158 
PROFESSOR W. C. WILLIAMSON ON THE ORGANISATION 
The exterior of the cortex of this plant was densely clothed with hairs. Though 
longitudinal sections through the specimen described present a somewhat obscure 
arrangement of tissues and organs, these hairs enable us to distinguish external 
surfaces from internal structures. The difficulty of doing this is the greater since 
two distinct stems are pressed closely together in the fragment of ganister in 
which my specimens are preserved, and also from the fact that the innumerable 
small cylindrical organs, cl, variously intersected, and each with an ill preserved 
vascular bundle in its interior, abound in all my preparations both within the cortex, 
and externally to it, as at cl, cl. Similar structures appear to exist in M. E-enault’s 
specimens of Anachoropteris, but that observer regards them as representing 
petiolar bundles. Mine, like his, are ill preserved ; but they more closely resemble 
adventitious roots than petiolar structures. A similar one M. Renault himself 
regards as a “ racine adventif ” These organs are about '066 of an inch in diameter. 
That no classification of these fossil Ferns based solely upon the transverse sections 
of their petiolar bundles is or can be of much value, is clearly shown when tested 
amongst those living Ferns the classification of which is chiefly based upon their 
sporangial reproductive organs. But, I think, I can show that we have here to do 
with a type of stem-structure which is remarkable, and which appears to throw them 
into something like a natural group recurring in several allied plants. 
In his memoir above referred to, M. Renault describes and figures"" a transverse 
section of the stem of his Zygopteris Brongniartii. In this section we find a central 
structure “tres probablement cellulaire” “ ou a ses prolongements qui, au nombre de 
six dans le Zygopteris, s’enfonyaient plus ou moins dans I’epaisseur de I’dtui ligneux 
a, a, forme par les cellules scalariformes.'’ {Loc. cit., pp. 164-5.) 
Specimens in my cabinet confirm M. PlENAULt’s suggestion that this central 
structure, with its thin radiating arms, is really a cellular one, being either a medulla 
or of a procambial character—but apparently the former—associated in either case 
with peculiarities in the primitive development of the vascular bundle which 
surrounds it. The presence of this peculiar cellular centre within the vascular axis 
constitutes a feature which seems to unite several otherwise distinct plants into a 
common group. It is obviously identical with the structure a, a, seen in my 
figs. 1, 2, and 5 of the present memoir. In my memoir. Part VIII.,t I figured, under 
the name of RacMopteris corriigata, transverse sections of a stem which has a struc¬ 
ture almost identical with that of ]\T. Renault’s Zygopteris Brongniartii, and in which 
the central cellular structure sends off five or six radiating and sometimes dicho- 
tomosing arms, partially subdividing the surrounding vascular cylinder into balf-a- 
dozen groups of cells. Unlike M. Renault’s similar example, the petiolar bundles 
given off by this central axis are not Zygopteroid in form, since they lack the two 
parallel extensions f', f" seen in Plate 1, fig. 4, of the present memoir; what 
* Plate .3, fig. I. 
t ‘ Phil Ti’ans.,’ vol. IG7, Plate 5, fig. 4, and Plate 6, fig. 13. 
