SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 777 



of Psilophyton princeps, as we understand it, are shown on pi. ix, figs. 97-110; 

 pi. x, figs. 112-114, 118-120; pi. xi, figs. 127, 128, 133a, b, c, and 134, 134a, b, c 

 of the memoir cited above.* 



Psilophyton princeps as thus limited t consisted of upright stems which 

 frequently dichotomised, the weaker branches often appearing as if borne laterally. 

 The stouter stems bore numerous spiny outgrowths but no definite leaves. The 

 growing regions of the stem were circihately coiled. The stems were marked by 

 longitudinal ridges. The finer branches appear to have been destitute of spines. 

 They dichotomised repeatedly, and their ultimate branches terminated in oval 

 sporangia which were borne singly or in pairs. 



The material of Psilophyton princeps is mainly in the form of impressions. 

 Dawson has, however, figured a few specimens in which the structure was imperfectly 

 preserved,} and has based a reconstruction § on such remains. The most interesting 

 of these is the axis given natural size in his fig. 133c, and magnified in fig. 134. 

 This had a wide cortex and a single central cylinder. Little of the structure of the 

 cortex was preserved, but the central cylinder is figured and described || as consisting 

 of " an axis of scalariform vessels surrounded by a cylinder of parenchymatous cells 

 and by an outer cylinder of elongated woody cells." Dawson's figures are of such 

 interest that they are reproduced in the accompanying text-figs. 1 and 2, together 

 with his description of the figures. 



The description of the central bundle of Psilophyton princeps as composed of 

 "scalariform vessels" surrounded by "woody fibres" is at first sight difficult to 

 understand, but seems to become intelligible in the light of the imperfectly preserved 

 examples of Rhynia described above on page 772. 



While we have not seen Dawson's original specimens, we venture to interpret 

 his published figures as exhibiting a stem with a stele consisting of a solid strand 

 of tracheides surrounded by a zone of phloem which had become partially decayed 

 and discoloured much as in the specimens of Rhynia shown in our PL X, 

 figs. 76-78. In particular, Dawson's fig. 134 (see text-fig. 2) may be compared 

 with our fig. 77, and his reconstructed fig. 127 (see text-fig. l) with our fig. 78. 

 Dawson's fig. 134a suggests that the tracheides in his specimens were annular 

 rather than scalariform. 



What Dawson speaks of as " the outer fibrous cylinder " or " bark fibres " would 

 correspond to the persistent outer cortex of Rhynia. The " cellular cylinder " in 

 text-fig. 1 would correspond to the inner cortex ; this has almost completely dis- 



* It will be observed that we exclude from Psilophyton -princeps Dawson's figures of " rhizomes," pi. x, figs. Ill, 

 115, 116, and 117. We also exclude at present from Psilophyton all the other species which have been referred to it 

 by Dawson and other writers, as they do not seem to show the characters necessary for their definite reference 

 to this genus. 



f The course we adopt is in general agreement with the views of Solm-Laubach, Fossil Botany, pp. 189-192, 

 and more recently of P. Bertrand in " Note preliminaire sur les Psiloxmytons des gres de Matringhen," Ann. Soc. 

 ge'ol. du Nord, yol. xlii, p. 157, 1913. 



| L.c, pi. xi, figs. 133 and 134. § L.c, pi. xi, fig. 127. || i.e., p. 37. 



TRANS. ROY. SOC. EDIN., VOL. LI, PART III (NO. 24). 113 



