THE FEMALE GAMETOPHYTE 103 



to her account, the megaspore mother cell divides to form two 

 dyad cells, of which the lower develops normally up to the 4-nu- 

 cleate stage. One of the micropylar nuclei is now said to form the 

 egg and the other a synergid; the second synergid is formed by one 

 of the chalazal nuclei which migrates upward; and the remaining 

 nucleus functions as the single polar. At the time of fertilization, 

 one of the synergid nuclei is said to become displaced by the in- 

 coming pollen tube and forced, as it were, to take part in triple 

 fusion. Owing to its unique and distinctive nature, this mode of 

 development was designated as the "Cypripedium type." 



The reinvestigations made by Prosina (1930), Francini (1931), 

 Carlson (1945), and Swamy (1945) have, however, shown that the 

 development does not end at the 4-nucleate stage but continues 

 further. Occasionally all 8 nuclei may be formed, but in any case 

 at least the 2 micropylar nuclei go through the next division, so 

 that the embryo sacs are 6 nucleate. 



The ovules and embryo sacs of the Podostemonaceae show several 

 interesting features to which a brief reference may be made here, 

 using Podostemon ceratophyllum (Hammond, 1937) as an example. 

 The outer integument appears first and forms the micropyle (Fig. 

 67 A). The megaspore mother cell (Fig. 67B,C), which is situated 

 directly below the epidermis, divides to form the two dyad cells 

 (Fig. 67D), of which the micropylar soon aborts although its nucleus 

 may occasionally divide (Fig. &7E). The nucleus of the chalazal 

 dyad cell divides to form 2 nuclei (Fig. 67F-H), of which the lower 

 promptly degenerates and disappears (Fig. 67 G). The remaining 

 nucleus undergoes two divisions, resulting in 4 nuclei (Fig. 677), 

 which organize to form two synergids, an egg, and a polar nucleus. 

 Occasionally the primary chalazal nucleus persists up to this stage 

 so that the 5-nucleate nature of the embryo sac is easily recognized. 

 More commonly, however, only 4 nuclei are seen and the fifth is 

 no longer recognizable at this stage (see also Razi, 1949). 



The following members of the Podostemonaceae are reported to 

 have tetranucleate embryo sacs: Podostemon subulatus, Hydrobium 

 (= Zeylanidium) olivaceum, Farmeria metzgerioides (Magnus, 1913), 

 and Weddelina squamulosa (Chiarugi, 1933). Here the lower dyad 

 cell is said to undergo only two divisions, resulting in 4 nuclei 

 which organize into the egg apparatus and a single polar nucleus 

 (Fig. Q7J-K, QSA-F). This type of development, sometimes called 



