THE FEMALE GAMETOPHYTE 99 



tomaceae (except Butomus), Alismaceae, and the tribe Viscoideae 

 of the Loranthaceae. It is also found in several members of the 

 Balanophoraceae, Liliaceae, Amaryllidaceae, and Orchidaceae, but 

 in other groups its occurrence is more or less sporadic. 



The chief variation in development is a tendency toward reduc- 

 tion in the number of nuclei at the chalazal end. This has been 

 very clearly demonstrated in the Alismaceae (Dahlgren, 19286, 1934; 

 Johri, 1935a,6,c, 1936a; Maheshwari and Singh, 1943), Butoma- 

 ceae (Johri, 19366, 1938a, 6), Podostemonaceae (Went, 1910, 1912, 

 1926), and some members of the Orchidaceae (see Swamy, 

 1949a). In the Alismaeae, of which Machaerocarpus calif ornicus 

 (Maheshwari and Singh, 1943) may be cited as an example (Fig. 

 64), the development usually proceeds normally up to the 4-nucleate 

 stage. After this only the 2 micropylar nuclei divide again, re- 

 sulting in a 6-nucleate stage comprising an egg apparatus, two 

 polar nuclei, and a single antipodal nucleus. In those plants in 

 which reduction has gone still further, only 5 nuclei are formed, 

 four at the upper end and the undivided primary chalazal nucleus 

 at the lower. The mature embryo sac therefore comprises an egg 

 apparatus, an upper polar nucleus, and a single antipodal nucleus; 

 a lower polar nucleus is absent. 



Special mention may be made of a few plants following the 

 Allium type of development. 



In the tribe Viscoideae, belonging to the Loranthaceae, this mode 

 of development seems to be of general occurrence and has recently 

 been described in some detail in Ginalloa (Rutishauser, 19376), 

 Korthalsella (Rutishauser, 1935, 19376 ; Schaeppi and Steindl, 1945), 

 and Viscum (Steindl, 1935; Schaeppi and Steindl, 1945). In all 

 these genera the central ovarian papilla has two or more arche- 

 sporial cells, each of which divides to form two dyad cells (Fig. 

 Q5A-C). Of these, the upper dyad cell is the larger and functions, 

 while the lower soon degenerates (Fig. 65 D). A peculiar feature 

 is that after the 4-nucleate stage there is a slow but steady curva- 

 ture of the embryo sac, which causes its lower end to bend out of 

 the papilla and proceed upward into the carpellary tissue (Fig. 

 65£"). Meanwhile, the 4 nuclei divide to form 8, one quartet being 

 situated at each pole of the embryo sac. The egg apparatus dif- 

 ferentiates in the originally lower pole, which is, however, now 

 situated at a higher level than the upper (Fig. 65F). 



