FERNS 



191 



answered question. The great interest of the Anthoceros 

 forms ( 99) is due to the fact that their sporophytes are 

 green and do chlorophyll work; and this has suggested the 

 thought that from such green tissue leaves have been de- 

 veloped, and thus a leafy sporophyte has been started. 



109. Sporangia. Upon the under surface of fern 

 leaves dark dots or lines are often seen (Fig. 182). These 

 are groups of sporangia, usu- 

 ally occurring along the veins 

 of the under surface, but some- 

 times in long lines along the 

 edge, the margin of the leaf 

 rolling in and protecting them, 

 as in maidenhair fern and com- 

 mon brake (Fig. 183). In ferns 

 having the groups of sporangia 

 away from the margin, each 

 group (sorus) is usually pro- 

 tected by a delicate flap (indu- 

 sium) growing out from the epi- 

 dermis, sometimes forming a 

 pocket (Fig. 182, A) and some- 

 times an umbrella-like or shield- 

 like covering (as in shield ferns) 

 (Fig. 182, B). The position and 

 the shape of the sorus and the character of the indusium 

 furnish useful characters in the classification of ferns. 



Most fern leaves do chlorophyll work and produce 

 sporangia, two very distinct kinds of work. In some 

 ferns, however, some of the leaves are sterile, that is, do 

 not produce sporangia, the other leaves doing both kinds 

 of work; while in other ferns certain leaves or leaf branches 

 are set apart to produce sporangia and do no chlorophyll 

 work, and vice versa, the two kinds of work thus being 

 divided among the leaves or leaf branches. Such a 



FIG. 183. Sporangia of ferns, show- 

 ing marginal lines of sporangia 

 protected by the inrolled margin 

 of the leaf: A , the common brake; 

 B, maidenhair fern. 



