342 



SOXI. 



PABT II. 



beneath, the epidermis or skin of the leaf, which is 

 forced up as the sporangia increase in size, in the form 

 of a whitish membrane, which con- 

 stitutes the indusiurn, or protecting 

 cover of the sori. While the fruit is 

 advancing to maturity, the indusium 

 separates partly or wholly from the 

 surrounding skin or epiderm, and 

 subsequently either shrivels or falls 

 off altogether. In some few species 

 the opening is in the centre of the 

 indusium, and then it surrounds the 

 sori like a cup; in other ferns, the 

 skin from both surfaces of the leaf 

 extends beyond the margin, includes 

 the sori between it, and fulfils the 

 office of an indusium. 



The sori, as already noted, take a 

 variety of forms, and are variously 

 situated. Some are round and dot- 

 Fig. 55. pinnniedPoiypo- like, some are oblong and straight, 

 some are hippocrepiform or horse- 

 shoe-shaped, while some are continuous in a line-like 

 band. The indusium, when present, takes more or less 

 exactly the form of the sorus. These peculiarities are 

 so well marked that they are taken advantage of in the 

 discrimination of genera. 



The sporangia or spore-cases are for the most part of 

 globular form, and are nearly or quite surrounded by a 

 strong elastic ring, which in some cases is continued so 

 as to form a stalk. When the spores are ripe, this ring, 

 by its elastic force, tears open the sporangia, and gives 

 free egress to their contents. The ring assumes various 

 forms. In one large group it passes vertically up the back 

 of the sporangium, and is continued to a point called 



