APOMIXIS 



335 



side the embryo sac and belonging either to the nucellus or the 

 integument. A common feature of the process is that the cells 

 concerned in such development become richly protoplasmic and 

 actively divide to form small groups of cells, which eventually push 

 their way into the embryo sac and grow further to form true em- 

 bryos. Frequently the zygotic embryo also develops at the same 

 time and is distinguishable from the adventive embryos only by the 

 somewhat lateral position and lack of a suspensor in the latter. 



A favorite and frequently quoted instance of adventive embryony 

 is that of Citrus (Strasburger, 1878; Osawa, 1912; Webber and 

 Batchelor, 1943), (Fig. 192) in which four or five embryos are com- 



A B C 



Fig. 192. Development of adventive embryos in Citrus trifoliata. A, micropylar 

 portion of embryo sac, showing fertilized egg, pollen tube, and endosperm nuclei; 

 some of the nucellar cells have enlarged and show prominent nucleus and dense 

 cytoplasm. B, same, more advanced stage. C, upper part of embryo sac, showing 

 several embryos lying in endosperm; only zygotic embryo has suspensor. (After 

 Osawa, 1912) 



mon and sometimes as many as 13 viable embryos can be found in 

 the same seed. Among other examples may be cited Euphorbia 

 dulcis (Carano, 1926), Sarcococca ilicifolia (Wiger, 1930), Eugenia 

 spp. (Tiwary, 1926; Pijl, 1934), Capparis frondosa (Mauritzon, 

 1934), Mangifera indica (Juliano, 1937), and Hiptage madablota 

 (Subba Rao, 1940). The chief variation in development concerns 

 the place of origin of the embryos. Whenever the nucellus is in- 

 tact, the adventive embryos originate from the nucellar cells, but 

 when it becomes disorganized the cells of the integument may take 

 over this function. Also, sometimes a single cell may become the 

 progenitor of an embryo, while on other occasions it is a small group 

 of cells. 



An especially interesting type of adventive embryony occurs in 

 the Scandinavian forms of the orchid Nigritella nigra (Afzelius, 



