452 



BOTANY 



PART II 



The sporangia are generally produced in large numbers, on the under side of 

 the leaves. The sporopliylls, as a rule, resemble the sterile, foliage leaves. In a 



few genera a pronounced heterophylly is exhibited : thus, 

 in the Ostrich Fern, Struthiopteris germanica {Onoclea 

 StnitJnoptcris), the dark brown sporopliylls are smaller 

 and less profusely branched, standing in groups in the 

 centre of a rosette of large foliage leaves. 



In the different families, differences in the mode of 

 development, as well as in the form, position, and 

 structure of the sporangia are manifested. 



The sporangia of the Polypodiaceae, in which family 

 the most familiar and largest number of species are 

 comprised, are united in groups or SORI on the under 

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

 projection of tissue termed the receptacle (Fig. 396, A), 

 and in many species are covered by a protective 

 membrane, the indusium, which is an outgrowth of 

 the tissue of the leaf (Fig. 396, B, C). Each spor- 

 angium arises by the division of a single epidermal cell, 

 and consists, when ripe (Fig. 398), of a capsule attached 

 to the receptacle by a slender multicellular stalk, con- 

 taining 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 tlio 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 aud determines the dehiscence of the sporangium by a transverse slit 



Fig. 397. — Smlopendnum rid 

 igare. (J nat. size.) 



Fifi. 398. — Sporangia. A, AxpkUnni Jllh: vuxs; there is a glandular hair at the base; U and C, 

 Ahophila armata, seen from the two sides; D, Aiieimia caiidnta; E, Ustnuiula regalls. 

 {A-D X 70 orig. ; K x 41), after LUrssen.) 



between tlie broad terminal cells of the annulus. Wlieu tlie pull exerted by the 

 cohesive jwwer of tlu; water suddenly gives way, the annulus returns by its 



