3 



Psilotopsida 



Sporophyte rootless, with dichotomous rhizomes 

 and aerial branches. Lateral appendages spirally 

 arranged, scale-like or leaf-like. Protostelic (either 

 solid or meduUated). Sporangia thick walled, 

 homosporous, terminating very short lateral 

 branches. Antherozoids flagellate. 



Psilotales 



Psilotaceae Psilotum 

 Tmesipteridaceae Tmesipteris 



This small group of plants is one of great interest to mor- 

 phologists because its representatives are at a stage of 

 organization scarcely higher than that of some of the earUest 

 land plants, despite the fact that they are living today. Their 

 great simphcity has been the subject of controversy for many 

 years, some morphologists interpreting it as the result of 

 extensive reduction from more complex ancestors. Others 

 accept it as a sign of great primitiveness. 



Two species of Psilotum are known, P. nudum {=P. 

 triquetrum) and P.flaccidum {=P. complanatum), of which 

 the first is widespread throughout the tropics and subtropics 

 extending as far north as Florida and Hawaii and as far 

 south as New Zealand. Most commonly, it is to be found 

 growing erect on the ground or in crevices among rocks, but 

 it may also grow as an epiphyte on tree-ferns or among 

 other epiphytes on the branches of trees. P. flaccidum is a 



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