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 Psilotacez, which is commonly associated 
with the club-mosses, includes two tropical genera, 
Psilotum and Tmesipteris. They are usually epi- 
phytes, ¢.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 Equisetinez, 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 
