288 



EMBRYOGENESIS IN PLANTS 



observations it may perhaps be inferred that there is some delay in 

 the estabhshment of polarised growth in adventive embryos. In 

 Citrus, many viable embryos may sometimes be found in the same 

 seed. Studies of adventive embryos, which may arise in various 

 positions, may prove of value in the causal analysis of morphogenesis. 



B 



Fig. 79. Examples of apomixis 



A, Liliiim martagon. The proembryo on the left has arisen from a synergid and is 

 hapioid; that on the right has developed from the zygote and is diploid (after 

 Cooper). B-D, Semigamy in Rudbeckia speciosa. B, Two-celled proembryo with 

 two small nuclei in basal cell. C, Three-celled proembryo show ing nuclei derived 

 from sperm in terminal cell. D, More advanced stage; the nuclei derived from the 

 sperm may occupy various positions (after Battaglia). E, F, Citrus trifoliata. 

 Zygotic and nucellar embryos; in F, only the zygotic embryo has a suspensor; the 

 embryos are surrounded by endosperm (after Osawa). 



Maheshwari states that if the nucellus is intact, the adventive embryos 

 will typically originate from some of its cells; but if the nucellus 

 becomes disorganised, they may originate in the integument. (For 

 other examples, see Maheshwari, 1950.) According to the species, 

 adventive embryonic development may take place with or without the 

 stimulus of pollination or fertilisation. The nature of those stimuli is of 

 primary interest to the embryologist. In Eugenia jaiubos (Myrtales: 

 Myrtaceae), adventive embryos originate without pollination, but only 

 if the ovum is fertilised do they attain to their full development (Pijl, 



