488 General Botany 



THE EQUISETUMS 



The equisetums constitute a small group of about twenty- 

 five living species that superficially bear little resemblance to 

 the ferns. Nevertheless, their life histories are quite similar. 

 Like the ferns they are representative of a very ancient phylum. 

 During the Carboniferous period there were allied plants that 

 formed extensive forests, with trunks 90 feet in height and 3 

 feet in diameter. The modern species are usually less than 3 feet 

 in height, although there are two tropical species that reach a 

 height of 10 to 15 feet and a South American species that attains 

 a height of 40 feet when partly supported by trees. 



The equisetums usually have columnar, upright, jointed 

 stems, externally fluted and internally characterized by long, 

 tubular air cavities. The upright stems arise as branches from 

 underground horizontal rhizomes. The leaves are scales ar- 

 ranged in whorls at the nodes. As the leaves are without chloro- 

 phyll, the photosynthetic work is carried on by the chlorenchyma 

 of the stems. In several species the upright stems bear a multi- 

 tude of slender whorled branches, whose brush-like character sug- 

 gests one of the common names, " horsetail." Another common 

 name, " scouring rush," is suggested by the fact that the cell 

 walls contain large amounts of silica and that in pioneer days 

 the plants were used to scour metal utensils. 



The roots are small and arise along the rhizomes mostly at the 

 nodes. The plants are essentially hydrophytes and are found 

 commonly on stream margins, swamps, and lake shores. A few 

 of the species occur in dry situations, but are there much dwarfed. 



The sporophyte. The sporophyte generation is the plant 

 we have just described. It is a perennial. The reproductive 

 structures consist of whorls of peculiar shield-shaped sporophylls, 

 each bearing five to ten sporangia, that together form a terminal 

 cone. 



Within the sporangia spores arise which are peculiar in 



