Knowledge of Lyginodendron . 7 1 
the appearance presented by the broad medullary rays of 
a Calamite stem (PL VI, Fig. 10). In the parenchymatous 
tissue internal to the tapered ends of the tracheid-rows, and 
to a less extent in the broad medullary-ray-tissues, there are 
found several patches of black substance which, no doubt, 
mark the position of secretory cells. Internal to the centri- 
fugal xylem there extends round the periphery of the pith 
a series of short rows of tracheids, often arranged in groups 
with a fan-like disposition and with the apex facing the 
centrifugal wood. These series of tracheids are separated 
here and there by distinct medullary rays. This inner and 
much smaller zone of secondary tissue constitutes the centri- 
petally-developed xylem. In PI. V, Fig. 6, the faint line 
immediately internal to the centrifugal wood, and surrounding 
the pith, marks the position of the centripetally-developed 
tracheids ; these are also semi-diagrammatically shown in 
PI. VI, Fig. 14X. In PI. VI, Fig. 10, the inner limit of the 
centrifugal wood is shown on either side of a bay forming 
a break in the outer ring of xylem, and probably due to an 
outgoing leaf-trace-bundle. In PI. VI, Fig. 16, a fairly well 
preserved portion of the centripetal xylem is figured ; it 
consists of rows of tracheids, sometimes as many as sixteen 
tracheids occurring in a single radial series, separated by 
bands of imperfectly preserved medullary-ray-parenchyma. 
The radial walls of these tracheids appear to have the same 
form of pitting as occurs in the centrifugally-developed xylem. 
Internally the centripetal ring is succeeded by very imper- 
fectly preserved tissue, but in some places it can be readily 
seen that a merismatic layer existed on the pith-side of the 
inner ring of wood. The crushed and partially disorganized 
tissue internal to the wood contains numerous dark-coloured 
patches of tissue (.?) and an abundance of secretory cells. 
The former, of which one (s) is shown in Fig. 16, internal to 
the centripetal wood, are nests of sclerous cells, of which 
several occur in the region of the pith (Fig. 10). In a longi- 
tudinal section the remains of phloem-tissues are found 
internal to the centripetal xylem, and in many places the 
