POLYEMBRYONY 345 



times by the statements that it is "inconsequential," a mere scale, that 

 "it has no vascular supply and no significance." But the fact that, in 

 some genera, it is a prominent part of the embryo — even as long as the 

 cotv^ledon — and has received much attention, indicates that it is probably 

 significant. It has been called a vestige of the second cotyledon, repre- 

 senting its sheath or its auricles — it is bilobed in some genera (Dan- 

 fhonia, Stipa, Eleiisine); it has been interpreted as a lobe of the scutel- 

 lum, of the coleorhiza (Fig. 130), of the hypocotyl, of the undifferen- 

 tiated body of the embryo, freed by the breaking out of the endogenous 

 root primordium. Granted that it is no more than a vestige, its position 

 on the axis — if "dii-ectly opposite the cotyledon" — would support the 

 view that it is a remnant of the second cotyledon. This interpretation 

 was prominent in the middle of the nineteenth century. A similar, but 

 usually larger, structure, is present in dicotyledons that have only one 

 cotyledon, and stages in the reduction of the second cotyledon are 

 present in some monocotyledons (Fig. 125). The epiblast is commonly 

 described as small and scalelike, but varies greatly in size and form. It 

 may even be of much the same size as the scutellum and is so de- 

 scribed for some of the bamboos; it is prominent in the embryos of 

 Zizania and Leerzia. 



Although through much of the nineteenth century, the epiblast was 

 considered the second cotyledon, in the twentieth century, this interpre- 

 tation has been discarded, and it has been called a lobe or an extension 

 of one of the other organs of the embryo or of the undifferentiated pro- 

 embryo. The interpretation of the epiblast as a vestigial second cotyle- 

 don receives some support by the growing view that the grasses are 

 related to the Commelinaceae, which have a reduced but well-marked 

 second cotyledon. The epiblast has been considered unusual among 

 grasses, but it is present in two-thii"ds of the genera; absence of an 

 epiblast is unusual in the family. Its presence in the primitive grasses 

 and its absence in the highest genera suggests that it is a primitive 

 structure disappearing within the Gramineae. The strongest evidence 

 of its cotyledonary nature would be its position directly opposite the 

 scutellum, but the attachment of the scutellum is not the attachment 

 of the cotyledon, which is just below the plumule (Fig. 130). The 

 morphology of the epiblast is uncertain. 



POLYEMBRYONY 



The term polyembryony is commonly used to describe the presence 

 of more than one embryo in an ovule or seed. The use of the term in this 

 way is morphologically loose, because of the varied nature and origin 

 of the multiple embryos. Some are sporophytic; others, gametophytic. 

 The sporophyte types are derived from tissues of the mother sporo- 



