and nucleus at each end, two along the sides, and four 

 in a central cluster. The Adoxa type has the nuclei arranged as in the 



