EMBRYOGENESIS IN FLOWERING PLANTS 289 



1934). According to Webber and Batchelor (1943), fertilisation is 

 necessary in most varieties of Citrus for the maturation of adventitious 

 embryos; and so, too, for the mango (Mangifera indica, Sapindales: 

 Anacardiaceae) (Juliano, 1934, 1937). An endosperm is nearly always 

 present in species in which adventive embryos are formed, an exception, 

 according to Archibald (1939), being Opimtia aurantiaca (Cactaceae). 



Ahhough formed from sporophyte tissue, adventive embryos 

 develop quite differently from normal terminal or axillary shoot buds. 

 The rudiment of a shoot bud is characterised by the early formation of 

 an axis bearing leaves: the adventive embryo, by contrast, passes 

 through the typical embryonic and seedling phases. In Citrus, the 

 shoot buds develop into almost thornless shoots, whereas nucellar 

 embryos, like the normal zygotic embryo, yield thorny seedlings. 

 While these morphological differences may be largely due to the 

 nutritional status of the environments of the two kinds of primordium, 

 genetical factors may also be involved. The effect of such factors could 

 perhaps be ascertained by observing the development of young, excised 

 nucellar embryos in suitable artificial media. The ontogenetic 'recapitu- 

 lation' shown by nucellar embryos is likely to be due to the same 

 factors which determine the normal embryonic development (Swingle, 

 1927). 



POLYEMBRYONY 



Polyembryony, or the occurrence of more than one embryo in the 

 seed, first discovered in oranges by Leeuwenhoek in 1719, has been 

 reviewed by Ernst (1918), Schnarf (1929), Webber (1940), Robyns and 

 Louis (1942), Gustafsson (1946), Maheshwari (1950, 1951, 1952), 

 Johansen (1950), and Lebegue (1952), Figs. 80, 81. Cleavage poly- 

 embryony results when the zygote or proembryo divides into two or 

 more embryos. This phenomenon, which is common in the gymno- 

 sperms, is only of sporadic occurrence in the angiosperms. Examples 

 are afforded by Erythronium anwricanum (Liliaceae) (Jeffrey, 1895; 

 Guerin, 1930); Tulipa gesneriana (LilisLCQaQ) {Emsi, 1901); Limnocharis 

 emarginata {Butomacea.e) (Hall, 1902); EuJophea epidendraea (Orchid- 

 aceae) (Swamy, 1943). Cleavage polyembryony is of fairly frequent 

 occurrence in the Orchidaceae. In Eulophea, the zygote may divide 

 irregularly forming a mukicellular mass, the more distal cells of which 

 grow out and form several individual embryos; or the filamentous 

 proembryo may branch and so yield several embryos; or small 

 proembryo buds may develop into separate embryos. Fig. 80d, e. In 

 Erythronium, the zygote divides irregularly and forms an embryonic 

 mass, the most distal cells of which grow out separately to form 

 embryos, Fig. 80c. 



