Pob. 
BRANCH OF LEPIDOPHLOIOS FULIGINOSUS. 225 
The transverse seetion shows that this secondary growth does not take place uniformly 
round the axis, but, as is very generally the case in the Lepidodendracee, it is more 
extensive on one side of the stem. In Pl. 24 figs. 6 & 7 it will be found largely, 
if not exclusively, on the lower side of the stele. 
The specimen under consideration, though more than twice the size of that described 
hy Williamson, has only a very little more secondary growth, confirming the belief that 
in this species the development of secondary wood did not take place to so great 
an extent as in other Lepidodendracee. At some parts of the circumference, indeed, 
there seems to be no secondary growth at all, and then there is a gradually increasing 
amount as one passes round the stem, decreasing again as one nears the starting-point. 
This eondition allows us to see not only the aetual secondary growth, but to follow 
out more or less the consecutive stages of its development. 
Even where there is no actual secondary tissue formed, one notices, on the outside 
of the dense parenchymatous tissue in which the leat-traces are at first embedded, a ring 
of more lightly coloured and thin-walled cells somewhat radially elongated. Within 
these, a little further on, will be found one or two rows of smaller squarish cells, and 
further on still they will be found to be more numerous and more regularly arranged. 
Seen in a transverse section, they are for the most part square in shape except the 
outermost layer just within the phloem, which, as stated above, is often considerably 
elongated in a radial direction (see Pl. 25. fig. 13) and very light in colour. This ring 
of radially elongated cells is probably not of a meristematic nature, as it occurs, as will 
be seen later on, in the lateral organs in which no secondary thickening takes place. 
Probably one or more of the rows of cells within the radially elongated cells were 
of a meristematic nature, differing somewhat from the flattened cambium-cells of recent 
plants. 
In those parts of the circumference where a considerable amount of secondary tissue 
has been formed, it will be noticed that certain rows of the secondary cells become more 
enlarged and appear to have somewhat thicker walls. "These cells are generally empty, 
and are in transverse section not unlike the secondary tracheids of other Lepidodendracee. 
They are somewhat irregularly arranged, as in the secondary wood of Lepidodendron 
Wiinschianum, and form slightly spindle-shaped groups of cells. 
A similar condition for the same species is figured by Williamson (1881), pl. 49. fig. 11, 
and in his description of this section (p. 290) he calls the larger empty elements “ barred 
vessels." An examination of a longitudinal section of our specimen (Pl. 25. fig. 11), 
however, shows that these larzer cells are short and square, and do not possess any 
sealariform marking. S r 
It is of course possible that we have in this specimen a fairly young condition of the 
stem before the lignification of the secondary tissue. If this is the case, then it is 
conceivable that tracheids may yet be formed from the secondary tissue. But neither 
the transverse nor longitudinal sections are suggestive of the formation of tracheids, 
the whole of the secondary tissue having the appearance of parenchyma. 
The very late development of secondary tracheids seems to be a characteristic feature 
of Lepidophloios fuliginosus, as far as can be gathered from the published accounts 
SECOND SERIES.— BOTANY, VOL. VI. 2K 
