THE EMBRYO 213 



produces parthenogenetic embryos. The observation was not 

 direct or conclusive, the inference being based upon the failure 

 to discover pollen-tubes although embryos were common, the 

 feeble development of endosperm, and the poorly developed 

 synergids, all of which is negative evidence. Treub suggests 

 that the stimulus that induces the egg to divide in this case 

 is the puncture made by the pollinating wasp Blastopliaga. 



There seems to be no doubt that other cases of partheno- 

 genesis will be discovered among Angiosperms, and that many 

 embryos supposed to be normal are parthenogenetic. There 

 seems to be no reason to doubt that if an envelop of cytoplasm 

 may result in the segmentation of the egg in Thalictrum, it may 

 often have the same result in other cases. For example, 

 Treub 25 observed that in certain Burmanniaceae (Gonyanthes 

 Candida and Burmannia javanica) the egg does not segment 

 until the embryo-sac is packed full of endosperm. Such a con- 

 dition might well repeat the results in Thalictrum. In fact, 

 all cases in which there is a long delay before the egg segments 

 may be suspected of occasional parthenogenesis. 



POLYEMBRYOXY 



Polyembryony in Angiosperms, while not so prevalent as in 

 Gymnosperms, is by no means a rare or recently discovered 

 phenomenon. As early as 1719, Leeuwenhoek found two em- 

 bryos in orange seeds. In Euonymous latifolius polyembryony 

 was discovered three times independently ; by Petit-Thouars in 

 1807, by Grebel in 1820, and by Treviranus in 1838. In this 

 species about one-half of the ripe seeds are said to contain more 

 than one embryo. A. Braun in 1859 gave an historical resume 

 of the subject, and cited sixty cases as known at that time. 

 The first demonstration of the real nature of certain cases of 

 polyembryony was made by Strasburger 12> 16 in 1878. He 

 found that in Funkia ovata, Noilioscordon fragrans, Citrus 

 Aurantium, and Coelebogyne iUcifolia the cells of the nucellus 

 above the apex of the embryo-sac become rich in contents, divide 

 and grow, and form several embryos that push the sac wall 

 before them and become placed in the seed like normal em- 

 bryos. In Fun~ki& the egg is fertilized, but seldom or perhaps 

 never produces an embryo, dividing a few times and then disor- 

 ganizing (Fig. 99). When pollination is prevented artificially, 



