22), and this consists of only 25 species. 

 These living forms are commonly called 

 horsetails. Also they may be referred to as 

 the scouring rushes. The stems contain many, 

 fine, sharp particles of silicate. This made 

 them useful as an abrasive material for clean- 

 ing pots and pans, especially in the early 

 Colonial days. 



Typically the dominant sporophyte (Fig. 

 31-22) displays well-developed vascular ele- 

 ments (both xylem and phloem). There is a 

 prostrate stem, from which true roots extend 

 downward into the soil and numerous 

 branches pass upward into the air. The stem 

 and its branches are characteristically hol- 

 low, and display conspicuous joints (nodes) 

 and longitudinal ribs. The leaves, at least in 

 modern species, tend to be very small scales, 

 which come off in whorls at the nodes (Fig. 

 31-22). Typically the spores are produced by 

 a cone, or strobilus. This consists of a com- 

 pact club-shaped group of sporangium-bear- 

 ing modified leaves (sporophylls) found at 

 the tip of a specialized branch (Fig. 31-22). 

 Sporulation, as in all higher plants, is meiotic 

 and each spore has the potential of giving 

 rise to a haploid gametophyte. Typically the 

 gametophyte is a semimicroscopic green 

 thallus, nutritively independent of the sporo- 

 phyte, but poorly adapted to terrestrial con- 

 ditions. For this reason and because the free- 

 swimming sperm require water if any is to 

 reach an egg, the Sphenopsida tend to be re- 

 stricted to swampy regions or to unusually 

 rainy localities. 



Apparently the Sphenopsida did not pro- 

 vide a direct evolutionary line leading to any 

 of the higher plants. Like other primitive 

 Tracheophyta, the Sphenopsida seem to have 

 arisen from the Psilophytales (p. 563) during 

 the Devonian period. They reached a peak 

 in the Carboniferous period (Fig. 31-23); but 

 by the end of the Paleozoic era, they had 

 almost vanished. 



The Pteropsida (Subphylum 4: Phylum Tra- 

 cheophyta). This highly successful group of 

 land plants is by far the largest in the Plant 

 Kingdom. It includes all ferns and all seed 



!-*%= 



Fig. 31-23. Petrified calamite fossil from a Carbon- 

 iferous formation. Many of the ancient Sphenopsida 

 were large, treelike forms that participated in the 

 formation of coal. (From The Plant World.) 



plants (Gymnospermae and Angiospermae). 

 The completely dominant sporophyte gen- 

 eration typically is represented by a sturdy 

 well-adapted land plant. It possesses very 

 efficient root, stem, and leaf systems, with 

 highly developed vascular elements in both 

 xylem and phloem. The leaves tend to be 

 large, complex structures, which have 

 evolved as flattenings at the ends of the stem 

 branches, rather than as small accessory out- 

 growths, as in the lower Tracheophyta. The 

 sporangia develop on the lower surfaces (or 

 margins) of the sporophylls and the sporo- 

 phylls usually are so highly specialized that 

 it is difficult to recognize them as modified 

 leaves. The haploid gametophyte generation 



