PTERIDOPHYTA 303 



The ripe sporangia open by a ventral cleft. The dehiscence is 

 caused, in part, by the contraction of the cells which form its outer 

 wall (Fig. 268). These cells develop upon their walls spiral thicken- 

 ings like those upon the walls of the tracheids. The expansion of 

 the elaters also contributes to the opening of the sporangium. 



Classification and Distribution 



The existing species of Equisetales are all referable to a single 

 order, Equisetacese, with one genus, Equisetum, which is repre- 

 sented in all parts of the world except Australia. From a study of 

 the fossil Equisetales, it is clear that many of these were much 

 larger and more specialized than their living descendants. These 

 specialized types may be referred to a well-defined order, Calamarieee, 

 the best known being the species of Calamites, which differed from 

 the Equisetaceae in being much larger and showing a secondary 

 thickening of the stems, now merely hinted at in such species as 

 E. telmateia. The leaves of some of these fossils have been described 

 under the name Anuularia, and the best known of their fructifications 

 belong to the type known as Calamostachys. These fossil Equisetales 

 have in many cases been preserved so perfectly that their internal 

 structure is readily made out. The earliest forms appeared in the 

 Devonian, and they reached their maximum during the Carbonifer- 

 ous, declining rapidly in importance in the later formations. Heter- 

 ospory has been demonstrated for some of them, but it never was as 

 well developed as in the Ferns and Lycopods. 



CLASS III. LYCOPODIALES 



The existing Lycopodiales, or Club-mosses, are intermediate in 

 number of species between the Equisetales and Ferns. About 450 

 species have been described. The great majority are tropical, but 

 several species of Lycopodium and Selaginella are common plants 

 in the cooler parts of the earth. 



The gametophyte is now well known in several species of Lycopo- 

 dium and Selaginella, and possibly in Psilotum, but is quite unknown 

 in the other genera. Of special importance are the investigations of 

 Treub, Goebel, and Bruchmann (13) upon Lycopodium. 



The sporophyte of all the existing species is of moderate size, 

 never exceeding a few metres in length and commonly is much 

 smaller. It usually consists of a creeping stem, with upright, leafy 

 shoots, but in some of the larger tropical species of Selaginella the 

 long, half-climbing stem is supported by other plants. Many tropical 

 forms are also epiphytes, and may have the roots absent (Psilotum). 



The existing Lycopodiales may be divided into three orders: 



