ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 721 
of the specimens the outer cortex is of the normal “ Dictyoxylon” type, with the 
parenchymatous bands already dilated. On the whole, we may say that these tiny 
stems have all the characteristic structure of Lyginodendron, except that the sclerotic 
nests are absent.* 
The small stems of the type with an almost continuous ring of primary wood 
(see Memoir XVII., fig. 11) are never quite so minute as those just described, 
the smallest found having a mean diameter of barely 4 millims. (C.N. 1137). The 
structure only differs from that of the typical Lyginodendron stem in having a 
continuous xylem-ring and a very small pith. Specimens of this kind are connected 
by an unbroken series of intermediate forms with the typical stems, which have a large 
pith with separate bundles at its periphery. These differences are quite irrespective 
of the amount of secondary thickening, and therefore cannot depend on the age of 
the branch. It might be easy to explain them as dependent on the order of the 
branch, but we have at present no evidence, that the stem of Lyginodendron branched 
at all. It may have done so, but not a single branching specimen has yet been 
detected. The supposed branches mentioned in earlier memoirs have all turned out 
to be either roots or petioles. We are, therefore, not justified in assuming the 
presence of a highly developed system of ramification such as would be necessary to 
account for the existence of branches of various orders. 
The explanation we would suggest is, that the small specimens, with a continuous 
ring of primary wood, may represent the basal, first developed portions of normal stems, 
Osmunda, as is well known, resembles Lyginodendron in the fact that the normal 
stem possesses a ring of collateral bundles, which are distinct, so far as their xylem is 
concerned. M. Lecterc pu SABLon in his interesting memoir on the development of 
the stem in Ferns, has shown that the lower internodes of the stem of Osmunda have 
a continuous ring of wood enclosing the small pith, this ring being only interrupted 
at the nodes. Itis only in the later-developed part of the stem that the pith enlarges 
and the bundles become permanently distinct.t 
A similar stage is passed through by the stems of many other ferns. Until we 
have been able to trace the transition in one and the same stem of Lyginodendron 
from the small pith and continuous xylem of the basal portion to the large pith and 
separate bundles of the upper stem, our suggestion must remain an hypothesis. In 
the mean time, however, it may serve as a provisional explanation, which is in 
harmony of what we know of the development of those recent Ferns which in their 
anatomy most nearly resemble our fossil. The fact that Lyginodendron had 
secondary growth in thickness, while the Ferns with which we have compared it 
have not, does not invalidate the comparison. We have good reason to believe that 
the secondary thickening in Lyginodendron and its allies was relatively a recently- 
* See C.N. 1139 and 1141 (same specimen), 1199, and especially 1885 D. 
+ Lucterc pu Saston, “Recherches sar la formation de la tige des Fougéres,” p. 9, figs. 25 and 26, 
‘Ann. des Sci. Nat. (Bot.),’ sér. 7, vol. 11, 1890. 
MDCCCXCV,—B, oA 
