338 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



rounded. Their walls become thickened and each cell undergoes two 

 divisions at right angles to each other to form a four-celled pro- 

 embryo. With further divisions the proembryo increases in size 

 and finally ruptures the wall of the parent cell and enters into the 

 cavity of the sac. Several other embryos are formed similarly but 

 owing to mutual competition only one or two reach maturity. 



An interesting feature, which has often been commented on, is 

 that although the adventive embryos have precisely the same 

 germinal constitution as sporophytic buds, their developmental 

 behavior is quite different. A sporophytic bud, whether terminal 

 or axillary, directly proceeds to the formation of a stem, leaves, and 

 flowers, while the adventive embryo recapitulates in a very striking 

 manner the morphological features of true seedlings, viz., presence 

 of cotyledons, radicle, plumule, epicotyl, and hypocotyl. In Citrus, 

 sporophytic buds produce virtually thornless plants, but the mi- 

 cellar embryos produce plants which are thorny, like the zygotic 

 seedlings with which they are associated. 14 



Swingle (1927) suggested that this extraordinary recapitulation 

 of a stage in ontogeny, already undergone by the generation which 

 produces the nucellar embryos, is probably due to some powerful 

 morphogenetic influence exercised upon the embryos by the "magic 

 bath" of the embryo sac. However, we have no knowledge so far 

 of the true nature of these influences, and it would be interesting to 

 know how the nucellar embryos would behave if they are removed 

 from the ovule at a comparatively early stage of development and 

 grown in artificial media. 



REFERENCES 

 Afzelius, K. 1928. Die Embryobildung bei Nigritella nigra. Svensk Bot. Tidskr. 



22: 82-91. 



. 1932. Zur Kenntnis der Fortpflanzungsverhaltnisse und Chromosomen- 



r zahlen bei Nigritella nigra. Svensk. Bot. Tidskr. 26: 365-369. 

 Archibald, E. E. A. 1939. The development of the ovule and seed of jointed 



cactus (Opuntia aurantiaca Lindley). South African Jour. Sci. 36: 195-211. 

 Battaglia, E. 1946. Ricerche cariologiche e embriologiche sul genere Rudbeckia 



(Asteraceae) . VIII. Semigamia in Rudbeckia laciniata L. Nuovo Gior. Bot. 



Ital. N.S. 53: 483-511. 

 14 It has also been noted (see Cook, 1938; Hodgson and Cameron, 1938; Frost, 

 1938) that Citrus clones continually propagated by cuttings eventually become 

 weak and sterile. Horticulturists rejuvenate the clone by using nucellar seedlings, 

 as the plants obtained from them show greater vigor and a more upright growth 

 than those obtained from cuttings. 



