508 



BOTANY 



PART II 



the upper side of the creeping branched rhizome. In other cases 

 the leaves may be simple and undivided, as in the Hart's-Tongue 

 Fern, Scolopendrium vulgare (Fig. 472). In the tropics many herbaceous 

 Ferns grow as epiphytes on forest trees (cf. p. 183). When young, 

 the leaves are coiled at the tips (Fig. 470), a peculiarity common to 

 the Ferns as a whole, and to the Water-Ferns. Unlike the leaves of 

 most Phanerogams, those of the Ferns continue to grow at the apex 

 until their full size is attained. Peculiar brownish scales (paleae, 

 ramenta), often fringed and consisting of a single layer of cells, invest 

 the stems, petioles, and sometimes also the 

 leaves of most Ferns. 



The sporangia are generally produced in 

 large numbers, on the under side of the 

 leaves. The sporophylls, as a rule, resemble 

 the sterile, foliage leaves. In a few genera 

 a pronounced heterophylly is exhibited : thus 

 in the Ostrich Fern, Struthiopteris germanica, 

 the dark brown sporophylls are smaller and 

 less profusely branched, standing in groups 

 in the centre of a rosette of large foliage 

 leaves. Blechnum spicant is another example. 

 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 SORT on the under side of the 

 leaves. They are borne on a cushion -like 

 projection of tissue termed the RECEPTACLE 

 (Fig. 471 A), and in many species are covered 

 by a protective membrane, the INDUSIUM, 

 which is an outgrowth of the tissue of the 



FIG. 472. Scolopendrium milgare. 



(inat. size.) leaf (Fig. 471 B, C). Each sporangium 



arises by the division of a single epidermal 



cell (Fig. 466), and consists, when ripe (Fig. 473), of a capsule at- 

 tached to the receptacle by a slender multicellular stalk, containing a 

 large number of spores, which only in a few genera (Asplenium, Aspidium, 

 Acrostichum, etc.) are surrounded by a perispore. 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 Polypodiaceae. 



