ORGANIZATION OF THE FOSSII, PLANTS OF THE COAL-MEASURES. 737 
centre of the cylinder, but we never find a definite pith. Secretory sacs sometimes 
occur in the conjunctive tissue. 
The best transverse sections of young roots show the primary phloém-groups quite 
clearly (see photograph 11 and fig. 20, ph.). They are distinguished at once from the 
conjunctive parenchyma by their smaller cells, with thinner walls, and by a peculiar 
brownish tint, which may be due to the presence of carbonaceous matter derived from 
the abundant cell-contents. These groups alternate in a perfectly regular way with 
the xylem groups (photograph 11, fig. 20), They are always separated from the 
adjacent xylem by one or more layers of conjunctive parenchyma. In longitudinal 
sections, the elongated phloém elements are even more sharply distinguished from the 
short-celled parenchymatous tissue (C.N. 1633). 
The limits of the central cylinder are not very well defined. It appears, however, 
that the pericycle was one cell only in thickness, at least, opposite the xylem-groups, 
though sometimes thicker opposite the phloém, and that the layer next beyond this, 
which has rather stouter walls, represents the endodermis (figs. 19 and 20). It is 
only in the rarest cases that any trace of the radial marks* characteristic of endo- 
dermal cells can be distinguished. 
The broad inner zone of the cortex, constituting the greater part of its bulk, con- 
sisted of lax somewhat lacunar parenchyma, traversed by an enormous number of 
secretory sacs, the dark contents of which give a characteristic appearance to the 
sections (see photographs 11-14, figs. 17, 19, and 20). The transverse septa in these 
sacs are very evident in longitudinal sections, proving that these organs were formed 
from cells and not from intercellular canals. 
In this cortical zone branched and septate fungal hyphe are often met with—a 
proof, if any were needed, that true Fungi (“Mycomycetes” of BrereLp) already 
existed in the Carboniferous epoch. 
The outer zone of the root is formed by the very well-marked “ epidermal layer,” 
consisting of two or more series of rather large thin-walled cells, which are often much 
better preserved than the rest of the cortex, so as to stand out conspicuously, when 
the more internal tissue has become disorganized (see the various figures in Memoirs 
VIL and XIII, also Plate 24, fig. 17 of the present paper). The general appearance 
of this layer is often suggestive of a velamen, but there are no special markings on 
the cell-walls. We find no signs, even in the youngest rootlets, of any cells exterior 
to this zone, which we may therefore assume to have been really superficial. No 
root-hairs have been observed, but the external cells are often somewhat papillate. 
We may speak of this zone as the outer cortex or epidermal layer indifferently, for we 
have no means of determining its strict morphological nature. 
The typical roots of Lyginodendron usually attain, before secondary thickening, a 
* They are visible in a small tetrarch root (O.N. 1234), previously figured on a small scale, 
Wixtiauson, “ Organization,” Part XIIL., Plate 24, fig. 22. 
MDCCCXCV.—B. 5 0 
