2l6 



THE ANATOMY OF WOODY PLANTS 



in the present connection to begin the discussion of the sporangium 

 with the consideration of the situation presented by the lowest 

 living gymnosperms, the Cycadales. Fig. 160 shows the organiza- 

 tion of a microsporangium in Zamia muricata. The structure in 

 question is covered on the outside by an envelope of thick-walled 

 cells which in the condition of maturity determine its dehiscence. 

 The mechanical structure is known as the annulus and is of great 



importance in bringing about the 

 distribution of the spores, particu- 

 larly in the lower Vasculares, in 

 which it takes the place of the 

 elaters found in many of the liver- 

 worts. The annulus is plainly an 

 epidermal structure, both because 

 it is actually the external layer of 

 the sporangium and because its con- 

 tinuity is interrupted by the pres- 

 ence of stomata. These can be seen 

 in the figure in profile view. 



The situation in regard to the 

 annulus in the Pteridophyta may be 

 briefly summarized. In lower forms 

 the thick-walled epidermal cells 

 which serve as the mechanism for 

 the opening of the sporangium are 

 massive in their development, while in the higher forms of the 

 vascular cryptogams the amount of mechanical tissue tends to 

 become more and more restricted. Fig. 1610 shows the structure 

 of the sporangium and its annulus in Selaginella. The mechanical 

 layer in this case is extensive and is almost coextensive with the 

 surface of the spore sac. In contrast to the conditions shown in 

 Selaginella are those presented by many of the ferns. In Fig. i6ib 

 is reproduced the organization of the sporangium of Polypodium 

 vulgare as an illustration of the higher type of annulus in the 

 Pteridophyta. The opening mechanism here constitutes an incom- 

 plete vertical ring and in consequence literally merits the name of 

 annulus. The further consideration of the types of annulus 



FIG. 160. Sporangium of 

 Zamia muricata. 



