POLY EM BRYONY 353 



The authors critically analyze the possible origins of the multiple 

 seedlings from a study of their chromosome numbers, stem color, 

 and distribution of sexes, and conclude that about one-fourth of the 

 diploid-diploid twin seedlings must have originated by a process of 

 cleavage polyembryony The remaining three-fourths are believed 

 to have arisen from two cells belonging either to the same embryo 

 sac or to two embryo sacs in an ovule. In addition a number of 

 "conjoined" twins were found, which showed varying degrees of 

 attachment to each other but were capable of developing into inde- 

 pendent plants. To explain their origin the following alternatives 

 are envisaged: (1) a partial fusion of two adjacent embryos, and 

 (2) an incomplete cleavage of one embryo. From the complete 

 identity in chromosome number and genetic characters between the 

 members of the conjoined type, it is concluded, however, that they 

 originated by an incomplete cleavage of a single initial embryo. 



Conclusion. It may be said that polyembryony, although fairly 

 widespread in angiosperms, is much less common in them than it is 

 in the gymnosperms. The reason for this is that in the latter there 

 are several archegonia, but in the former there is only one cell in 

 the ovule (the egg) which is normally capable of giving rise to an 

 embryo. Sometimes, however, the proembryo may become sepa- 

 rated into two or more portions (cleavage polyembryony), or more 

 than one cell of the embryo sac may develop into an embryo. Less 

 frequently there may be two or more embryo sacs in an ovule, each 

 of which may give rise to embryos. A fourth source of poly- 

 embryony is the "budding" or proliferation of the cells of the nu- 

 cellus or integument (adventive embryony). The adventive em- 

 bryos are diploid and similar to one another as well as to the plant 

 from which they arise. Embryos produced by the cleavage of a 

 single zygote are also identical in all essential respects. Embryos 

 arising from two or more cells of one or separate embryo sacs may, 

 however, have the same or different chromosome numbers. Even 

 when developmental stages in embryogeny are not available, it is 

 possible in some cases to infer the mode of origin of the polyem- 

 bryonate condition on genetical evidence. 



REFERENCES 



Akerberg, E. 1939. Apomictic and sexual seed formation in Poa pratensis. 



Hereditas 25: 359-370. 

 . 1943. Further studies of the embryo and endosperm development in 



Poa pratensis. Hereditas 29: 199-201. 



