THEORETICAL CONCLUSIONS 413 



and gymnosperms is also monosporic. An additional argument in 

 favor of the primitive nature of the monosporic 8-nucleate type is 

 that all the other types can be easily derived from it while the 

 reverse is almost impossible. The Oenothera type presents no diffi- 

 culty; here only two divisions intervene between the functioning 

 megaspore stage and the differentiation of the egg apparatus, and 

 all the 4 nuclei are restricted to the micropylar part of the embryo 

 sac. In the Allium type, wall formation does not occur after Meio- 

 sis II, and even if it does occur the cell plates soon dissolve, so that 

 each dyad cell (or at least the functional one) contains 2 megaspore 

 nuclei. Only two further divisions are now required to give rise 

 to the 8-nucleate stage. In the tetrasporic types no permanent 

 walls are laid down after any of the meiotic divisions. As a result 

 all the 4 megaspore nuclei lie in a common cavity and may take 

 up varying arrangements, one pair of nuclei lying at the micro- 

 pylar end and the other at the chalazal (2+2), or one nucleus at 

 the micropylar end and three nuclei at the chalazal (1+3), or one 

 nucleus at each end and two at the sides (1 + 1 + 1 + 1). The mega- 

 spore nuclei may undergo two divisions or only one. The 2+2 

 position apparently leads to the Peperomia and Adoxa types; the 

 1+3 position to the Drusa, Fritillaria, and Plumbagella types; 

 and the 1 + 1 + 1 + 1 position to the Penaea and Plumbago types. 

 Assuming then that the monosporic 8-nucleate embryo sac is 

 the fundamental type, there are three principal theories as to its 

 homologies : 



1. The embryo sac of angiosperms is derived from a form like 

 Gnetum in which all the nuclei of the embryo sac possess the same 

 morphological value, and any of them can function as an egg and 

 give rise to an embryo. First put forward by Hofmeister and 

 Strasburger, this view may for convenience be called the Gnetalean 

 theory. 



2. The embryo sac of angiosperms is derived by reduction from 

 the female gametophyte of some gymnosperm and consists of only 

 two archegonia without any prothallial tissue (Fig. 214). Accord- 

 ing to this view the micropylar quartet represents one archegonium 

 (the synergids are equivalent to neck cells and the polar nucleus 

 to the ventral canal nucleus), and the chalazal quartet represents 

 the second but nonfunctional archegonium (Porsch, 1907). 



3. The micropylar quartet represents two archegonia and the 



