402 



BOTANY 



Water-Ferns. Unlike the leaves of Phanerogams, Fern leaves continue 

 to grow at the apex until their full size is attained. The leaves of 

 the common Polypodium mdgare are pinnate, and spring singly from 

 the upper side of the branched rhizome, which creeps 'amongst Moss 

 or on rocks. In other cases the leaves may be 

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

 Fern, Scolopendrium vulgare, at one time officinal 

 and designated Herba linguae cervinae (Fig. 333). 



In the tropics many herbaceous Ferns grow 

 as epiphytes on forest trees. Peculiar brownish 

 scales (paleae, ramenta), often fringed and consist- 

 ing of a single layer of cells, invest the stems, 

 petioles, and sometimes also the leaves of most 

 Ferns (Fig. 113, p. 98). 



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 hetero- 

 phyll is exhibited : thus, in the Ostrich Fern, 

 Struthiopteris germanica, the dark-brown sporo- 

 phylls 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 num- 

 ber of species are comprised, are united in groups 

 or SORI on the under side of the leaves, at the ends 

 of or between the branches of the nerves. They are borne on a cushion- 

 like projection of tissue termed the receptacle (Fig. 332, 5), and in 

 many species are covered by a protective membrane, the indusium, 

 which is an overgrowth of the tissue of the leaf (Fig. 332, 3-5 ; 

 Fig. 334, A, i). Each sporangium arises by the division of a single 

 epidermal cell, and consists, when ripe (Fig. 334, B-E), of a capsule 

 attached to the receptacle by a slender multicellular stalk, containing 

 a large number of spores with a ribbed or warty thickened exine 

 (Fig. 334, F). 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. Through the contraction, on loss of water, of the thin 

 outer walls of the cells composing the ring, it springs backwards, and 



Fig. 333. — Scolopmdrium 

 (} nat. size.) 



