PITjENOGAMIA. 157 



considerable thickness and height. They contain flat fibro- 

 vascular bundles, usually arranged in a circle. When the 

 stems become thick with increase of growth, a net-work 

 of anastomosing bundles is formed in place of the central 

 bundle. Ferns appeared in the Devonian Age, repre- 

 sented by twelve genera, belonging to extinct families. In 

 the Carboniferous Age they were much more numerous, 

 but decreased to the present time. These plants are very 

 ornamental, but otherwise of comparatively little value 

 economically. The largest and commonest genera are 

 Asplenium, Aspidium, Botrychium, Cystopteris, etc. 



3. Lycopodineae. The stems of the Club-Mosses are 

 solid, leafy, and mostly erect. The leaves are simple, 

 small, sessile, imbricated, and resemble those of the Mosses. 

 The spores are produced in sporangia, situated in the axils, 

 and are appendages of the leaves. In some of the genera 

 (JLflfGopodium, etc.) the spores are all alike; in others 

 {Selaginella, etc.) there are two kinds — large spores 

 (macrospores) and small spores (microspores). The plants 

 of this class, now generally terrestrial, and only a few 

 inches high, were numerous in the Devonian and Carbon- 

 iferous Ages. Some of them {Lepidodendron, etc.; Fig. 

 345) were of gigantic size, but the order to which they 

 belonged became extinct in the Permian Period. Several 

 species of I/ycopodium occur in the United States. Many 

 species of Selaginella, which are mostly tropical, are culti- 

 vated for ornament. 



PH^NOGAMIA. 

 179. The seventh and last division is called Phaenero- 

 gamia, and includes all the common flowering plants ; as 

 herbs, shrubs, and trees. The reproductive organs (Fig. 



