CHAPTER XIII. THE PTERIDOPHVTA. 35 I 



CHAPTER XIII.— THE PTERIDOPHYTA (Cont'd). 



Class II. — The FiLiciNiEiE, or Ferns. 



These are plants with solid, mostly unbranching, or but spar- 

 ingly branching stems, which, in our species, are all subterranean : 

 but in some tropical or sub-tropical forms rise above ground and 

 form scaly trunks, sometimes of considerable size. They all 

 increase in length by the division of a single apical cell. 



The leaves are more highly developed than in any other 

 group of vascular cryptogams. They are ample, petioled, some- 

 times stipulate, and commonly, though not always, fork-veined ; 

 they usually unfold circinately, increase in length by an apical 

 growth, and often branch into very compound forms. 



The fibro-vascular bundles are of the concentric type, and of 

 that variety of it which has the xylem tissues located centrally, 

 and ensheathed by the phloem. In the stems the bundles are 

 usually disposed in a single circle, as has already been explained. 

 See Vegetable Histology. 



The sporangia are always borne on the leaves, either on those 

 of the ordinary form, or on those slightly modified for the pur- 

 pose. They are either borne at the margins or on the under 

 surface, most commonly in groups or clusters, each called a sorus. 

 The sporangia are cellular sacs, whose walls consist of a single 

 layer of cells, and enclose usually a considerable number of 

 spores. 



The class is subdivided into two groups, those which produce 

 but one kind of spores, and those which produce two kinds, iso- 

 sporous and heterosporous Filicineae. The former is represented 

 by one order, the Filices, or true Ferns, and the latter also by one, 

 Rhizocarpece, or Pepperworts. 



Order A. Filices. This constitutes by far the largest order 

 of vascular cryptogams, and includes all the plants we ordinarily 

 call Ferns. The spores in germinating produce at first a fila- 

 ment, one end of which soon expands into a flattened tissue 

 consisting of one stratum of cells. By further growth it becomes 



