342 THE BIOLOGY OF FLOWERING PLANTS 



series of evolutionary changes. It has one other important 

 and constant difference from the microsporophyll of the 

 gymnosperm in the mechanism of its dehiscence. 



At maturity the pollen grains lie, usually completely 

 separated, packed together in the sporangial cavities. As 

 we have said, the stamen usually opens by longitudinal 

 slits. In the gymnosperms the opening is also by slits 

 though their direction is more varied. The actual opening 

 is partly a passive process, due to the weakening and dis- 

 solution of the cell walls along the line of dehiscence ; partly 

 it is due to the mode of drying out of a special layer of 

 cells of the anther wall, the walls of which are peculiarly 

 thickened. This opening mechanism is very ancient ; it is 

 already highly specialised in the fern sporangium, with 

 its annulus. In the gymnosperms, the specialised cell layer 

 belongs to the epiderm of the sporangium ; it is an 

 exothecium. The sub-epidermal layer in the angiosperms 

 is effective, and is termed an endothecium. 



In a good many angiosperms the active layer appears to 

 be an exothecium, but in such cases we are dealing with a 

 secondary reduction phenomenon and not with a primary 

 condition homologous with the gymnospermous exothecium. 

 In the Ericaceae a series of forms may be traced with typical 

 endothecium at the one extreme, e.g. in Clethra, and an 

 apparent exothecium at the other, e.g. in Erica. The details 

 of the reduction are treated of by Goebel and by Staedtler 

 (1923). In some submerged aquatics the endothecium fails 

 to develop and special modes of opening are found. In 

 Zannichellia a swelling of the inner cells of the wall bursts 

 the epiderm open. 



The endothecium consists of a layer of cells with their 

 long axes at right angles to the surface of the anther wall. 

 The inner walls of these cells are strongly thickened, and 

 thickened bands run up the side walls tapering off and 

 ceasing at the outer wall (Fig. 48). Steinbrinck (1906) has 

 elucidated the way in which this peculiar type of thickening 

 aids dehiscence. As the cells dry out the cohesion of the 

 diminishing water tends to produce collapse. At the inner 



