THE TWO MAJOR TYPES OF ANGIOSPERM EMBRYOS 363 



one; in the other, the monocotyledonous type is considered primitive, 

 the dicotyledonous type derived by the spHtting of the one cotyledon. 

 Anatomical evidence has been used in considerable part in support of 

 these theories of evolutionary relationship, but usually without con- 

 sideration of the fundamental anatomy of cotyledons as a whole — in all 

 angiosperms as well as in gymnosperms. (A pseudodichotomous struc- 

 ture has long been recognized in many angiosperm cotyledons and has 

 been used as evidence in the claim that cotyledons are organs sui 

 generis, and not of leaf rank. ) 



Underlying the problem of whether the possession of one or of two 

 cotyledons in the embryo is primitive, lies the almost universal presence 

 of two cotyledons in the lower seed plants — even in SeJaginella, there 

 is a pair of opposite, cotyledonlike organs. (The polycotyledonary em- 

 bryos of some conifers seem to have been derived from dicotyledonous 

 types. ) Comparison of angiosperm embryos with those of lower taxa 

 should give evidence of the basic primitive number; none of the lower 

 taxa has one cotyledon. The dominance of the two-cotyledonary state in 

 embryos generally is evidence of the two-cotyledonary ancestry of the 

 monocotyledons. The monocotyledons are not an isolated group of in- 

 dependent origin; their very close relationship with the dicotyledons — 

 their derivation from ancient dicotyledons or from a common two- 

 cotyledonous stock — is hardly to be questioned. The monocotyledons 

 need not be called "syncotyledons," as has been suggested by those 

 who believed there is evidence of fusion of two cotyledons to form the 

 single cotyledon. 



Evidence in support of the theory that the monocotyledons have 

 been derived from dicotyledonous stock has been obtained from dicoty- 

 ledonous taxa in which one cotyledon has obviously been reduced or 

 lost. Furthermore, some monocotyledons have what appears to be a 

 vestigial cotyledon opposite the normal one. In the mid-twentieth cen- 

 tury, this structure has either been overlooked or considered "without 

 .'■•gnificance." Evidence in support of the view that the monocotyle- 

 donous embryo is the primitive type has come chiefly from anatomical 

 structure — from the double vascular supply of the cotyledon of some 

 monocotyledons, especially those considered the more primitive. The 

 single cotyledon is considered, under this theory, to be terminal and 

 two lateral cotyledons to have been formed by the longitudinal splitting 

 of the terminal one. 



In the light of the large amount of information about cotyledons now 

 available — much of it in the older literature — the dicotyledonous type 

 is now generally accepted as primitive; the monocotyledonous type, as 

 derived by the loss of one of the ancestral pair. 



Nature of the Cotyledon of the Monocotyledons. At the beginning of 

 the twentieth century, the cotyledons were described as "primarily 



