210 MORPHOLOGY OF ANGIOSPERMS 



traversed by two opposed vascular bundles, which suggest the 

 fusion of two organs and a derivation from the dicotyledonous 

 condition. This position is further strengthened by the well- 

 known tendency among certain Dicotyledons for the cotyledons 

 to become more or less completely fused (see Chapter XV). 



The whole problem, however, is too indefinite as yet, and 

 the data are too few to permit well-grounded conclusions, but it 

 is well worth consideration. 



PARTHENOGENESIS 



The term parthenogenesis was once very loosely applied, 

 including all cases of the appearance of embryos without fer- 

 tilization. Strictly, however, it includes only those cases in 

 which the normal egg produces an embryo without fertilization, 

 and this phenomenon has thus far been demonstrated in only 

 three angiospermous genera, to be described below. Apogamy, 

 being the production of a sporophyte by a gametophyte without 

 the act of fertilization, of course includes parthenogenesis, but 

 the production of sporophytes by gametophytic structures 

 other than the egg may for convenience be distinguished as 

 vegetative apogamy. In this category would be included all 

 cases of embryos derived from unfertilized synergids, antip- 

 odals, and endosperm, the last-named structure being included 

 or not dependent upon one's view as to its morphological char- 

 acter. When an unfertilized synergid produces an embryo, it 

 might be claimed that it is not a case of vegetative apogamy 

 but of parthenogenesis, since the synergid is to be regarded as a 

 non-functioning egg. This simply serves to illustrate the fact 

 that categories are essentially arbitrary and artificial. A third 

 category includes those cases in which embryos are produced by 

 the tissue of the nucellus or of the integument. This is not 

 apogamy, although it has often been so called, for it is a case in 

 which a sporophyte is produced by sporophytic tissue, and can 

 be included under the general name of budding. In addition 

 to the normal method, therefore, embryos appear among Angio- 

 sperms in three ways, namely, by parthenogenesis, by vegetative 

 apogamy, and by budding. In most cases vegetative apogamy 

 and budding are associated with polyembryony, and they will 

 be considered under that head. The three well-authenticated 

 cases of parthenogenesis among Angiosperms are as follows: 



