314 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



the plant. But since this is merely a form of vegetative reproduc- 

 tion, it will not be considered in the present account. 



NONRECURRENT APOMIXIS 



The development of a haploid cell of the gametophyte into the 

 embryo is a comparatively infrequent occurrence, and as mentioned 

 before, the plants arising in this way are sterile. Several years ago, 

 Kusano (1915) and Haberlandt (1922) observed mitoses in the un- 

 fertilized egg cells of Gastrodia and Oenothera, but the development 

 was found to stop at a very early stage. Fully developed haploid 

 plants, first found in Datura (Blakeslee et at., 1922), have now been 

 recorded in several genera 4 but their exact origin remains obscure 

 except in a very few cases. 



For the first critical account of the origin of haploid embryos, 

 we are indebted to C. A. J0rgensen (1928). He treated 90 flowers of 

 Solarium nigrum with the pollen of S. luteum, resulting in 43 fruits 

 with 70 seeds. From these arose 35 S. nigrum seedlings of which 

 seven turned out to be haploid. 5 A developmental study showed 

 that the pollen of S. luteum readily germinated on the stigmas of 

 S. nigrum and the pollen tubes reached the embryo sacs in the 

 normal way. One of the sperms also entered the egg but failed to 

 fuse with the female nucleus and soon degenerated. Its presence 

 sufficed, however, to stimulate the haploid egg to develop parth- 

 enogenetically and give rise to an embryo (Fig. 177). 



In some species of Lilium (Cooper, 1943), in Bergenia delavayi 

 (Lebegue, 1949) and in Erythraea centaurium (Crete, 1949), one of 

 the synergids may divide in a small number of the ovules. Twin 

 embryos are thus produced, one diploid and the other haploid (Fig. 

 178). However, in most cases of this kind the synergid embryo 

 usually degenerates at an early stage, and only the zygotic embryo 

 is seen in the mature seed. 



In some recent studies of Orchis mocidata, Epipactis latifolia, 

 Platanthera chlorantha, Cephalanthera damasonium, and Listera ovata, 

 Hagerup (1944, 1945, 1947) found a number of haploid embryos. 

 He observed that while some ovules received more than one pollen 

 tube (Fig. 179), others received none at all, or the tube arrived too 



4 For detailed references see Kostoff (1941). 



6 The remaining seedlings were diploid and are believed to have arisen by a 

 process of endoduplication or nuclear division without cell formation, followed by 

 a fusion of the spindles in the next division. 



