250 



PLANT MORPHOLOGY 



sisting of three lower sterile segments arranged in one plane and three 

 upper fertile ones. Each fertile segment was a sporangiophore bearing 

 four sporangia. The strobilus of Cheirostrobus was the most complex one 

 in all pteridophytes. 



All the Sphenophyllales were homosporous. Nothing is known of the 

 gametophyte generation. 



A 1/ B 



Fig. 207. Sphenophyllum dawsoni. A, diagram of longitudinal section of cone, showing 

 three whorls of sporophylls and, above, whorl of sporophylls in surface view seen from the 

 inside; B, diagram of one-half of a single whorl of sporophylls and sporangiophores. (A, 

 after Scott; B, after Hirmer.) 



3. Equisetales 



The Equisetales, often called horsetails, are herbaceous plants compris- 

 ing a single surviving genus, Equisetum, with about 25 species cosmopoli- 

 tan in distribution. Although related to Paleozoic forms, this order 

 became prominent in the Mesozoic. Equisetites, one of the Triassic 

 horsetails, had a stem 20 cm. in diameter and in general was built on a 

 vastly grander scale than modern forms. The common horsetails of 

 temperate regions grow in swamps, meadows, forests, and sandy wastes. 



Sporophyte. The largest living species, Equisetum giganteum, of trop- 

 ical America, reaches a height of 12 m. but has a weak stem only about 

 2 to 3 cm. in diameter at the base. Most of the other species are less than 



