The Phylogeny of Angiospeems 



of the stem and in the character of its vascular bundles are far more difficult to connect 

 genetically than to refer to a polyphyletic origin, with all that that implies. One of 

 the strongest arguments against the monophyletic theory comes from historical 

 testimony. The "Proangiosperms" of the Lower Cretaceous, so far as known, appeared 

 associated with undoubted Monocotyledons, and merged gradually into recognizable 

 Dicotyledons, without indicating any relationship to the Monocotyledons. The 

 emerging of Dicotyledons from this vague group either indicates that they originated 

 independently, or that the Proangiosperms were transition forms between Mono- 

 cotyledons and Dicotyledons. The latter alternative is inconceivable, especially since 

 the most primitive Dicotyledons are now recognized to be more primitive than any of 

 the Monocotyledons. 



It is of interest to note that recent anatomical investigations contradict the current 

 view that Monocotyledons are the more primitive, and Dicotyledons derived from 

 them, and show, so far as the development of the vascular system is concerned, that 

 Monocotyledons are derived from Dicotyledons in case they have a common 

 phylogeny. 



All the testimony available, morphological, historical, and anatomical, seems most 

 consistent when interpreted in favor of the polyphyletic origin of Angiosperms. 



The second phase of the problem is to determine whether the Angiosperms have 

 been derived from Gymnosperms or from Pteridophytes. The general question is the 

 same whether one believes in their monophyletic origin or not. The older view is 

 that Angiosperms are phylogenetically related to Gymnosperms, and Gnetum has been 

 regarded as the nearest living representative of a transition condition between 

 Gymnosperms and Angiosperms. The argument is based upon such angiospermous 

 characters in Gnetum as the absence of archegonia, the organization of eggs while the 

 gametophyte consists only of free cells, the presence of a perianth and true vessels, 

 and the dicotyledonous leaves. This showing is certainly strong, and especially in the 

 structure of the embryo sac does Gnetum show characters that may well illustrate a 

 stage in the Angiosperm phylum ; but that it actually represents the group from which 

 the Angiosperms were derived seems unlikely. In fact, the historical argument 

 against such a claim is very strong, for there is no evidence that Giietum or allied 

 forms existed among the numerous Angiosperms of the Cretaceous and Tertiary. If 

 it were related in any way to the origin of so dominant a group as the Angiosperms, 

 it seems probable that it would have left some evidence of its existence. Nor is it of 

 special avail to claim that fossil Gnetales may be found in the tropics or in the 

 Southern Hemisphere, for the great uniformity of climate during early periods has left 

 the records of tropical vegetation in the temperate and even boreal regions of today ; 

 so that the strata of the tropics are not likely to reveal prominent types of vegetation 

 unrepresented in the strata of temperate regions. 



The argument from the presence of a " perianth " is particularly vulnerable, since 

 the structure so-called merely represents the bracts common among Gymnosperms, and 



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