776 DR R. KTDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE" PLANTS 



the existing group of the Psilotales, and on the other hand with one less perfectly 

 preserved Devonian plant, Psilophyton princeps, Dawson. Even these comparisons 

 will not be developed fully until Asteroxylon has been described in the next paper 

 of this series. 



The Psilotales, with the two existing genera Psilotum and Tmesipteris, have 

 always presented difficult morphological problems. They are rootless, the under- 

 ground parts consisting of rhizomes bearing rhizoids. The leaves are small and 

 without vascular system in Psilotum, larger and with a vascular supply in Tmesi- 

 pteris. Their reproductive organs consist of bi- or tri-locular synangia, subtended 

 by a pair of leaf-like lobes. This fertile structure has been variously interpreted as 

 a bifid sporophyll subtending an adaxial synangium or sporangiophore, or as a lateral 

 branch bearing a pair of leaves and terminating in a synangium. 



The Psilotacese agree with Rhynia and differ from all other Pteridophyta in the 

 absence of roots and (if the second interpretation be accepted) in the position of 

 the synangia terminating a short branch. There is, further, a striking general 

 agreement between Psilotum and Rhynia in the plant consisting of more or less 

 delicate subterranean rhizomes, bearing a system of xerophytic stems, and in the 

 occurrence of both dichotomous and lateral branching. We also find in the rhizome 

 or in the ultimate branches of Psilotum a parallel to the simplicity of the stele 

 of Rhynia. 



The Psilotaceae exhibit, however, some important points of difference from 

 Rhynia, such as the presence of leaves, the specialisation of the fertile branches 

 or sporophylls, the more complicated anatomy, the synangia, and the shape of the 

 spores. 



The comparison will not be carried further at present, but it may be noted in 

 passing that it would lead us to regard the Psilotacese as having preserved many 

 primitive characters and not as reduced. On this view the Psilotaceee would be the 

 little modified survivors in the existing flora of a type of plant that existed in early 

 geological times, the most fully known example of which is now Rhynia Givynne- 

 Vaughani. It does not, however, follow that a direct line of descent is to be drawn 

 between Rhynia and the Psilotacese as we know them:* 



With regard to the comparison of Rhynia with extinct plants, it is only necessary 

 at present to consider Psilophyton princeps, Dawson, including under this the 

 variety ornatum, which probably merely represents the lower portions of the 

 stems. We take the description of this species as given by Dawson in the " Fossil 

 Plants of the Devonian and Upper Silurian Formations of Canada," t though the 

 plant has been described by him in several of his other works. The main characters 



* The primitive nature of the general organisation of the Psilotacew is clearly held on other grounds by C. Eg. 

 Bertrand in his Recherches sur les Tmesipteride'es, Lille, 1883, pp. 313-316; and by Lignier in his " Equise tales 

 ct, Sphenophyllales leur origine filicineenne comnmne," p. 95, Bull. Soc. Linn, de Normandie, 5 e ser., vol. vii, 1903, 

 ]>. 93, Caen. 



t Geological Survey Canada, Montreal, 1871. 



I 



