PROGRESS IN PLANT MORPHOLOGY 6ll 



stamen, carpel, gametophytes, vascular tissue — and its wood structure is 

 that of an advanced angiosperm. Casuarina is a highly specialized and reduced 

 angiosperm; its simplicity is that of reduction, not of primitiveness. The 

 maintenance, therefore, of the Prephanerogamae as a valid taxon is absurd. 



Our opinion of the nature of the primitive flower has changed completely 

 in recent years. Formerly, the simplest flower was considered the most primi- 

 tive, and the flowers of the Amentiferae and those of some of the lower 

 aquatic monocotyledons^unisexual, with few sporophylls, and naked — were 

 accepted as primitive. Now, the bisexual flower with numerous sporophylls, 

 with an elementary perianth — well demonstrated by the woody Ranales — is 

 generally accepted. The claim that the primitive flower was unisexual with 

 few sporophylls is, however, still maintained by some botanists. 



Our understanding of the stamen has perhaps changed more than that of 

 any other floral organ. The stamen has long been considered a simple organ 

 as compared with the carpel, but that concept must be changed. We now 

 know that it is fundamentally a laminar organ, very like the carpel, as we 

 might expect from comparison with the micro- and megasporophylls of the 

 other major taxa. In these primitive laminar sporophylls, the two pairs of 

 wall-less microsporangia are deeply sunken near the center of the sporophyll. 

 The angiosperm microsporangia have long been interpreted as typically 

 superficial, with heavy walls, each pair marginal on a slender axis. We now 

 see that it is the pairs, not the individual sporangia, that appear marginal. 

 Reduction of the ancestral lamina by narrowing of the blade has so displaced 

 one member of each pair that, commonly, these two sporangia stand on the 

 sporophyll surface opposite that on which they were ancestrally borne. The 

 genus Eupomatia shows anatomical evidence that, in some taxa at least, 

 more than a displacement of two of the sporangia has occurred; the sporo- 

 phyll has been folded, as has the carpel, and complete evidence is present in 

 anatomical structure. The theory that the stamen is a folded organ like the 

 carpel was proposed more than one hundred years ago, but soon forgotten. 

 The anther-sac wall, which is commonly considered the sporangium waU, 

 represents the reduced blade of the broad ancestral sporophyll. The embedded, 

 wall-less character of the microsporangium is probably of great importance in 

 our search for the ancestors of the angiosperms. 



The carpel is usually considered a tightly closed structure, but the primitive 

 carpel is a loosely closed, or still open, laminar organ, without style or stigma, 

 receptive to pollen along the entire ventral margin. Placentation was originally 

 laminar; the ovules, like the microsporangia, are not primitively marginal or 

 submarginal. Nor, when all evidence is considered, are the ovules of any 

 angiosperm taxon borne morphologically on the receptacle, as formerly 

 generally believed. 



On the new evidence of primitiveness, the most primitive dicotyledons are 

 not the Amentiferae (a heterogeneous lot at best) but the woody Ranales 



