448 



BOTANY 



In tlie different families, differences in the mode of development, as well as in 

 the form, position, and structure of the SPOI:AM;I.\ are manifested. 



The sporangia of the Polypodiaceae, in which family 

 the most familiar and largest number of species are com- 

 prised, are united in groups or SOKI on the under side 

 of the leaves. They are borne on a cushion-like pro- 

 jection of tissue termed the UKCKPTACLK (Fig. 407 A], 

 and in many species are covered by a protective 

 membrane, the IXDUSIUM, which is an overgrowth of 

 the tissue of the leaf (Fig. 407 B, C). Each spor- 

 angium arises by the division of a single epidermal 

 cell, and consists, when ripe (Fig. 409), of a capsule 

 attached to the receptacle by a slender multicellular 

 stalk, containing a large number of spores. The wall 

 of the capsule is formed of a single layer of cells.' A 

 row of cells with strongly thickened radial and inner 

 walls, extending from the stalk over the dorsal side and 

 top to the middle of the ventral side of the capsule, are 

 specially developed as a ring or ANNULUS, by means of 

 which the dehiscence of the sporangium is effected. 

 This type of annulus is characteristic of the Poly- 

 podiaceae. 



On drying of the wall of the sporangium the co- 

 hesion of the remaining water in the cells of the 

 annulus draws in the thin outer walls of these cells ; 

 this causes the annulus to shorten and determines the 

 dehiscence of the sporangium by a transverse slit 

 between the broad terminal cells of the annulus. 

 When the pull exerted by the cohesive power of the 

 water gives way, the annulus returns by its own 

 elasticity to its original position, thus effecting the 

 dispersal of the spores. The sporangium remains 

 open owing to the drying and contraction of the thin cell walls. 



FIG. 408. Scolopendrium vul- 

 gare. (J nat. size.) 



FIG. 409. Sporangia. A, Aspidium Jttix mas ; there is a glandular hair at the base ; li and < , 

 Alsophila armata, seen from the two sides ; D, Aneimia caudata; E, Osmunda rrgalls. 

 (A-D x. 70 orig. ; E x 40, after LURSSEN.) 



The form and insertion of the sori, the shape of the indusium when present, 

 or its absence, all constitute important criteria for distinguishing the different 



