222 BOTANY. 



plex in structure, until finally the full form is reached, and 

 still later the full size. This stem, bearing leaves and 

 roots, constitutes the asexual plant (sporophore), which is 

 sharply contrasted with the sexual plant (gametophore) in 

 structure, size, and duration, the latter being short-lived, 

 small, and of simple structure, while the former is long- 

 lived, often of large size, and of great complexity of 

 structure. 



413. The classification of ferns is based almost wholly 

 upon the structure of the asexual plant. Four orders, in- 

 cluding about 3500 species, are usually recognized, as 

 follows : 



414. The Adder-tongues (Order 38, Ophioglossace^) 

 include a few species of fern-like plants, in which the 

 spores develop from cells in the tissue of the leaves. Those 

 portions of the leaves which produce spores are much 

 changed in size and shape (Fig. 131, /) and are strikingly 

 different from the foliage segments. The spore-cases {eu- 

 sporangia) are rounded, and split open by a simple fissure 

 of the tissues. The leaves are of slow growth, and are 

 straight or folded (not rolled) in the bud. The sexual 

 plant is known in few cases, but it appears to be a rounded 

 body, with little, if any, chlorophyll, growing a little below 

 the surface of the ground. 



Two genera, Opliioglossum, Adder- tongues proper, and Botrychi- 

 um, the Moonworts, are represented in tlie United States by ten or 

 eleven species. 



415. The Ringless Ferns (Order 29, Marattiace^) 

 constitute an interesting group, of mostly tropical ferns, 

 now including but few species (20 to 25), but in geological 

 times represented by many species. Their spore-cases 

 are eusporangiate, i.e., they develop from internal leaf -cells, 



