CHAPTER XII. — THE PTERIDOPHYTA. 347 



cellular bodies, with walls composed of a single layer of cells, 

 and borne on the surface of the prothallium. In the interior a 

 number of small rounded cells are produced, each of which 

 emits a spirally coiled antherozoid, provided at its anterior or 

 smaller end with numerous vibratile cilia. Both organs are 

 usually borne on the under surface of the prothallium, and the 

 antherozoids are set free when the surface is bathed in water, 

 and through this medium they swim about, finding their way 

 ultimately to the mucilage discharged at the orifice of the arche- 

 gonium, and penetrate through it to the germ-cell. The latter 

 now begins to develop in the bottom of the archegonium, and 

 by means of a root-like process which it sends into the tissues 

 of the prothallium, it for a time derives nourishment from it, 

 but soon forms roots, stems and leaves of its own, and becomes 

 independent. 



In the Mosses, under the same circumstances, only a spor- 

 gonium, which remains attached to the parent plant, and 

 seems a part of it, is produced ; but here it develops into a 

 conspicuous and highly organized plant, usually with an unlimi- 

 ted period of growth, and possessing roots, stem and leaves. 

 This plant at maturity bears, usually either on the ordinary 

 leaves, or on others specially modified for the purpose, a multi- 

 tude of spores corresponding to those produced in the capsules 

 of Mosses. These are borne in organs called sporangia, which 

 may be regarded as modified hairs. 



Great differences exist in different members of the Series, in 

 the degree of development which the prothallium attains. In 

 the Ferns, and other cf the lower members of the group, it is 

 comparatively well developed, while in some of the higher forms 

 it tends to disappear entirely, being reduced to one, or a very 

 few cells, which do not emerge from the coats of the germinat- 

 ing spore. 



Some of the species produce but one kind of asexual spores, 

 while others produce two sets, one of large size, called macro- 

 spores, which, in germinating, produce prothallia bearing arche- 

 gonia only, and a smaller, called microspores, whose prothallia 

 produce antheridia only. The former species are called isospo- 

 rous, the latter, heterosporous. • 



Three distinct classes are usually recognized: (i) the Equi- 

 setinea, (2) the Filicineoz, and (3) the Lycopodineoz. 



