ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 16 & 
narrower cells were parenchymatous. The analogy of Heterangium tilieoides, in which 
the phloém is perfectly preserved, supports this view. MM. Berrranp and Renavuur 
however, have come to the opposite conclusion in the case of Poroxylon, the phloém 
of which much resembles that of Lyginodendron.* 
The medullary rays broaden out somewhat in passing through the phloém zone. 
The secretory sacs are more frequent here than in the xylem portion of the rays (see 
figs. 1,5,and 7). Occasionally the medullary rays of the wood show signs of tangential 
dilatation, owing perhaps to the long-continued growth of the pith, but this is 
exceptional. 
We may sum up our results as to the secondary tissues in the stem as follows : 
1. The stem of Lyginodendron constantly formed a large amount of secondary wood 
and bast by means of a normal cambial layer. 2. The secondary wood consisted of 
tracheides and xylem-rays; the former possessed numerous bordered pits on their 
radial walls. 3. The secondary bast-zone consisted of groups of phloém separated by 
the phloém-rays. In the phloém larger and smaller elements alternated regularly. 
It is probable, though not certain, that the larger elements were the sieve-tubes. 
4. The secondary tissues bear a general resemblance to those in the stem of 
Cycadeze. 
5. Pith and Pericycle. 
The pith of Lyginodendron consists of moderately thin-walled parenchyma, in 
which large groups or nests of dark-coloured tissue are imbedded. These almost 
black masses, which also occur in the pericycle, and sometimes in the cortex, give 
a most characteristic appearance to the sections (see photographs 1 and 4, fig. 1, 
&e.). It is not easy to determine the nature of the tissue of which they are 
composed. The cells in question do not differ greatly in form from those of the 
thin-walled medullary parenchyma. Their walls, however, are decidedly thicker and 
their arrangement is more regular, showing a fairly definite longitudinal seriation. 
Their cavities are more or less completely filled with carbonaceous contents. It is 
a question whether these carbonaceous bodies represent the actual contents of the 
cells or whether they are due to the degradation of the inner layers of the much 
thickened cell-walls. We incline to the latter view: (1) because the apparent 
contents often show a distinctly laminated structure and (2) because the fibres of 
the outer cortex, which were certainly sclerenchymatous, are often filled with similar 
material. We will, therefore, provisionally speak of the dark-coloured groups as the 
sclerotic nests. 
The pith further contained numerous thin-walled sacs, with more uniform 
carbonaceous contents, which we regard as probably of a secretory nature. They are 
sometimes isolated, sometimes arranged in vertical rows (see Plate 21, figs, 2 and 3, 
* ‘Recherches sur les Porowylons,’ loc. cit., p. 289. 
