VEGETATIVE REPRODUCTION. 229 



in many species thicken their walls along one or more spiral 

 lines. These sterile cells are called elaters (fig. 11, A, A'). 

 They serve to entangle the spores in clusters when they are 

 set free. The sporangium opens at maturity by splitting at 

 the apex, sometimes into two, commonly into four or more, 

 parts (fig. 64). 



322. Mosses. — In most mosses the sporangium is developed 

 within the enlarged upper part of the sporophyte, to which 

 the name capsule is given. In the peat mosses it is cap- or 

 thimble-shaped (fig. 230), while in most of the true mosses 

 it is a hollow cylinder (fig. 231). It opens by the falling off 

 of the sterile upper end of the capsule, which separates as a 

 lid and thus allows the spores to escape from the upper 

 end of the cylindrical sporangium. By the time the spores 

 are mature, the sterile central tissue of the capsule, which 

 forms the columella (c, fig. 231), shrivels and often almost 

 disappears, so that the capsule seems to be a cup or urn, filled 

 with loose spores. In the younger stage (fig. 232) the orig- 

 inal form is shown. 



323. Ferns. — In the ferns the sporangia are usually nu- 

 merous, stalked, free, and often associated in clusters 

 called sori. They are either produced upon the under sur- 

 face of the foliage leaves or upon specialized leaves.* The 

 sori are often arranged in elongated clusters or lines 

 (fig. 233). Each sorus, or a cluster of them, may be pro- 

 tected by a special outgrowth from the cells in its neighbor- 

 hood, called an indusium (figs. 233, 234). Each sporangium 

 consists of a stalk composed of two or four rows of cells ex- 

 panding above into a body composed of a single outer layer 

 enclosing the spore-producing cells, and at maturity the 

 spores themselves. The walls of a row of cells more or less 

 completely encircling the body of the sporangium become 



* It must be remembered that the entire plant, consisting of root, stem, 

 and leaves, is the homologue of the capsule and stalk of the mossworts. 



