OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
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in the group. This cluster is surrounded by a ring of very small woody fibres, the 
Phloem of the German botanists, the innermost cells being the smallest and the outer 
ones the largest in the series. Around this central bundle is a cylindrical air-cavity 
traversed by numerous detached columns of cells, which ascend as they pass inwards 
from the cortical layer to the fibro-vascular bundle, to which latter they serve as a series 
of flying buttresses, sustaining it in its position. Externally to this air-cylinder is the 
bark, Avhich varies in its composition in different Lycopods. In Selciginella Martensii , 
denticulately and Wallichii the inner part of the bark consists of a dense mass of paren- 
chyma, with large cells and thin transparent walls, and with a few large chlorophyl- 
grains in each cell. Yet more externally this parenchyma gradually passes into a thin- 
walled fusiform prosenchyma, the walls of the cells becoming thicker as we proceed 
outwards, until at their external surfaces they present a woody structure, forming the 
outermost envelope of the stem. But on turning to the leaves we find something more : 
the substance of each leaf is parenchymatous, besides which it has a true epidermal 
layer of sinuous cells and stomata on its under surface*. 
In Lycopodium chamcecypctrissusy though the central fibro-vascular mass is more 
complex than that just described, it is, as Sachs justly points out, essentially the same ; 
but the air-cylinder of the Selaginellce is absent, as is also the inner parenchyma of the 
bark. The prosenchymatous layer is very thick, and closely embraces the fibro-vascular 
bundle ; its component cells also are much more thickened by ligneous deposits in their 
interiors than in S. Martensii. Another important difference exists in the fact that the 
parenchyma of the leaves now extends itself over the entire stem, forming an outer 
cortical layer ; but this is not invested by any true epidermis, such as is seen covering 
the leaves. 
If we turn from these general features to some special points in the development of 
these plants, we shall find that new light is thrown upon the fossil forms. The young 
growing bud at the tip of a Lycopod is composed externally of ordinary parenchyma ; 
but in its interior we find formed at the earliest period a central column of what Sachs 
designates procambium , a solid cylinder^ of very delicate, vertically elongated cells, the 
transverse section of which has in most species an elliptical outline. I have carefully 
traced the development of these procambial tissues in many Lycopods, and can thoroughly 
confirm the accounts given of them by Sachs. Where the first pair of leaves is given 
off in S. Martensii , a slender spiral or scalariform duct passes from each leaf into this 
procambial layer, through which the two vessels descend vertically into the stem at points 
corresponding, as Sachs correctly indicates, with the two foci of the ellipse, where it joins 
some vessels already formed in the stem itself. The second pair of leaves contributes a 
second set of vessels, which in like manner enter the procambial cylinder. We thus 
obtain, partly from the stem itself and partly by successive additions from the various 
leaves, two parallel columns of fibro-vascular tubes separated by the central mass of 
procambium. Descending still lower into the matured parts of the stem, we find that, 
* Selagineila clenticulata appears to have the same structure as S. Martensii. See Sachs's ‘Lehrbuch,’ fig. 89 A. 
