24 THE PSILOPHYTON FLORA [CH. 



Dawson's earlier conclusions on this point. These authors also 

 all agree in finding in Psilotum the nearest existing type of habit. 



We now pass on to consider the question of the identity of 

 Rhynia with Psilophyton. These authors, after discussing this 

 question, conclude that the two genera are distinct. They 

 compare Rhynia with P. princeps and conclude that the latter 

 differs from the former "in the presence of spines, in the more 

 profuse dichotomous branching, in the subordination of some 

 of the branches to a sympodial main axis, and in the absence, 

 so far as we know, of lateral adventitious branches 1 ." 



The weak point of this argument is that it does not take into 

 account species of Psilophyton other than P. princeps. As we 

 have seen, several other species of this genus, e.g. P. elegans and 

 P. robustius, have neither visible spines nor scales, while the latter 

 has " slender alternate branches " arising from a relatively robust 

 axis 2 . The agreement of Rhynia with Dawson's account of P. prin- 

 ceps and P. elegans appears to us to be so close that we have no 

 doubt as to a generic identity, at least, existing between these types. 



In support of this contention as to the generic identity of 

 Rhynia with Psilophyton, we have one constructive addition to 

 make to the discussion, and this is perhaps important. Rhynia 

 was not a spineless type, despite Kidston and Lang's assertion. 

 The small "lateral protuberances or bulges" of those authors 

 are the spine-like emergences. In order to realise this, it is only 

 necessary to compare the Figs. 7 and 8 on PL III of the paper 

 of these authors, which show the surface of stems of Rhynia 

 enlarged fourteen times, with the similar figures of Halle's 

 Psilophyton princeps (i.e. the very spiny type, Dawson's P. 

 ornatum) on PL 2, figs. 3 and 5 of Halle's paper, which are of 

 nearly equal magnification ( x 16). These figures are reproduced 

 here in Fig. 7. This comparison is we think conclusive, and it 

 also settles once and for all the correlation of the external 

 morphology of the spines with their internal anatomy. 



We may add that on microscopic examination, certain impres- 



1 Kidston and Lang (19 17), p. 779. [In a more recent paper, Trans. Roy.Soc. 

 Edinb.Vol. 52, 1920, p. 603, Kidston and Lang have described a second species 

 of Rhynia, R. major which shows no trace of adventitious branches. A. A.J 



2 Dawson (1859), Fig. 2 a, p. 481. 



