THE STAMEN 139 



cells, are not fiberlike, but the term is an old one and will probably 

 continue in use. Histologically, the fibrous layer may be added to be- 

 low the sporogenous and tapetal cells, by transformation of parenchyma 

 cells of the connective so that the inner tissues are completely encircled 

 by the fibrous layer. The parietal layers are continuous with sterile cells 

 of the connective. 



In further development, the epidermis usually remains simple but its 

 cells may be velvety papillose, as in Gloxinia and Gladiolus, and the 

 hairs may cover the entire anther, as in species of Calijcanthus. Stomata 

 — doubtfully functional — are frequent on the connective, rare over the 

 sporangia. Where the fibrous layer is strongly developed, the epidermal 

 cells may be tabular and thin, appearing collapsed and scalelike. They 

 may be early deciduous or lost, as dehiscence approaches, over the 

 entire anther or over restricted areas. 



As cells of the fibrous layer mature, lignified or suberized thickenings 

 develop in the walls, contrasting strongly with the thin, cellulosic wall 

 areas between. The thickenings take the form of bars, spirals, or an- 

 nular bands and often resemble the thickenings of the cell walls of 

 protoxylem, to which they have even been considered morphologically 

 equivalent and, therefore, of phylogenetic significance. But they are 

 doubtless of only ecological importance, representing a structural mech- 

 anism involved in dehiscence (Fig. 68). Under drying, the thick and 

 thin wall areas shrink unequally, and the layer, with attached layers, is 

 ruptured. Continued drying of the wall tissues brings about retraction of 

 the borders of the opening; changes in atmospheric humidity cause 

 opening and closing of the rupture. In some genera, as in Liliinn, the 

 rupture is through a stomium, a group of specialized epidermal cells 

 similar to those of the stomium of some ferns. It was long believed, with 

 the existence of the stomium as evidence, that dehiscence was brought 

 about by the epidermis alone. But the epidermis may be lacking at time 

 of dehiscence, especially over the lines of rupture. 



Where the sporangia are deeply sunken, the sporogenous cells are 

 covered externally by only two layers, the epidermis and the fibrous 

 layer. The fibrous layer extends a short distance beyond the sporangium, 

 as in Degeneria (Fig. 67) and Himantandra. With increasing specializa- 

 tion of the anther, the fibrous layer extends farther and farther around 

 the anther lobes, developing internally from parenchyma of the con- 

 nective where it is often irregular in thickness. The parietal layers are 

 often flattened by enlargement of the tapetal and sporogenous cells 

 and may be crushed and absorbed. This is especially true where there 

 is but one layer. Where the connective is greatly reduced, the fibrous 

 layer may extend all over the anther. In anthers with poricidal or val- 

 vular dehiscence, the fibrous layer is usually absent or very weak. 



