OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
287 
is closely invested by a second ring of barred vessels ( d ), arranged in radiating lines, and 
the products of an exogenous process of growth. The thickness of this exogenous vas- 
cular zone, in the specimen under consideration, is about the same as that of the medul- 
lary one which it incloses. Each radiating line commences, at its inner extremity, at one 
of the very small vessels corresponding with those at the periphery of Plate XLI. fig. 8. 
From this starting-point new vessels have been added to the peripheral end of the line, 
as occurs in the case of the wood-cells of coniferous plants ; but here each succeeding 
vessel has been somewhat larger than the one preceding it, so that many of the outer- 
most ones of this cylinder have a mean diameter of -005. Each of the radiating rows 
consists of from thirteen to seventeen vessels. On making vertical sections of this spe- 
cimen new elements revealed themselves. Plate XLII. fig. 10 represents a small portion 
of a radial section crossing the two cylinders. To distinguish these latter from each 
other I will now employ terms used in my previous memoir, designating the inner one 
the medullary cylinder and the outermost the ligneous zone. In fig. 10 part of the 
former is represented by c and the latter by d. The drawing shows the gradual 
increase of size in the vessels of the ligneous zone as we proceed from within outwards. 
At d' we find the very small vessels from amongst which the radiating exogenous 
series originates ; and we now find that large and well-defined bundles of vessels (in) 
spring from the same series, but which curve rapidly outwards so as to proceed hori- 
zontally, and at right angles to their original course, to the periphery of the ligneous 
zone. These vessels are very small, not averaging more than ‘0006 in diameter ; but as 
considerable numbers of them are aggregated to form each bundle, the latter attains 
to conspicuous dimensions. That they are identical in character with those already 
noticed as observed in the young leaflets I have no doubt ; but it is also obvious that the 
bundles have now become very much enlarged, though no corresponding enlargement 
has taken place in the individual vessels. This increase in the size of the bundles is 
explained by the fact, that whilst in the specimen represented in Plate XLI. fig. 1 the 
largest leaflets are not more than ’055 in diameter, in that under consideration (Plate 
XLII. fig. 9) they have expanded to more than double that size, or T2. Medullary rays 
also now make their appearance in the ligneous zone ; but as I propose to describe these 
more fully when speaking of the matured stem, I will not dwell upon them now. The 
greater part of the bark has disappeared from this specimen ; all the inner parenchy- 
matous layer is gone, and most of the prosenchymatous one. All that remains consists 
of the parenchymatous subepiderm with its leaf-petioles (Plate XLII. fig. 9, l), and with 
a small portion of the prosenchyma of the outer layer, i, attached to its inner surface. 
In the transverse section the cells of the latter have now be nun to assume the radiating 
linear position which I described in my last memoir as so commonly characterizing this 
tissue amongst the Lepidodendroid plants. 
The specimen last described has obviously been a stem or branch, with a diameter of 
about 1-| inch ; but other examples in my cabinet lead us up from this one to stems of 
much larger size. 
