THE STRUCTURE OF FERNS. 17 



kinds, it is projected beyond the margin, and the little cases 

 of seeds are collected around its free extremity. More 

 commonly, however, the veins stop within the margins, and 

 the seed- cases grow in round or elongated clusters, situate 

 at their ends or along their sides, and protruded through 

 the skin of the lower surface of the fronds. 



No flowers are produced, but the plants bear, generally, 

 great abundance of seed-like bodies, which are technically 

 called spores, and are contained in little cases of very sin- 

 gular construction. Collectively, these cases and their 

 contents are called the fructification. The seed-cases, as 

 already remarked, are attached in the different species to 

 certain determinate thickened portions of the veins, which 

 points of attachment are called the receptacles. Each 

 separate mass or cluster of the seed-cases is called a sorus, 

 but as they are generally spoken of collectively, the plural 

 term sori becomes much more frequently used. 



The seed-cases called also spore-cases, or sporangia, or 

 tlieca are mostly minute roundish-oval bodies, containing 

 one cavity, and nearly surrounded by an elastic vertical 

 band or ring, which is continued from the base so as to 

 form a short stalk, by which they are attached. TV hen 

 they have reached maturity, the elasticity of the ring 



