126 MORPHOLOGY OF THE ANGIOSPERMS 



be absent and the wall two-layered, consisting only of epidermis and 

 fibrous layer, or of the fibrous layer alone, when the epidermis has 

 been lost ontogenetically. 



The fibrous layer is associated, mechanically, with dehiscence and 

 varies in position, extent, and structural details with type of dehiscence. 

 In the primitive types, the fibrous layer covers the sporogenous tissue 

 and extends laterally over adjacent sterile tissue somewhat beyond the 

 fertile tissue, as in Degencria and Himantandra (Figs. 67 and 51); in 

 advanced anthers, the fibrous layer may be more extensive, covering 

 the protuberant areas and, in some high types, the entire anther. In 

 anthers with apical dehiscence, the fibrous layer is usually absent, ex- 

 cept in the region of the pores. In the elaboration of mechanical de- 

 hiscence, a series apparently exists in increase in extent of the fibrous 

 layer and in simplicity of wall structure, accompanying increase in pro- 

 trusion and freeing of the anther sacs. The anther wall becomes simpler 

 by reduction of cell layers; the layers between the tapetum and the 

 fibrous layer are reduced to one layer, and this also may be lost in the 

 highest types. Even the epidermis may be lost — in part or wholly — and 

 the fibrous layer take its place as the outermost layer, its cells remain- 

 ing fibrous or becoming epidermislike. The loss of layers may be 

 ontogenetic — occurring early or late in the development of the anther 

 — or phylogenetic. Ontogenetic breakdown of the inner layers occurs 

 at about the time of sporocyte formation. In delicate anthers, like those 

 of the grasses, the wall of the anther sacs may have lost all but the 

 outer two layers, the epidermis and a one-layered endothecium. Rarely, 

 the epidermis may degenerate early or be lost just before dehiscence 

 and the anther wall described as one-layered. This one-layered wall 

 consists, at least in part, even in highly specialized anthers, of connec- 

 tive or laminar cells. There is, morphologically, no microsporangium 

 wall in angiosperms — an important character of these plants. (Anther- 

 wall structure is further discussed under Dehiscence of the Anther.) 

 The anther lobe or sac is sometimes called a sijnangium, but, mor- 

 phologically, this is inaccurate, especially when a "significant" resem- 

 blance is seen to the synangium of the Bennettitales. The cycadophyte 

 synangium consists of two connate sporangia; the angiosperm anther 

 lobe consists of two sporangia individually encased in sporophyll tis- 

 sues, sometimes widely separated by them. 



In the massive reorganization of the tissues of the fertile part of the 

 laminar stamen to form the anther, the areas in which the pairs of 

 sporangia lie become protuberant — progressively more and more 

 markedly so, with increased specialization in sporophyll form. The 

 sporangia of the more primitive stamens, sunken in the mesophyll, are 

 wall-less (Fig. 67). With the reduction of the lamina, the sterile tissues 



