24 PSILOTALES [CH. 



parison between the sporophylls of that genus and Spheno- 

 phyllum as those of the latter can hardly be regarded as other 

 than foliar organs. 



Both members of the Psilotales may, as Boodle has 

 suggested, be regarded as descendants of a common parent in 

 which the aerial stems possessed a fluted or stellate cylinder of 

 mesarch xylem. There can be no doubt as to the significance 

 of the morphological resemblances between the Psilotales and 

 the genqra Sphenophyllum and Cheirostrobus, but the position 

 of Tmesipteris and Psilotum in the plant-kingdom may probably 

 be best expressed by adopting the group-name Psilotales rather 

 than by transferring the recent genera to the Sphenophyllales. 

 One of the most striking differences between the Psilotales and 

 the genus Lycopodium is in the form of the sporophylls and 

 sporangia ; in Lycopodhim a single sporophyll bears a unilocular 

 sporangium, but in the Psilotales the sporophyll may be 

 described as a bilobed structure homologous with a foliage- 

 leaf, bearing a sporangiophore which consists of a short stalk 

 terminating in a bilocular or trilocular synanguim ; the short 

 stalk receives a special branch from the vascular bundle of the 

 sterile portion of the sporophylP. 



Fossils described by authors as being closely 

 allied to Psilotum. 



A search through palaeobotanical literature reveals the 

 existence of a very small number of specimens which have been 

 identified as representatives of the Psilotales. An inspection of 

 the material or published drawings leads one to the conclusion 

 that practically no information of a satisfactory kind is 

 available in regard to the past history of the two southern 

 genera Psilotum and Tmesipteris, which are regarded by some 

 botanists as relics of an ancient branch" of pteridophytes. 



In 1842 Miinster^ instituted the genus Psilotites for a small 

 impression of a slender branched axis from Jurassic rocks near 

 Mannheim in Germany which he named Psilotites filiformis; 



1 Bower (94) ; (08). ^ Bertrand (81) p. 254. 



3 Munster (42) p. 108, PI. xni. fig. 11 ; PI. xv. fig. 20. 



