294 



SPORANGIA OF FILICALES 



the concentric type (Fig. 221) and usually form a hollow net- 

 work in the stem (Fig. 220, .r). Certain cells of the cortex and 

 pith often become modified into strengthening cells of stereome 

 (Fig. 221, st). Roots arise near the base of the leaves, and in 

 some of the tree ferns form a thick mat-like covering on the 

 stems. They originate from the endodermis of the bundles and 

 possess a root cap and radial arrangement of the vascular bundles 

 as in higher plants. 



(a) Structure and Character of the Sporangia. The spor- 

 angia, instead of being produced in the tissues of special branches 

 as in Ophioglossum, are borne in curiously constructed capsules, 

 usually situated on the under surface of the ordinary green 

 leaves (Fig. 222). The sporangia-bearing leaves are usually 



FIG. 222. Arrangement of the sporangia: A, lobe of leaf of Dryopteris 

 with sporangia grouped in circular sori, s. B, sorus enlarged, showing the 

 shield-like membrane or indusium, in, covering the sporangia, sp. C, lobe 

 of leaf of Asplenium with elongated sori, s. 



called sporophylls, meaning spore-bearing leaves. In some cases 

 the sporophylls are highly modified, being entirely given up to 

 spore production and therefore quite different from the green 

 leaves (Fig. 227). The sporangia are usually associated in 

 groups or sori (sing, sorus) on the vascular bundles and pro- 



