674 TEXTBOOK OF BOTANY 



histories, however, are very similar. Like the ferns and lycopods, they 

 are the modern descendants of an ancient phyhmi of plants. During the 

 Carboniferous period equisetum-like plants with trunks 90 feet in height 

 and 3 feet in diameter formed extensive forests. Fossil equisetum- 

 relatives have been found also in rocks of the Devonian age. The mod- 

 ern species are usually less than 3 feet high, although there are two 

 tropical species that may grow to a height of 10 or 15 feet, and a South 

 American species that is said to attain a height of 40 feet when partly 

 supported by trees. 



Equisetums occur today in the tropics and semi-tropics, and in the 

 temperate and arctic zones. None has as yet been reported from 

 Australia or New Zealand, from the islands of the Indian Ocean, or from 

 Antarctica. The plants grow best in mildly acid habitats; in the north 

 they are likely to be found in peat bogs. Elsewhere they are found gen- 

 erally in sandy areas, particularly along streams and rivers, or where 

 there is an underground source of water. One species, Equisetum kansa- 

 num, is found in prairie patches and the more moist situations of the 

 plains. 



The horsetails — or scouring rushes, as they are often popularly called 

 — have columnar, upright, jointed stems, externally fluted and internally 

 characterized by long, tubular air cavities. The central axis of the young 

 stem is pith, but this soon disappears, and the older stem is hollow. Lat- 

 erally fused colorless scales occur in whorls at the nodes (Fig. 318). 

 Photosynthesis takes place in the chlorenchyma of the stem. Some of 

 the ancient fossil species had true foliage leaves. Stomates occur regu- 

 larly in the furrows of the stems, and their arrangement is used in the 

 classification of the various species. 



The name horsetail is probably suggested by the brush-like character 

 of the numerous slender whorled branches occurring on the upright 

 stems of a few species. The name scouring rush refers to the accumula- 

 tion of silica in the walls of the stem tissues. In pioneer days these were 

 gathered and used for scouring metal utensils. The roots are small and 

 adventitious, and arise along the perennial rhizomes, mostly at the nodes. 



The sporophyte. The plant described above is the sporophyte phase of 

 Equisetum. It is perennial, although the erect aerial branches may be 

 annual. The peculiar shield-shaped sporophylls are arranged in whorls 

 within a terminal cone and each bears five to ten sporangia. 



Within the sporangia are numerous spores which are peculiar in hav- 

 ing four long appendages that coil around the spore when moist and 



