42 Transactions. 



simply as haustorial protuberances of the surface cells of the foot. If 

 there is no evidence forthcoming that the absence of the root is due to 

 reduction, other than a certain degree of probability arising out of the 

 present habit of the plants, coupled with the fact that in other isolated 

 pteridophytic classes we seem to trace signs of reduction, we must ask, 

 Is there anything to adduce in favour of the theory that the absence of a 

 root in the Psilotaceae is a primitive feature ( In this particular character 

 the Psilotaceae stand alone amongst existing Pteridophytes. The funda- 

 mental differences between the various classes of Pteridophytes in the 

 manner in which the root is differentiated in the embryo shows that those 

 classes have been distinct from one another from a far-distant period, 

 and accordingly if one of them shows the total absence of a root from 

 its embryo this may quite conceivably be due to the preservation in the 

 one particular line of descent of a primitive character of vascular plants. 

 Such a theory will, of course, best be substantiated by direct evidence 

 from the fossil record. Such evidence has lately been brought forward 

 by Kidston and Lang in their account of the fossil plant Rhynia Gwynne- 

 Vaughani (1917). It must suffice here for me to mention briefly those 

 points in their paper which bear directly upon the present subject. The 

 authors themselves state that they have reserved to a later paper their 

 own discussion of the relation of their plant to the important questions 

 concerning the differentiation of primitive Pteridophytes into stem, root, 

 and leaf {ibid., p. 775). 



Rhynia Gwynne-Vaughani occurs in the Old Red Sandstone of Aberdeen, 

 and is, as its investigators point out, " the most ancient land-plant of which 

 the structure is at all fully known." Fortunately, the plant was preserved 

 in large numbers as it grew, and Kidston and Lang have been able to 

 elucidate fully its general habit of growth, external form, and structure. 

 The plant was leafless and rootless, the branched cylindical stems being 

 differentiated into underground rhizoid-bearing rhizomes and tapering aerial 

 stems. Branching of the stem was by the dichotomous division of its 

 apex, or more frequently by the formation on the stem of adventitious 

 lateral branches. The vascular system of the plant consisted throughout 

 of a simple cylindrical stele composed of a slender solid strand of tracheides, 

 with no distinction of protoxylem and metaxylem, surrounded by a zone 

 of phloem. The possession of these general characters leads Kidston and 

 Lang to compare Rhynia with the existing Psilotales ; but the presence of 

 certain other characters, such as the total absence of leaves, the consistent 

 simplicity of the stele, and especially the single large sporangia borne 

 terminally on short stalks, has decided them to recognize a new pterido- 

 phytic class (to which they propose to give the name " Psilophytales ; ') 

 somewhat resembling the modern class Psilotales, and embracing with 

 Rhynia certain Devonian plant fossils. The authors note that the com- 

 parison which they institute between Rhynia and the Psilotf ceae " would 

 lead us to regard the Psilotaceae as having preserved many primitive 

 characters, and not as reduced. On this view the Psilotaceae would be 

 the little-modified survivors in the existing flora of a type of plant that 

 existed in early geological times, the most fully known example of which 

 is now Rhynia Gwynne-Vaughani. It does not follow, however, that a 

 direct line of descent is to be drawn between Rhynia and the Psilotaceae 

 as we know them " (ibid., p. 776). 



It might, of course, with some reason be argued that the simple morpho- 

 logical nature of Rhynia was due to reduction ; but, all things considered, 

 it is more likely that the characters of this ancient plant are primitive 



