FERTILIZATION 205 



were eight in addition to the two which took part in fertiliza- 

 tion. 



The presence of extra sperms inside the embryo sac, whether they 

 are derived from one or more than one pollen tube, may result in 

 two kinds of abnormalities. Either some of the supernumerary 

 sperms enter the egg, resulting in a polyploid offspring, or more 

 than one cell of the egg apparatus may be fertilized, resulting in 

 multiple embryos. In Monotropa hypopitys (Strasburger, 1884), 

 Iris sibirica (Dodel, 1891), and Gagea lutea (Nemec, 1912), and in 

 Oenothera nutans pollinated by 0. pycnocarpa (Ishikawa, 1918), two 

 sperms were sometimes observed to enter the egg. Michaelis 

 and Dellinghausen (1942), who obtained a triploid plant from a 

 cross between Epilobium hirsutum and E. luteum, considers it 

 probable that it resulted from the entry of an E. luteum sperm in- 

 to the egg after it had already been fertilized by an E. hirsutum 

 sperm. 



Fertilization of more than one cell of the egg apparatus has been 

 reported in several plants 6 of which Sagittaria graminea (Johri, 

 19366) and Crepis capillaris (Gerassimova, 1933) may be cited as 

 examples. In Sagittaria fertilization usually occurs normally, one 

 male gamete fusing with the egg and the other with the two polar 

 nuclei. But the synergids often assume an egg-like appearance 

 (Fig. 116 A) and sometimes a second pollen tube enters the embryo 

 sac, releasing two additional sperms (Fig. 1165). Although an 

 actual fertilization of the synergids was not seen, the presence of 

 two pollen tubes and three proembryos in the upper part of the 

 embryo sac (Fig. 1162?) leave no doubt that this may happen. 



In Crepis capillaris, Gerassimova (1933) observed some embryo 

 sacs with two to five eggs in addition to the two synergids (Fig. 

 117A). Usually only one of the eggs gives rise to an embryo (Fig. 

 117J5) and the others eventually degenerate and disappear, but if a 

 pollen tube carrying more than two sperms enters the embryo sac 

 there is a possibility of the production of additional embryos. 



6 Tischler (1943) has recently given a complete list of such plants but in some 

 cases the inference is based merely on the presence of a second or third embryo 

 beside the zygotic embryo. It is now known that even unfertilized synergids can 

 undergo a few divisions and sometimes develop into fully mature haploid embryos, 

 or the zygotic embryo may itself give rise to additional embryos by a process of 

 cleavage or budding (see Chaps. 9 and 10). 



