214 



PLANT MORPHOLOGY 



enous tissue was dome-shaped, capping a sterile columella, as in Anthoc- 

 eros and Sphagnum. Nothing is known of the garaetophyte generation. 

 Some botanists see in the simpler Psilophytales a resemblance to the 

 liverwort Anthoceros and regard this group as a connecting link between 

 the bryophytes and pteridophytes. Others feel that a closer relationship 

 exists between the Psilophytales and the algae and that the Psilophytales 

 were derived directly from alga-like ancestors. But, regardless of their 

 origin, there is general belief that the group may have been ancestral to 

 the other great pteridophyte lines — the lycopods, horsetails, and ferns — 

 all of which are represented in the later Devonian deposits (Fig. 258). 



2. Psilotales 



The Psilotales are a modern order including only two genera. Psilo- 

 tum, with two species, occurs in tropical and subtropical regions in both 



D CAB 



Fig. 170. Psilotales. A, upper portion of shoot of Psilotum nudum with sporangia, 

 slightly reduced; B, closed sporangium of same, enlarged; C, open sporangium; D, cross 

 section through an unripe sporangium, showing three locules containing spore mother 

 cells; E, shoot of Tmesipteris tannensis, slightly reduced; F, a sporophyll of same, enlarged. 

 {After Wetistein.) 



the Eastern and Western Hemispheres. Tmesipteris, with a single spe- 

 cies, is confined to Australia, New Zealand, the Philippine Islands, and 

 parts of Polynesia. The Psilotales were formerly classified with the 

 Lycopodiinae, but they are now generally regarded as being more closely 

 related to the extinct Psilophytales. 



