ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 767 
rest has disappeared, having, perhaps, been sufficiently replaced by the newly 
acquired secondary tissues. 
Platyzoma, one of the Gleicheniaceze, resembles Lyginodendron in possessing a 
pith,* but the nearest analogy to Lyginodendron among recent Ferns is probably to 
be found in Osmunda, which has recently been investigated afresh by ZENeTTI.t 
Osmunda is a monostelic Fern with a large pith, collateral bundles in the stem, and 
a concentric bundle in the petiole. The leaf-trace bundles, however, unlike those of 
Lyginodendron, become concentric immediately on leaving the stele. Another 
difference from the fossil genus is that in Osmunda the phloém forms a continuous 
ring, which does not seem to have been the case with the primary phloéin of Lygino- 
dendron, The position of the protoxylem is also different: in Lyginodendron,as we have 
seen, all bundles are mesarch, with the protoxylem-strands embedded in the primary 
wood nearer its outer than its inner surface. In Usmunda the bundles only have 
protoxylem at all in the upper part of their course.{ In the bundles, where they 
first enter the stele, the protoxylem lies on the inner concave side of the xylem 
horse-shoe. Lower down, where the horse-shoe becomes a closed curve, the proto- 
xylem comes to be surrounded by wood, so that here we have a certain resemblance 
to the Lyginodendron bundle, though perhaps only an accidental one.§ 
The example of Osmunda is, at any rate, sufficient to show that the general 
primary structure of Lyginodendron is consistent with Fern affinities. 
So far we have considered only the points in which our two genera approach the 
Ferns. We have now to call attention to the remarkable relation which their struc- 
ture shows to that of Cycadez. 
The secondary tissues strongly recall those of Cycads; the resemblance shows 
itself especially in the simple structure of the secondary wood, the great number of 
the medullary rays, and the tracheides with multiseriate bordered pits on their radial 
walls. The structure of the phloém also agrees well with that of Cycadez, and in 
the case of Heterangium tilieoides the agreement extends to the details of the sieve- 
plates. The rarity of the tangential divisions in the medullary rays, causing an 
apparent interruption of the cambium at these points, is another peculiarity which 
both our fossil genera share with Cycadez. || 
So far as the secondary tissues are concerned, the Cycadean characters apply to 
both Heterangium and Lyginodendron, though, on the whole, more marked in the 
latter. The primary structure of the stele in Heterangiwm bears no resemblance to 
* Some account of the anatomy of Gleicheniaces will be found in Porraunt, ‘“ Recherches anato- 
miques sur les Cryptogames vasculaires,” ‘ Aun. des Sci. Nat., Bot.,’ Ser. 7, T. 18, p. 171, 1894. 
+ ‘ Botanische Zeitung,’ 1895, Abth. 1, p. 53. 
¢ An interesting analogy for this is found in the fossil Purorylon ; see Bertrano et Renautt, “ Re- 
cherches sur les Porcaylons,” p. 283. 
§ See Zunerri, loc. cit., woodcut 2, p. 57, Plate 2, figs. 2 and 3. 
ij See pz Bary, “ Comparative Anatomy,” d&ec., p. 611. 
