ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 755 
2. The secretory sacs, which are so conspicuous in petioles of Lyginodendron, ave 
either absent, or, at least, much less noticeable in those of Heterangium.* 
2. Form and Structure of the Leuf. 
We are only able to obtain a very general idea of the form of the leaf in H. Grievii. 
Several of the preparations show a confused mass of petioles of various orders, inter- 
mixed with fragments of leaflets. The petioles vary in diameter from 4 millims. to 
0°4 millim., or even less, All alike show the characteristic structure of the cortex, 
which distinguishes these petioles from that of any other known plant, with the 
exception of Lyginodendron, from which they can be separated by the characters 
mentioned in the last paragraph. In some of the specimens the branching of the 
petioles is shown. Plate 26, fig. 23, represents a small portion of one of the prepara- 
tions in question, but gives only an imperfect idea of the complexity of the mass of 
foliage which it contains. Sections of leaflets are found among the petioles, and are 
sometimes in connection with their finer branches. The structure of the leaflets is 
never well preserved, but the form of the sections indicates a decided resemblance to 
the leaflets of Lyginodendron. 
Although the evidence available does not admit of our attempting any detailed 
reconstruction of the form of the Heterangium leaf, yet one conclusion of great 
importance follows with certainty from the facts observed. The leaf of H. Grievi 
must have been a highly compound one, with a much branched rachis, bearing 
numerous small leaflets. In other words, the foliage of our plant was certainly of a 
Fern-like character, and was totally different from any leaves known among the 
Lycopodiacez. 
Our knowledge of the structure of the leaf is practically limited to that of the 
petiole and its branches. It is true that in some of the preparations from the 
Dulesyate specimens beautiful sections of leaflets are shown.t Unfortunately, 
however, these preparations contain specimens of Lyginodendron, as well as of 
Heterangium, so it is impossible to decide for certain to which plant the leaflets 
belong. 
The sections of the petiole are not equal to those of Lyginodendron, but the chief 
points in the structure can be determined. The vascular bundle was no doubt con- 
centric. The central mass of wood is always surrounded by a zone of thin-walled 
tissue, which, though imperfectly preserved, can hardly be interpreted otherwise than 
as phloém.t 
* For specimens showing leaf-bases in connection with the stem, sce the series C.N. 1240-1247; also 
C.N. 1281, 1286, 1294, 1915 A, and 1915 M. 
+ See C.N. 1915-1915 C. 
{ Transverse sections of the petiole are contained in the following preparations: C.N. 1244, 1283, 
1289, 1300. The last-named is figured in Winramson, “ Organization,” Part IV., Plate 28, fig. 46. It 
was then termed “a young twig.” For longitudinal sections see especially C.N. 1286, 1287, and 1292, 
5 BE 2 
