OF THE FOSSIL PLANTS OF THE COAL-AIEASURES. 
95 
plants. That 44 laminas of my example A can be represented by TL20 similarly 
arranged ones in the section Q shows a remarkable expansion of the medullary border 
that was produced not by the mere enlargement of the existing tissues, but by the 
intercalation of new ones. That such must have been the case is, as I have already 
observed, demonstrated by the dissociation of the primarily contiguous trachese of 
the central axis a of fig. 10, already described. We realise the extent of this separa¬ 
tion when we compare the intervals separating the bundles a of fig. 13 with the 
corresponding ones a' of fig. 12. In estimating the sizes of those intervals we must 
remember that fig. 12 is enlarged more than fig. 13 by at least four times. 
I repeat, therefore, that however brought about, wm have two separate and inde¬ 
pendent proofs of the origination and development of the medullary area. First in 
the separation of the clusters of trachem which primarily formed one united cluster, 
and secondly in the enormous increase in the number of the vascular laminm, the inner 
extremities of which, though commencing their growth at dlfierent periods of the 
plant’s life, alike start from the medullary border of the vascular zone, and extend to 
its periphery.* 
The origin of the cellular medulla in the young branches of Lyglnodendron presents 
no real difficulties. Plate 26, fig. 24, of my Memoir, Part IV., shows that these 
branches were derived from the exogenous xylems of the parent stems. The medullary 
rays of those woody zones are large and multicellular (Plate 13, fig. 3, h"), hence we can 
easily understand how a few small examjjles of these cells could be enclosed amongst 
the trachese of a central axis like fig. 10, a, and yet escape detection. Personally, I 
have no doubt that such was their origin. There is no reason for supposing that the 
area a of examples like fig. 11 was not normally occupied by the young medulla. 
The cells in such examples have merely failed to be preserved. In one of my slides 
(Cabinet number 1883) is an obliquely transverse section of a very young branch, in 
which the trachem are readily distinguishable by the conspicuous reticulations of their 
walls; running through the central axis of this branch is a very thin line, consisting 
of a small number of minute, but distinct cells. These germs would amply suffice for 
commencing a merlstemic action, which would produce such a medulla as I have 
described above. But another question is less easily answered. What has occasioned 
the expansion of the medullary area and its surrounding tissues ? Has it been due 
to the centrifugal pressure of the growing medulla alone, or have other forces taken 
part in the process ? 
It is scarcely necessary to remind any botanist that though, as recent exogenous stems grow, an 
enoi'mous inci’ease takes place in the number of the radial laminjE of the wedges composing their woody 
zone, little or no increase is seen in the number of those laminae, of which the inner extremities reach 
the central medulla, or the point which that medulla originally occirpied. All the additions are inter¬ 
calated more or less peripherally. 
