140 MORPHOLOGY OF THE ANGIOSPERMS 



The Thin-walled Anther. It was long ago stated that the microsporan- 

 gium of the pteridophytes and gymnosperms, with the exception of 

 Ginkgo, had only an exothecium (epidermis); that the angiosperms, in 

 contrast, had both an exothecium and an endothecium (cell layers be- 

 tween the epidermis and the sporogenous and tapetal tissues). The 

 validity of this distinction was questioned when it was found that the 

 anther sacs of many angiosperms had walls of only one or two cell 

 layers. It had been pointed out that thin-walled anthers belonged chiefly 

 to taxa then generally considered primitive — Casuarinaceae, Piperaceae, 

 Proteaceae, Urticaceae — and the similarity in sporangium structure to 

 that of the gymnosperms gave support to the view that these taxa were 

 among the most primitive angiosperms. But it was, even at that time, 

 argued that these families were highly specialized in many other char- 

 acters, that the simple wall structure of the anthers was the result of 

 reduction in specialization. Only with the general recognition that these 

 families are not liighly primitive, and that the woody Ranales are the 

 primitive angiosperms, has the phylogenetic significance of the thin- 

 walled anther become apparent. 



The deeply sunken sporangia of laminar stamens, without a sporan- 

 gium wall and with their sporogenous tissues surrounded by sterile 

 cells of the microsporophyll, have important phylogenetic significance. 

 They set the angiosperms well apart in method of microspore-bearing 

 from other vascular plants, except the primitive eusporangiate ferns. 

 The multiseriate anther wall suggests possible relationship of the angio- 

 sperms to ancient eusporangiate ferns. 



In the 1920s, the uniseriate anther wall was correctly recognized as 

 a reduced structure, but the interpretation received little attention. The 

 evidence presented then can well be outlined here, because it supports 

 the growing opinion that the families in which it occurs are advanced, 

 not primitive. 



Reduction of the wall layers comes about by ontogenetic or phylo- 

 genetic loss of the innermost or the outermost layers, or of both of these 

 layers. Loss of the epidermis may occur early in ontogeny of the anther 

 or just before dehiscence, or vestiges may remain, especially over the 

 line of fusion of the two sporangia in a sac (Fig. 59C, G, H). Scattered 

 remnant epidermal cells may persist over the entire anther sac. Where 

 the epidermis is degenerate or reduced, the fibrous layer takes its place 

 as the outermost layer, retaining its characteristic cell-wall thickenings, 

 as in Casuarina. 



Variety of structure in thin-walled anthers and the evidence of re- 

 duction from thicker-walled types are shown in the following examples. 

 All stages in the reduction can be seen in the Urticiflorae. Fictis — per- 

 haps alone in the order — shows a typical anther wall, with well-de- 



