APOMIXIS 337 



tetrads) which may undergo a secondary increase in number by 

 further proliferation, budding, or cleavage. 



Adventive embryony may be completely autonomous, i.e., inde- 

 pendent of pollination and fertilization, or it may be induced by one 

 or both of these factors. The former condition prevails in Al~ 

 chornea ilicifolia (Strasburger, 1878), Euphorbia dulcis (Carano, 

 1926), and Sarcococca pruniformis (Wiger, 1930). In Nigritella 

 nigra (Afzelius, 1928) also, neither pollination nor fertilization are 

 essential, but the occurrence of pollen tubes in the ovary seems to 

 accelerate the tendency towards the production of adventive em- 

 bryos. In most other plants, either pollination, or pollination 

 followed by fertilization, is an important factor in stimulating the 

 development of adventive embryos, although their exact roles have 

 not been properly elucidated. In the orchid Zygopetalum mackayi 

 (Sussenguth, 1923) unpollinated flowers were found to degenerate 

 and fall off shortly after blooming, but on treating the stigmas with 

 pollen from Oncidium the adventive embryos developed to maturity 

 a.nd viable seeds were formed. Here the foreign pollen, although 

 quite incapable of effecting fertilization, exercised some kind of a 

 chemical influence which affected the growth of the embryos in a 

 favorable manner. Eugenia jambos (Pijl, 1934) seems to illustrate 

 the next step, for in this plant the adventive embryos may originate 

 quite independently of pollination but do not attain their full de- 

 velopment unless fertilization has taken place. In most varieties 

 of Citrus also (see Webber and Batchelor, 1943) fertilization is con- 

 sidered necessary for the maturation of the adventive embryos, and 

 a similar condition occurs in the "carabao" mango (Juliano, 1937). 



In all cases of adventive embryony there is a formation of the 

 endosperm, whether it originates as the result of triple fusion or 

 without it. The only exception is Opuntia aurantiaca (Archibald, 

 1939). Here the egg, synergids, and antipodals are said to de- 

 generate, and later also the polar nuclei, so that an endosperm is not 

 formed at all and the whole embryo sac is reduced to an irregular 

 darkly staining cavity. 13 The nucellar tissue becomes more mas- 

 sive, however, and certain cells lying below the nucellar cap and 

 bordering on the cavity of the embryo sac enlarge and become 



13 It should be noted, however, that Ganong (1898), who found nucellar em- 

 bryony in another species— 0. vulgaris — records normal fertilization and the forma- 

 tion of an "abundant endosperm." 



