394 



PART III. THE CLASSIFICATION OF PLANTS. 



ment ; in the Schizaeaceae the annulus is a dome-shaped group of 

 cells at the apex of the ovoid sporangium. 



As the development proceeds, the formation of the spores takes 

 place in the interior of the sporangium. The archesporial cell 

 undergoes repeated division, with the result that usually sixteen 

 cells are formed, which are the mother-cells of the spores. At 

 this stage the tapetal cells undergo disintegration, so that the 

 mass of spore-mother-cells floats freely in the liquid thus pro- 

 duced. Each mother-cell then undergoes division to form four 

 spores; the planes of division may be either such that the 

 developing spores all lie in one plane, when the mature spores are 

 bilateral (e.g. many Polypodiaceae, as Aspidium, Asplenium, Ne- 

 phrolepis, Blechnum, Polypodium, etc. ; Gleicheniaceae ; Schizaea); 



or such that the spores are ar- 

 ranged in a pyramidal manner, 

 when the mature spores are tetra- 

 liedral (Hymenophyllaceae; Cya- 

 theaceae; most Schizaeaceae; some 

 Polypodiaceae, as generally in the 

 Pterideas ; Osmundaceae : see p. 

 126). 



The sporangium of the Os- 

 mundaceaa differs in various re- 

 spects from that of the other 

 Leptosporangiate Ferns ; its ori- 

 gin from a single superficial cell 

 cannot be so easily traced, as the 

 mother-cell does not project, and 

 the succession of cell-divisions is not regular; the archesporial cell 

 is frequently not tetrahedral in form, and the resulting spore- 

 mother-cells are more numerous than in the other Ferns (as many 

 as 128) ; the sporangium is somewhat pear-shaped, with a rudi- 

 mentary annulus consisting of groups of cells situated laterally 

 towards the upper surface (Fig. 261). 



The sporangium may be sessile (Gleicheniacese, most Schizaeaceae, 

 Hymenophyllaceae) ; or shortly stalked (Lygodium, Cyatheaceae, 

 Osmundaceae) ; or it may have a usually rather long slender stalk 

 consisting of two or three longitudinal rows of cells (Polypodiaceae) ; 

 this is dependent upon whether the originally-formed stalk-cell 

 developes further or not. 



The spores are set free by the dehiscence of the sporangium; 



B 



st 



FIG. 261. Osmunda regalis. A Fertile 

 pinna with naked marginal sori(s). Some 

 mesophyll is, however, developed at the 

 base (nat. size). B A single sporangium 

 ( x 200) : st the short stalk ; r the annulus ; 

 d the longitudinal slit. 



