DEVELOPMENT OF PLANTS 311 



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 

 leaves. Therefore, Bower estimates that the shield fern produces 

 annually upwards of 50 million spores. This makes a striking 

 comparison when we consider the spore production in the Bryo- 

 phyta, and it is evident that ample provision is made for the 

 maintenance of the race notwithstanding the postponement and 

 specialization in spore production. 



The sporangia vary considerably in structure in the various 

 genera. In the simplest forms, they consist of a uniform layer of 

 wall cells inclosing a mass of cells, the majority of which become 

 spore mother cells, producing four spores each. In other cases, 

 a few of the wall cells are thickened, forming a rudimentary 

 annulus (Fig. 224, A), but in the majority of our common ferns, 

 the sporangium consists of a delicate stalk, supporting a rather 

 spherical spore-bearing capsule (Fig. 224, B). The walls of this 

 capsule consist of a single layer of thin-walled cells except for a 

 row of thickened cells, the annulus, that extends from the stalk 

 over the capsule nearly to the opposite side (Fig. 224, B, an). 

 At the latter point, the annulus ends in a few rather weakened 

 cells, two of which, the lip cells, are conspicuousfor their larger size 

 (Fig. 224, B, I). It is to be noticed that the cells of the annulus 

 are thickened on their inner and radial walls while the outer 

 walls remaifi comparatively thin. When the spores are mature 

 and lie loose in the capsule, the cells of the annulus that have 



