26 THE PSILOPHYTON FLORA [CH. 



scattered that they are invisible macroscopically. As they 

 increase in size and in number, they gradually become more and 

 more macroscopic, at first resembling chaffy scales, later spinous 

 outgrowths. These facts explain the different appearance of 

 shoots of the same plant to which we have already referred 

 (pp. 21, 22) when discussing Halle's conclusions. 



In view of these facts, impressions of Psilophyton which, when 

 without emergences, must appear to be very featureless fossils, 

 can no longer be regarded as doubtful objects or as unworthy 

 of serious consideration. The genus is now as important as any 

 from a phylogenetic standpoint, as we shall see later. 



It may eventually prove convenient to retain the term Rhynia 

 as a type of internal structure and if this is necessary such would 

 seem to be the chief justification of its existence. As a generic 

 term Psilophyton has undoubted priority. 



ARTHROSTIGMA. 

 (Figs. 8, 9.) 



Arthrostigma, Dawson, 1871 1 . Axis very stout, bifurcating 

 and giving off lateral members, irregularly furrowed or ribbed 

 longitudinally, bearing numerous large and long scattered, 

 straight, sometimes falcate, spine-like organs. Axes possessing 

 a slender central strand of vascular tissue. Fructification un- 

 known. 



Distribution. Lower Old Red, Scotland; Lower Devonian, 

 Canada, Norway, and (?) Belgium; Middle Devonian, Bohemia. 



According to Halle 2 , this plant displays considerable variations 

 in the shape of the spine-like organs and their manner of attach- 

 ment. There are first of all cylindrical impressions with a radial 

 arrangement of their leaf -like projections, though, in a few 

 instances, cases of a pseudo-verticillate arrangement occur. 

 "There can sometimes be noted a very fine but distinct vein- 

 like line running through the leaf," . . .which " no doubt represents 

 a vein or vascular strand." No leaf scars on the stem can be 

 seen in the case of the Norwegian specimens. 



Secondly, there are stems with " unusually densely and regularly 

 placed leaves." A third type consists of stouter specimens "with 



1 Dawson (1871), p. 41. 2 Halle (1916), p. 7. 



