OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
311 
however, the German botanists especially have restricted the application of this term to 
a more special set of phenomena than was previously clone. They now limit the expres- 
sion cambium to a cellular layer which originates in a peculiar way, and which develops 
new tissues in a manner equally special, both processes being illustrated by what occurs 
in the majority of Dicotyledonous and Gymnospermous Exogens. In these plants.the 
young aerial buds and the tips of the young leaves and roots severally contain the special 
homogeneous parenchyma to which Sachs has given the name of “ procambium.” The 
foliar fibro- vascular tissues are developed, in the first instance, in this procambium ; and 
on tracing each bundle so formed down into the stem we find, in Dicotyledons and 
Gymnosperms, that its fibro-vascular elements are produced on both the central and the 
peripheral sides of the procambial mass. For those tissues which are produced on its 
inner or medullary surface, corresponding with the new wood of English botanists, some 
Germans assign the name of Xylem ; whilst to the tissues formed on the outer or peri- 
pheral side they give the name of Phloem, which is the equivalent of our English liber 
or endophlceum. These two elements of permanent tissue are developed centripetally, 
so far as each isolated string of procambium is concerned, until they almost meet in the 
centre, having used up in their growth a considerable portion, if not all, of the pro- 
cambial cells. At this stage the detached fibro-vascular bundles are separated from each 
other by some of the primitive cells constituting the primary medullary rays. The 
growth of the second year commences by the extension of the cambium-tissue, as inter- 
fascicular cambium, across the outer ends of these primary rays by the usual process of 
cell-fission, to which the German botanists give the general name of Meristem. Instead 
of the cambium continuing as a circle of isolated vertical strings, it now forms a con- 
tinuous cylinder, which repeats, on an enlarged scale, the operations of the previous year, 
with the addition of lengthening the preexisting medullary rays by adding new mural 
cells to the outer extremities of those in the Xylem layer, as well as to the inner ends 
of others separating the Phloem bundles. The Germans designate the latter the Phloem 
rays, in contradistinction to the Xylem rays separating the growing wedges of true wood. 
As I have already shown, nothing that exactly corresponds with the details of these 
processes has taken place amongst the fossil Cryptogams which I have described ; hence 
I cannot affirm that the latter possessed a cambium ring in the sense to which I have 
just referred. But that a process of new cell-growth has led to the development of a 
succession of enlarging woody zones, each in its turn enclosing more or less completely 
the preexisting ones, is certain. 
But there are many obscurities which make it difficult to ascertain what are the exact 
analogies subsisting between the growth-processes in the recent Dicotyledons and fossil 
Cryptogamic plants. In the former, the primary ring of vascular bundles in the stem 
consists of an aggregation of individual leaf-bundles. The equivalents of this foliar 
series, as I have shown in the preceding pages, are to be found in my medullary vascular 
cylinder, which in the fossil Lycopodiacete is mainly, if not wholly, composed of pro- 
longations of the true foliar bundles. So far as origin and position are concerned, this 
