208 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



sperms fused with the secondary nucleus. A different type of 

 abnormality, reported sometimes, is that the triple fusion nucleus 

 undergoes one or two divisions and then the male nuclei discharged 

 from a second pollen tube fuse with some of the endosperm nuclei 

 (Fig. 116Z)). 



When two or more pollen tubes are discharged inside an embryo 

 sac, it is also possible that the sperm fusing with the egg is derived 

 from one tube and the one fusing with the secondary nucleus from 

 another. This "heterofertilization" has not yet been cytologically 

 demonstrated, but Sprague (1932), has inferred it in Zea mays on 

 genetical grounds. In a study of the inheritance of scutellum color 

 he found several kernels with white aleurone but a colored scutellum, 

 and their progeny segregated for aleurone color in ratios similar 

 to those obtained from hybrid kernels with colored aleurone. He 

 therefore concludes that in such cases the egg and polar nuclei had 

 been fertilized by sperms of unlike genotypes. 



Single Fertilization. Although double fertilization is the rule in 

 angiosperms, the question arises whether development can proceed 

 with only a single fertilization, i.e., if there is syngamy without 

 triple fusion or triple fusion without syngamy. Cooke and Shively 

 (1904) stated that in Epiphegus virginiana endosperm formation 

 begins before fertilization, and Anderson (1922) reported the same 

 in Martynia louisiana. In Ramondia nathaliae and R. serbica 

 (Glisic, 1924) syngamy occurs regularly, but triple fusion is said 

 to be "facultative" and is frequently omitted. Wiger (1935) stated 

 that in some members of the Buxaceae and Meliaceae, endosperm 

 formation is entirely independent of fertilization. 

 1 All these reports are, however, of a doubtful nature (see also 

 Mauritzon, 1935). Without going into details it may be said that 

 some of the above workers seem to have overlooked the pollen tube, 

 and others mistook the unfused polar nuclei for the first pair of 

 endosperm nuclei. It is only rarely that development can take 

 place without triple fusion. Guignard (1921) reported a case in 

 Vincetoxicum nigrum in which the zygote had divided several times 

 while the polar nuclei were still lying unfused and the second male 

 gamete had not yet been discharged from the pollen tube. More 

 recently, Dahlgren (1930, 1939) has figured embryo sacs of Mitella 

 pentandra and Zostcra marina (Fig. 118) in which a several-celled 

 embryo is associated with an undivided secondary nucleus, and 



