OF THE FOSSIL PLANTS OF THE COAL-MEASTTEES. 
389 
lower part of the figure the latter tissues are yet further deflected right and left, to allow 
a large vascular bundle to pass out between them. This bundle and the surrounding- 
tissues are shown, still further enlarged, in Plate XXIV. fig. 12. The section has 
divided the vessels in the centre of the bundle transversely ; but the more peripheral ones 
trail across the field of view in irregularly radiating lines, which, at their peripheral ends, 
appear to merge with the curving vessels of the parent axis. Having obtained another 
specimen similar to that just described, I made a series of tangential sections of it, and 
found that the divergent bundle of vessels diminished rapidly in size as I approached 
the medullary axis, and became almost, if not wholly, wanting before reaching the inner 
surface of the ligneous zone, affording an additional illustration of the exogenous deve- 
lopment of both stem and branch. The section Plate XXVI. fig. 24 throws additional 
light upon these arrangements: this is part of a transverse section of a stem, but the 
section has passed somewhat obliquely through one of these branches ; besides which, 
owing to the curvilinear direction followed by the branch, the section has divided its 
upper half almost transversely, whilst its lower or inner portion is intersected more longi- 
tudinally. The peripheral portion of the section shows clearly that this branch con- 
sisted of laminae arranged in radiating order ; whilst the more central part demon- 
strates that these laminae consist of true vessels, which are derived from the exogenous 
ligneous zone. 
But there is yet another peculiar lateral appendage found in this plant, as seen in a 
transverse section of a stem represented in Plate XXIII. fig. 7. In the centre of this 
figure we observe that the vascular laminae bounding two of the medullary rays are 
forced asunder, to permit the escape of a large mass of tissue (n), some of which 
emanates from the left-hand ray, but the greater portion of it comes from the one to 
the right. It will be observed that in this instance there is no disturbance of the regular 
order of the vascular laminae of the exogenous zone beyond their being thrust asunder 
at their peripheral margins ; neither is there the slightest indication of what appears 
obvious in Plate XXVI. fig. 24, viz. a direct connexion between the vascular tissues of 
the parent axis and those of the branch. As the latter proceeds outwards, it passes 
through the parenchymatous (//.) and prosenchymatous (k) layers of the bark, contracting 
rapidly in diameter as it does so, and terminates abruptly at the circumference of the 
outer bark (A). 
Unlike the previous examples, this divergent structure consists of a mass of cells, many 
of which are so elongated and fusiform as to assume a pseudo-vascular aspect. But each 
cell and pseudo-vessel, unlike the cells of the parent medulla and medullary rays, is beau- 
tifully reticulated like the true vessels of the central axis — an important feature to be 
borne in mind on endeavouring to assign a probable function to this organ. Equally 
important is it to remember that the tissues composing this appendage, instead of being 
derived from the vascular laminae forming the two lateral boundaries of one medullary 
ray, have every appearance of originating in some metamorphosis of the mural cells of two 
such rays. We can clearly trace the pseudocambial layer of the parent axis accom- 
