334 INTRODUCTION TO EMBRYOLOGY OF ANGJOSPERMS 



both of the synergids, and less frequently the antipodal cells, may 

 give rise to embryos (diploid apogamy). In some plants such de- 

 velopment takes place without the stimulus of pollination (auton- 

 omous apomixis) and may begin even in the bud stage. More com- 

 monly, however, the stimulus of pollination is essential and even 

 triple fusion occurs more or less regularly. This condition is called 

 pseudogamy. 10 



There are a few reports of embryos arising from the cells or nuclei 

 of the endosperm. Rosenberg (1907), Schnarf (1919), and Gent- 

 scheff (1937) reported such an occurrence in some species of Hier- 

 acium. However, in a later paper Rosenberg (1930) withdrew this 

 interpretation. He now thinks that here several aposporic embryo 

 sacs develop in the same ovule and fuse with one another so that 

 the boundaries between them get lost, and the embryos, although 

 really arising from the egg cells of the different embryo sacs, become 

 included in a common mass of endosperm tissue. The reports of 

 Billings (1937) and of Jeffrey and Haertl (1939) about the origin of 

 endosperm embryos in Isomeris and Trillium are also open to criti- 

 cism. 11 To date there is, therefore, no established case of the origin 

 of embryos from the endosperm. 



Mention must finally be made of the peculiar phenomenon called 

 "semigamy" which has been discovered very recently by Battaglia 

 (1946, 1947). Here a sperm nucleus enters the diploid egg cell but 

 does not fuse with its nucleus and divides independently to form a 

 few daughter nuclei. Embryonal chimaeras are thus produced in 

 which most of the cells and nuclei are diploid but a few are haploid. 

 The endosperm, however, is pentaploid, being formed as the result 

 of a fertilization of the secondary nucleus by one male nucleus. So 

 far this condition has been reported only in two species of Rud- 

 beckia, R. laciniata and R. speciosa (Fig. 191), but it is possible that 

 it occurs in other apomicts and has been overlooked. 



ADVENTIVE EMBRYONY 



In adventive embryony there is no alternation of generations and 

 the embryos originate from the diploid cells of the ovule lying out- 



10 For a more detailed discussion of pseudogamy, see Hafliger (1943) and Fager- 

 lind (1946). 



11 Swamy (1948), who has recently studied the development of the embryo and 

 endosperm in Trillium undulalum, refutes the occurrence of endosperm embryos in 

 this species but states that adventive embryony is frequent. 



