THE PTERIDOPHYTES 281 



or by the recurved leaf margin, which is known as a false indu- 

 sium (Fig. 232). Each species of fern has a regular position 

 in which its sori appear, and in some cases their arrangement 

 and relation to the veins are used in distinguishing species 

 from one another. 



In most of the common ferns the sporangia are of the 

 form shown in Fig. 233. Each consists of a stalk, at the free 

 end of which is a flattened capsule. Within the capsule, by 

 division of the tissues, numerous asexual spores are formed. 



FIG. 233. A fern sporangium, showing its behavior in the process of 



distributing its spores 

 Much magnified. After Atkinson 



The capsule wall is extremely thin and consists of a single layer 

 of cells. From the end of the sporangium stalk over the cap- 

 sule, and halfway to the stalk again, is a row or ring (annulus) 

 of cells, which have heavy walls on all but the outer side. 

 At the tip of the annulus is the capsule mouth, about which 

 are two pairs of specially formed cells known as the lip cells. 

 When the spores within the capsule are ripe, the indusium 

 becomes dry and turns up sufficiently to expose the sporangia. 

 The annulus upon a ripe capsule then begins to dry. Its outer 

 wall, being thin, permits its cells to contract as the whole 



