THE STAMEN 125 



though the similarity is obscured. Primitively, both are laminar; in 

 advanced forms, submarginal; in specialization, the laminar form is lost 

 in microsporophylls, greatly reduced in megasporophylls; the mega- 

 sporangia are primitively many; the microsporangia always four (higher 

 numbers represent divided sporangia or united sporophylls ) . Reduction 

 to two or one sporangia is uncommon in stamens, common in carpels. 

 Parallelism in the two types of sporophylls is to be expected. Number 

 and distribution of sporangia are correlated with form of sporophylls. 



Primitively, the sporangia were deeply embedded in the tissues of the 

 sporophyll — Degenerla, Michelia, Himantandra, Ceratophijlhim (Figs. 

 67, 51, and 49). In the specialization of the anther, progressive reduc- 

 tion and reorganization of the sterile tissues have freed the sporangia 

 from their burial in the lamina, by the formation of anther lobes 

 (Fig. 56). But some of the sterile tissues remained as protective layers 

 about the sporangia and formed the heavy anther-lobe walls. The spo- 

 rangia within the anther lobes are wall-less, as in the laminar sporo- 

 phyll. Each lobe of the anther contains the pair of sporangia of one 

 side of the ancestral stamen. As the anther sacs became strongly pro- 

 tuberant, they were gradually freed from the connective (Fig. 56f/, e, 

 f), until ultimately nearly free [versatile anthers (Fig. 56/)]. The separa- 

 tion began at either the distal or the proximal end, or at both ends 

 simultaneously, with resulting variations in point of attachment. Where 

 the anther lobes are nearly "free," they are attached to a small median 

 remnant of the connective, the wall is thin, and the anther consists 

 largely of sporangia, two in each half ( Fig. 56/ ) . An anther of this type, 

 freed at both ends, is typically X-shaped, with the ends of the halves 

 often spreading — many grasses. Even where the anther lobes are nearly 

 free and the form and position of the sporangia are prominent, the 

 sporangia themselves are not free — in the sense of superficial — for they 

 remain sheathed by a few layers of sporophyll tissue. 



Anther-sac Wall and "Sporangium Wall" 



The wall of the anther sac is usually described as made up of an epi- 

 dermis, a hypodermal or fibrous layer, and one to several parenchym- 

 atous layers. The term exotheciiim has been used for the two outer wall 

 layers — the epidermis plus the fibrous layer — and endotheciuni for layers 

 between the fibrous layer and the sporogenous tissues. Also, the epi- 

 dermis alone has been called the exothecium, and all layers within it, 

 the endothecium. There has been no consistency in the use of these 

 terms; they cause constant confusion in interpretation and have no 

 morphological value. In unspecialized anthers, the epidermis resembles 

 that of the filament; in highly specialized anthers, with strongly pro- 

 tuberant sporangial areas, the intermediate parenchymatous layers may 



