POLY EM BRYONY 349 



Woodworth (1930) has called attention to the frequent occurrence 

 of polyembryonate seeds in Alnus rugosa. Meiosis was found to be 

 disturbed and only 2 to 3 per cent of the pollen grains were viable. 

 Pollen tubes were not observed, and bagged catkins produced per- 

 fectly normal and viable seeds, similar to those obtained from un- 

 bagged catkins. Embryo sac formation was not preceded by meio- 

 sis, and more than 50 per cent of the seeds showed diploid egg em- 

 bryos at the micropylar end of the ovule. A few seeds had an 

 embryo oriented in the opposite direction, suggesting its origin from 

 an antipodal cell. One ovule showed three embryos at the micro- 

 pylar end, two of which are believed to have originated from syner- 

 gids and the third from the egg. Nucellar embryony was frequent. 

 Some of the embryos were found sunken in the endosperm and are 

 believed to have originated from the cells of the latter. Several 

 ovules showed more than one embryo sac, each giving rise to one or 

 more embryos. From the occurrence of four to seven cotyledonary 

 buds on certain embryos, it further appeared that originally sepa- 

 rate embryos could sometimes fuse to form a composite structure. 



Woodworth 's work, although of much interest, lacks the early 

 stages in embryonal development, and therefore some of his con- 

 clusions about the origin of the polyembryonate condition need 

 confirmation. In particular, his inference that endosperm cells 

 may also give rise to embryos must be regarded as very doubtful 

 (see page 334). 



In Atraphaxis frutescens, Edman (1931) has described some 

 interesting cases of polyembryony. Haploid embryo sacs are pro- 

 duced only rarely and must be fertilized before they can give rise to 

 embryos. More often, meiosis fails and the embryo sacs are there- 

 fore diploid. Frequently two or more occur in the same ovule, and 

 each of them may produce an embryo. Figure 190C shows two 

 embryo sacs lying side by side, the left with two overlapping em- 

 bryos and the right with one embryo at the micropylar end. Figure 

 190 A shows a different condition, in which the second embryo sac 

 has arisen from a chalazal cell and is inversely oriented in relation 

 to the normal embryo sac. A similar but more advanced condition 

 is seen in Fig. 190£>i,B 2 , where a well-developed and nearly mature 

 embryo is present in the upper embryo sac, but the chalazal embryo 

 sacs and embryos failed to keep pace and are in the process of 

 obliteration. In addition, embryos of nucellar and apogamous 



