PTERIDOPHYTA 147 



main alive and active up to the time the spores are 

 ripe. 



A third genus, Phylloglossum, allied to Lycopodium, 

 includes a single species from Australia, and is appar- 

 ently a very primitive type, as it resembles closely the 

 embryonic condition of some species of Lycopodium. 

 Unfortunately, all attempts to germinate the spores 

 have failed, and the gametophyte is entirely unknown. 



The order Psilotacese, which is commonly associated 

 with the club-mosses, includes two tropical genera, 

 Psilotum and Tmesipteris. They are usually epi- 

 phytes, i.e. grow upon the trunks and branches of 

 trees, and Tmesipteris shows some evidences of being 

 partially parasitic. The sporangia are large and all 

 alike, but as yet nothing is known of the nature of the 

 gametophyte produced from them, so that it is impossi- 

 ble to compare it with that of the other Pteridophytes, 

 and at present the systematic position of these curious 

 plants must be regarded as doubtful. 



Like the Equisetinese, the club-mosses were once 

 much more abundant than at present, and many of 

 them far exceeded in size and complexity any of the 

 existing species. Members of this class probably ex- 

 isted as far back as the upper Devonian, and in the Car- 

 boniferous rocks they form one of the most conspicuous 

 features of the fossil flora. The most striking forms 

 are the species of Sigillaria and Lepidodendron, which 

 reached tree-like dimensions and showed a secondary 

 thickening of the stems like that of the living conifer- 

 ous trees. Many of these fossil Lycopods are preserved 

 in an extraordinarily perfect manner, so that the histo- 

 logical details are perfectly recognizable and can readily 



