466 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
London, a section of a Lepidodendron of the type of L. Harcourtii, in which nearly 
every one of the cells is in a state of simultaneous meristemic division. Fig. 20 repre¬ 
sents a small portion of this section. The entire stem has a diameter of about 175 ; 
the vascular cylinder is about '37 and the medulla about ‘3 in diameter. Fig. 20, a, 
are the large vessels of the vascular cylinder. At b we have the thick, older, cell-walls 
of the medullary cells, whilst at c we have the newly formed septa by which each older 
cell is becoming divided into two or more new ones. The older cells exhibit the 
form of regular parenchyma; the new ones are extremely irregular in size and form, 
and would obviously require to undergo a considerable expansion, leading to a steady 
enlargement of the entire medulla, before they attained to the regular forms of the 
parent cells. Every fact observed thus far indicates that the vascular cylinder, a, 
develops centripetally, and that the multiplication of the medullary cells here demon¬ 
strated to have existed was preparatory to the conversion of the outermost of them 
into new vessels. The known facts of enlargement in the sizes of and in the number 
of vessels composing the vascular cylinder thus receives a probable explanation.'”' 
Halonia. 
In my memoir, Part II., p. 222, et seq., I described the organization of some Halonice 
in which the central axis consisted, as in most of the young Lepidodendroid branches, 
of a central medulla surrounded by a vascular, non-exogenous cylinder. Thanks to 
Professor Young and Mr. J. Young, of the Glasgow University, I am now able to 
describe another interesting form of Halonia from the Arran deposits of Laggan Bay. 
In my descriptions of the Lepidodendroid plants from that locality (Memoir, Part X., 
p. 494) I pointed out that in all the small Lepidodendroid twigs, occurring so abun¬ 
dantly in those beds, the central axis was a solid, non-cylindrical, bundle of vessels 
( loc . cit., Plate 14, figs. 1 and 2). Fig. 21 represents a section of the Arran Halonia 
which has a mean diameter of about - 87—a small portion of its peripheral cortical 
tissue having disappeared. The central axis, a, consists of a solid rod of barred 
vessels—resembling, in this respect, the young twigs with which it was found asso¬ 
ciated in the Arran beds. The surrounding cortical layers consist, as usual, of an 
innermost bark at b, composed of rather compressed but regular parenchyma, the cells 
of which are small, averaging about ’0012 in diameter. The space, c, seems to have 
been occupied by very similar cells, only a few of which remain. At d we have a 
middle bark, composed of larger and coarser parenchymatous cells, and at e is the 
usual prosenchymatous layer, the small cells of which are arranged in radiating series. 
In the specimen from which my sections were prepared the protuberances so charac- 
* Though this addition to the number of the vessels appears to be made at the centripetal border of 
the vascular cylinder, a, it must not be supposed that this addition reduces the diameter of the medulla. 
Unlike what occurs amongst Phanerogams, the medullce of many Lepidodendra obviously continue to 
enlarge long after the development of the exogenous zone.—July 8, 1883. 
