162 
PROFESSOR W. C. WILLIAMSON ON THE ORGANISATION 
end of which disj^lays features identical with those of the latter figure. It exhibits 
three of the radiating rootlets (?) d, d, and d' •, but d is fortunately intersected trans¬ 
versely, and enables us to identify several isolated transverse sections of these roots 
scattered through some of my slides, three of which are figured in figs. 13, 14 (Plate 4), 
and 15 (Plate 1). 
Fig. 13 has a mean diameter of ‘0025 of an inch ; fig. 14 of 'Ol, and fig. 15 of -025. 
In each of these examples we have the conditions seen in fig. 11 , g, viz., a 
central bundle, a, invested by the two cortical zones d' and e', as well as the 
apparent bundle-sheath h, enclosing the phloem c. In fig. 15, which seems to 
represent a younger but smaller rootlet, the separation of the cortex into two 
zones is less distinct than in figs. 13 and 14. Its component cells also exhibit the 
tendency to arrange themselves in the concentric cycloidal circles so common amongst 
young rootlets of this character. The vascular bundles of all these rootlets are of 
the diarch type, though possibly they may also be regarded, both in structure 
and development, as resembling the concentric bandies of Lycopodiaceous stems. 
This plant, which I propose to distinguish as Rachiopteris hirsuta, is wholly distinct 
from any which I have hitherto described. Can it be identified with any living type ? 
The young branches of the living Marsilece, M. quadrifolia and M. scdvatrix, are clothed 
with hairs absolutely identical with those of fig. 10, and longitudinal sections of 
these branches display similar irregularities of ramifications to those shown in fig. 9 ; 
rootlets, branches, and bases of fronds being alike cut through in sections made in 
almost any one plane, and their rootlets also remind us strongly of those seen in 
figs. 11, 12, 13. Without attaching an undue importance to these resemblances, the 
sections of Rachiopteris hirsuta undoubtedly suggest closer relationships with the 
Marsilece than with any other living j)lants with the organisation of which I am 
familiar. The specimens described are from the Halifax deposits. 
I have at various times discovered other forms of roots or rootlets in these Halifax 
Carboniferous beds, some of which at least are sufficiently interesting to be put upon 
record, as showing the early period at which certain types of these organs made their 
appearance on the earth. The first of these I discovered in some slides also prepared 
for me by Mr. Isaac Earnshaw, of Oldham. 
Fig. 16 (Plate 4) represents a longitudinal section through a very delicate root, of 
which I have a number of fragments. Their most characteristic feature resides in 
the circumstance that them secondary rootlets are given off in numerous verticils, c, c. 
The cellular parenchyma of which they consist exhibits extremely limited differentia¬ 
tion. A primary vascular bundle, a, composed of barred vessels, runs down the 
centre of the primary axis ; and secondary ones, composed of vessels of smaller 
dimensions, a', a, bend downwards and outwards into the secondary rootlets. The 
jDarenchyma, h' , immediately surrounding the primary bundle consists of long, narrow’, 
thin-walled cells, wdiich are invested by an external bark, h, composed of a larger and 
more strongly marked form of parenchyma. 
