DEVELOPMENT OF PLANTS 325 



portions of it turn out here and there, thus forming the vascular 

 bundles that extend into the leaves. Such an arrangement is 

 seen in Botrychium (Fig. 216). These spaces formed in the ring, 

 called foliar gaps, are often quite large so that the cylinder of 

 vascular tissue in the stem presents the appearance of a lattice 

 work^ — note the vascular tissue at the right-hand end of the stem 

 in Fig. 220. In cross section such a cylinder would appear as a 

 series of more or less widely separated strands (Fig. 221). This 

 latter figure shows that strands arising from the lattice-like 

 cylinder may also extend into the pith-like region — note the three 

 large strands in Fig. 221, v. This explains the confusing 

 arrangement of strands often seen in the cross section of fern 

 stems and previously noted in monocotyledons, p. 96. Certain 

 cells of the cortex and pith often become modified into strengthen- 

 ing 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 sporangia, 

 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 

 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- 

 tected by a membranous outgrowth of the epidermis, known 

 as the indusium (Fig. 222, B). Each sporangium originates 

 usually from a single epidermal cell, which by repeated divisions 

 (Fig. 223) produces a capsule or sporangium that contains the 

 spores. A sporangium of the shield fern contains 48 spores and 

 there are fully 100 sporangia to a sorus and 20 sori on each lobe 

 or pinna of the leaf. A well-developed leaf would have on an 

 average 50 pinnae and a healthy plant would bear about 10 



