FROM THE LOWER COAL-MEASURES. 59 
tubes; we may hope that more perfect longitudinal sections may ultimately furnish 
decisive evidence. The position of the supposed sieve-tubes in the outer part of the 
phloem may seem to cast some doubt on their nature, as in most cases we are accustomed 
to find the small-celled protophloem towards the periphery. We have, however, an 
analogy in the Marattiaceze, where, as Miss Shove and the late Mr. Brebner have 
shown, the protophloem is internal, and the large sieve-tubes occupy a peripheral 
position *. 
With reference to the large size of the supposed sieve-tubes in the leaf-traces and 
petiolar bundles, compared with those of the stele, I have observed a similar difference 
in the case of Lygodium pinnatifidum in preparations shown me by Mr. Boodle. In 
other species of Lygodiwm the sieve-tubes of the leaf-traces, and of that part of the stele 
which is in direct communication with them, are of larger size than those in other 
parts of the stele. 
The cortical tissue, which is identical with that of the leaf-bases, requires no detailed 
description. The short-celled ground-parenchyma contains numerous saes with carbon- 
aceous contents, and is traversed by secretory canals and strands of sclerenchyma ; the 
latter, as already mentioned, are for the most part grouped around the vascular bundles. 
The hypoderma is essentially of the Myeloxylon Landriotii type, as in JMedullosa 
anglica and other species (Pl. 9. fig. 16). 
The Petioles. 
A peculiar patchy form of preservation is characteristic of the vegetable remains 
found in roof-nodules. This is seen to some extent in the specimen of the stem of 
Sutcliffia (Pl. 7. fig. 5), though fortunately the greater part of the structure is intact in 
this case. The specimens of the petioles are much less favourable, for here a large part 
of the area of the sections is occupied by merely mineral substance, and the organic 
structure is limited to a peripheral band and to scattered patches of the internal tissue, 
though the preservation at these favoured spots is often excellent, as fig. 22 (Pl. 10) 
shows. 
Attention has already been called to the great size of some of these petioles, which 
reach 12 em. or more in diameter. Some of the sections appear to have been cut 
through the place where a petiole forked. The general structure is similar to that of 
the leaf-bases attached to the stem. The bundles, however, are more numerous, and 
often of large size, attaining a diameter of 3mm. Here, as in the leaf-bases, the lateral 
fusion of bundles is a common phenomenon. The larger and more central bundles are 
of the type described above as radially symmetrical. The example figured (fig. 22) 
is a unilateral strand from near the periphery. There is nothing to add to the 
description of the bundles already given, for their structure in the leaf-base and petiole 
is identical. There is, however, a slight difference in the distribution of the scleren- 
chymatous strands around the bundles. In the leaf-base each bundle has a ring 
of these strands surrounding it, while in the petiole they often form a horseshoe 
* Shove, 1900, p. 513, pl. 29. figs. 23, 25, & 28; Brebner, 1902, p. 542, pl. 23. figs. cs 
