. Practical botany. Botany. THE PTERIDOPHYTES 291 foliage leaf. In the sensitive fern (Fig. 241) and ostrich fern and some others the sporophyll and foUage branches rise separately from the rhizome. This division of labor in fern leaves, resultmg in development of distinct sporophylls and foliage leaves, is a great advance. Setting apart special structures for special pieces of work (division of labor) ordinarily increases the quality and quantity of V70rk done. 266. The water ferns. Thewater ferns are not really members of the true fern class but are closely related thereto. Their water hab-
. Practical botany. Botany. THE PTERIDOPHYTES 291 foliage leaf. In the sensitive fern (Fig. 241) and ostrich fern and some others the sporophyll and foUage branches rise separately from the rhizome. This division of labor in fern leaves, resultmg in development of distinct sporophylls and foliage leaves, is a great advance. Setting apart special structures for special pieces of work (division of labor) ordinarily increases the quality and quantity of V70rk done. 266. The water ferns. Thewater ferns are not really members of the true fern class but are closely related thereto. Their water hab- itat is striking. Marsilia (Fig. 242) has peculiar leaves, looking like the four-leafed clover, and these float upon water or stand slightly above it. The plant is fairly abundant in greenhouses and park pools. Salvinia (Fig. 243) and Azolla are also widely distributed free-floating water Fig. 243. A water fern (Salvinia) The plant has two rows ol hairy leaves and one row ol water leaves (which look like roots). Natural size HORSETAILS OR SCOURING RUSHES (EQUISETINE^E) 267. General characteristics. This class now consists of one genus QEquisetum) and a few species (variously estimated at from twenty to thirty). The class and closely related classes were once abundantly represented, and as tree-like forms were a prominent part of the earth's flora. Fossil remains of equise- turns and their relatives tell interesting stories of these tree- like forms which lived during the chief coal-forming periods. In studying present-day forms we are observmg the remi:iants of the former abundant plant life of this class. Equisetums now live chiefly in regions unfavorable to most plants, — around open marshes, in sandy wastes, and along railway embankments. They have hard, rough, siliceous, and. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly
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