VI 



PSILOTALES 87 



not deny that there are points of similarity between the Psilotales 

 and both those groups. These resemblances however appear to 

 us to be far outweighed by the differences, and the latter seem 

 to be of a more fundamental nature than the former. 



In comparison with the Lycopsida, we would point to the 

 microphyllous habit and the presence of a pericaulome, as 

 features in common. On the other hand in the anatomy of their 

 sporophyte and its fructifications the Psilotales are quite unlike 

 living or fossil Lycopods. 



In recent years the view that this group may be related to 

 the Sphenopsida rather than to the Lycopsida has undoubtedly 

 gained ground, despite the vast differences in habit. In certain 

 points in the anatomy of the shoot and in the type of sporophyll 

 it is possible to institute some comparison between the Psilotales 

 and the Sphenophyllales 1 . We are now inclined to regard these, 

 however, as cases of parallelism of development and not to 

 attach any special phyletic arguments to such resemblances 

 which are far outweighed by more important dissimilarities 2 . 

 We should be inclined to regard the Psilotales as a quite inde- 

 pendent race, also of algal origin, which appeared on the scene 

 long after the other races which we are discussing here, possibly 

 in Mesozoic times or even later. We know of no evidence (not- 

 withstanding Kidston and Lang's views of Psilotum to which 

 we shall shortly refer again) of the existence of any plant in 

 Palaeozoic times which has any real claim to inclusion in this 

 group. Certainly none occurs among the known members of the 

 Psilophyton or Archaeopteris floras. The Psilotales, like the 

 Bryophyta, are thus a much later group in point of age than the 

 Sphenopsida, Lycopsida and Pteropsida. That the Psilotales 

 retain certain features which may be primitive we should agree, 

 though certainly in Psilotum and probably also in Tmesipteris 

 other characters, which might be regarded at first sight as 



1 Scott (1909), Vol. IT, p. 626. 



2 The most recent advances of our knowledge of the Psilotales which we 

 owe to Lawson (1917) do not help us here since they are concerned with 

 the gametophytes. The latter do not appear to be very remote from those 

 of Lycopsida, but since the corresponding structures of the Sphenophyllales 

 are not known and probably never will be known, we have no grounds for 

 any comparison in this respect. 



