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 1 to such an ex-tent that the thought that Psilophyton 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 

 which 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 plants 

 have ever existed. 



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

 be kept within such narrow bounds. 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 presents 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 j. 

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

 that the xylem of the axis is reduced to a single protoxylen^ 

 strand, a state of affairs unknown in any other, however highl^ 

 reduced plant, whether living or fossil ? How is it that a lack o'f 



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

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

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

 shire, Parts IT and in. Trans. Roy. Soc. Edinb. Vol. 52, 1920, pp. 603, 643. 



A. A.] 



