OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
203 
medium exhibit the same laminae arranged, as in L. Jidiginosum, in a very irregnlai', 
undulating manner. In both these cases this irregularity is due to the excessive 
development, amongst the vascular laminae, of a cellular parenchyma. This feature, 
common to the two plants, is suggestive in both of a rudimentary type of exogenous 
growth ; one, however, in which tlie L. intermedium has attained to a more advanced 
stage than L. fidiginosum has done. In the types in which the exogenous zone has 
reached a yet higher condition the number of the disturbing cells has been very much 
reduced, such only remaining as could be utilised as muriform medullary rays. How^ 
far a yet lower Lepidodendroid state has existed, in which no form of exogenous 
growth was developed at any period of life, cannot yet be determined. Thus far, 
however, we have obtained no specimen of L. Ilarcourtii which possesses such a zone, 
though I have a stem of that plant which is 3^ inches in diameter. Nevertheless, 
that still larger stems may yet be discovered, showing exogenous growths of xylem, 
is suggested by the Arran plant, of which I have sections fully 3 inches in diameter, 
in which no such growth has yet made its appearance ; whilst other stems of very 
much larger dimensions have the exogenous cylinder fully an inch in thickness 
between medulla and cortex. It now becomes more than probable that at one stage 
or another of their development all the Carboniferous Lepidodendroid stems grew 
exogenously. In some cases, as in fig. 15 of the present Memoir, such a growth took 
place when the medullary vascular cylinder of a branch was not more than the one- 
fortieth of an inch in diameter, whilst in others, as in the Arran plant represented in 
Plate 14, fig. 5, of my Tenth Memoir, the exogenous growth, though present, had 
made very small progress when the medullary vascular cylinder was fully an inch and 
a half in diameter. 
The various instances in which I have now been able to trace the developmeni. of a 
true medulla in the Lepidodendra throws, I think, some light upon the physiological 
character of that development, as well as upon its homologies amongst living plants. 
All botanists are aware, though many geologists may not be, that thennedulla of an 
ordinary exogenous stem makes its appearance in a very different manner from that 
seen in the Lepidodendra. The tip of a growing twig consists of a mass of what is 
termed “primitive tissue,” viz., of undifferentiated parenchymatous cells. Almost, 
though not quite, simultaneously, a ring of vascular bundles is formed, which separates 
the lower portion of that pi’imitive tissue into an inner mass, the medulla, or pith, and 
an outer ring of cortex. The medulla thus formed, though of small size, is merely a 
downward prolongation of the mass of apical cells, the two being absolutely continuous. 
As we trace this medulla downwards into the lower part of a shoot of the first year 
we find that the medulla increases in size up to a certain point, the distance of 
which point from the growing tip varies in different species of plants. As we thus 
descend we find that the increase in size is due to an increase partly in the number 
of the cells and partly in the diameter of the individual cells. Still lower dowm the 
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