THEORETICAL CONCLUSIONS 429 



In a slightly later publication, Coulter (1915) extended this view 

 to include the embryo of grasses. He interpreted the scutellum as 

 the functional cotyledon arising from the peripheral cotyledonary 

 ring and the epiblast 6 as a second and greatly reduced cotyledon. 

 From Leersia and Zizania, where the epiblast is a very conspicuous 

 structure, he traces a gradation to the condition in Zea, in which 

 this organ is practically nonexistent. 



Turning now to the dicotyledons, we find that the seedlings of a 

 number of genera and species show only a single cotyledon. The 

 best known of these are: Ranunculus ficaria, Corydalis cava, 

 Abronia, Carum bulbocastanum, Blumium elegans, Erigenia bulbosa, 

 and several members of the Gesneriaceae. The embryogeny of 

 these plants should be especially instructive in giving us an indica- 

 tion as to whether the monocotyledonous state is the more primi- 

 tive or the dicotyledonous. Unfortunately most of the informa- 

 tion available on them deals with the morphology and anatomy of 

 the seedling rather than with the actual development of the 

 embryo. 



Mention may, however, be made of the work of Metcalfe (1936) 

 on Ranunculus ficaria. The embryo is only a small club-shaped 

 mass of cells embedded in the endosperm (Fig. 176). Further 

 development takes place after the seeds are shed. It is interesting 

 to note that during this process a small parenchymatous hump, 

 supplied with a procambial strand, arises in the position in which 

 the second cotyledon would be expected to originate if one were 

 present. The position and mode of origin of this hump strongly 

 suggest that it is the rudiment of the second cotyledon which fails 

 to develop further. 



In conclusion it might be said that, although embryology does 

 not throw any light on the ancestry of the angiosperms, it indicates 

 that the group is probably monophyletic in origin. There are no 

 essential differences between the monocotyledons and dicotyledons 

 as regards the development and organization of the male and fe- 

 male gametophytes and the endosperm, and the process of fertiliza- 

 tion is the same in both the subgroups. Further, the differences in 

 the organization of the embryo are not fundamental, for there are 

 some dicotyledons in which only one cotyledon develops fully and 

 the other becomes arrested, and some monocotyledons in which 



8 See p. 289. 



