i4o8 A TEXTBOOK OF THEORETICAL BOTANY 



mature sac has the normal three nuclei at the micropylar end and eleven 

 antipodals. 



Drusa opposittfolia, the type species, is a member of the Umbelliferae, 

 but the same conditions in the embryo sac have been found also in some 

 species of Riihia, Crucianella, Maianthemum, Chrysanthemum and Ulmus, 

 among others. Irregularities in number may occur through the failure of 

 certain nuclei to divide. 



9. Tetrasporic, sixteen-nucleate sacs. The Penaea type. The four mega- 

 spore nuclei undergo two successive divisions and the sixteen nuclei formed 

 arrange themselves in four groups of four, one at each end of the sac and 

 one at each side. One nucleus from each group moves to the centre, so that 

 there are four polar nuclei. In this and some related cases where a number 

 of polar nuclei exist, their fusion and fertilization produce a highly polyploid 

 primary endosperm nucleus, which seems frequently to be associated with 

 subsequent abortion of endosperm formation. 



The remaining three nuclei in each group are isolated by membranes 

 and form three groups like the normal micropylar group. Only the latter, 

 however, seems to be functional in fertilization. 



This type is found in a number of members of the small family Penaea- 

 ceae (Myrtales) and the same or closely similar conditions have been des- 

 cribed from several of the Malpighiaceae and Euphorbiaceae. 



10. Tetrasporic, sixteen-nucleate sacs. The Peperomia type. The four 

 megaspore nuclei undergo two successive divisions and the sixteen resulting 

 nuclei are at first distributed all over the sac. Generally two form the micro- 

 pylar group, one as the oosphere and the other as one large synergid. Eight 

 nuclei fuse in the centre of the sac, as polar nuclei, and the remaining six 

 are cut off singly by membranes all round the chalazal part of the sac and 

 may be regarded as antipodals. There are variations, however, even in the 

 type species, P. pelhicida, and more than one strain may exist in the species, 

 for in some cases the sac is pear-shaped instead of spherical, and these sacs 

 all seem to have two synergidae. In P. hispidula no fewer than fourteen 

 of the sixteen nuclei are involved in fusion at the centre of the sac, and there 

 are no antipodals. 



Apart from Peperotnia (Piperaceae), the same type of sac is only known 

 in Gunnera, a peculiar genus of Haloragidaceae, with a southern distribution. 



The telescoping of the two processes of sporogenesis and gameto- 

 genesis in the ovule has gone further than in the microsporangia of the 

 anther, where spores are always formed as distinct structures, and it illus- 

 trates how far the reduction of the gametophyte has gone in Angiosperms. 

 The end of one generation flows, as it were, directly into the beginning of 

 the next and is marked by nothing but the nuclear change in meiosis. Among 

 the Gnetales we may find approaches towards the same condition, but falling 

 short of it, and, indeed, the only parallel is the remote one of Fiicus. 



Apospory as known in the Filicales is not a true parallel, since in this 

 process there is no meiosis and certain somatic cells of the sporophyte 

 develop directly into a diploid prothallus. Apospory of a similar kind is 



