48 THE PSILOPHYTON FLORA [ch. 



plexity, particularly in the possession of a vascular strand in the 

 main axis. 



This anatomical fact, of the greatest interest and importance, 

 seems to have mesmerized both Halle and also Kidston and 

 Lang^ to such an extent that the thought that Psilojjhytoti might 

 be a Thallophyte, does not appear to have ever occurred to them. 

 The position is much the same as when Brongniart argued that 

 Sigillaria, because it possessed secondary wood, must be a 

 Gymnosperm ! The possession of a vascular strand in the main 

 axis does not appear to us to be a necessary sign of Pteridophytic 

 affinity. Thallophyta are living to-day which possess a well- 

 marked phloem. Thallophyta may have existed in the past 

 Avhich possessed a xylem strand. If they were terrestrial and 

 not hydrophytes, it is highly probable that this was the case. 

 Psilophyton was undoubtedly terrestrial. It is an immensely old 

 type taking us back to days when terrestrial Algae of a high 

 grade may have existed, though now long since extinct. At any 

 rate it would be a very rash conclusion to deny that such ])lants 

 have ever existed. 



The Thallophyta are a race of plants which can by no means 

 be kept within such narrow boimcls. A race which among at 

 least some of its members had evolved alternation of generations, 

 a cormophytic habit, and true phloem, would if necessity arose 

 be quite capable of evolving a lignified conducting tissue. If 

 some of its members were at one time land plants, such a 

 necessity would be obvious. 



We have next to enquire whether Psilophyton j^resents any 



evidence of being an extreme case of Pteridophytic reduction 



rather than a Thallophyte. Here it appears to us that both the 



morphological and anatomical evidence is emphatically against 



the former view. If Psilophyton is such a reduced type, how is it 



that the xylem of the axis is reduced to a single protoxylem 



strand, a state of affairs unknown in any other, however highly 



reduced plant, whether living or fossil ? How is it that a lack of 



1 [It should be noted tliat the present memoir was completed before the 

 ajjpearance of the following 'papers by Kidston and Lang : On Old Red 

 Sandstone Plants showing Structure, from the Rhynie Chert Bed, Aberdeen- 

 shire, Parts II and iii. Trans. Roy. Soc. Edinb. Vol. 52, 1920, pp. G03, 643. 



A. A.] 



