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98 DR. D. H. SCOTT ON THE HETERANGIUMS 
for water-storage. As only the wood is known, we have no information as to 
the outward course of the leaf-traces. On the existing evidence, there is 
nothing to connect the genus with the polydesmie species of Heterangium, 
though Megalowylon appears to represent, in a different direction, an advance 
on the original protostelic type of structure. 
Polydesmy is common enough among plants grouped under Pteridosperms 
or Cycadofilices, but the only families w hich seem near enough to Heterangium 
to deserve mention here are the Medullose:e and the Calamopityez. 
A simple Medullosean stem, such as that of MJedullosa anglica, has been 
described as essentially a polystelic /eterangium. Each stele, in fact, 
repeats the characters of the Heterangivm monostele ; the position of the 
protoxylem-groups is mesarch, but approaching exarchy, as in our polydesmic 
Heterangiums. Several distinct bundles are given off from the stele to form 
the trace of a single leaf and they divide up freely on their passage outwards 
(Scott, 1899, p. 194; 1914 ; de Fraine, 1914). In all these points there is 
a clear analogy with the plants which form the subject of this paper, the 
polydesmy, of course, going much further in Medullosa than in any species 
of Heterangium. 
Suteliffia, though not strictly polystelic, otherwise differs from Heterangium 
more widely than does a Medullosa of the anglica type. The stele is exarch, 
the emission of leaf-traces takes place through the intermediation of bulky 
meristeles, and the trace-bundles are concentric throughout (Scott, 1906 ; 
de Fraine, 1912). The polydesmie Heterangiums no doubt show some slight 
approach towards certain characters of the Medullosew, but not enough to 
justify one in supposing that they are on the line of descent of the latter 
family. — 
A comparison with the Calamopityez is interesting. These are monostelic 
plants whieh until recently have been assumed to have had a true pith, 
surrounded by a ring of primary mesarch xylem-strands, the stem-strueture 
thus presenting a close analogy with that of Lyginopteris. In a joint paper 
on Kentucky fossil plants, of Lower Carboniferous age, Prof. Jeffrey and I 
have, however, shown that the structure in the species Calamopitys americana 
was essentially protostelic, the “mixed pith” containing a varying proportion 
of tracheides among the parenchyma (Scott & Jeffrey, 1914, pp. 318, 326). 
There is reason to suspect that the same may be true of the Thuringian 
species, C. annularis (Unger). This brings the species in question nearer to 
the Heterangium type of structure. The petiole (Aalymma) has long been 
known to be polydesmic ; our work on C. americana showed that the leaf- 
trace, single at its origin, almost immediately divides into two strands * 
further subdividing in the cortex. In C. Saturni, Unger, previously 
* Since the paper by Prof. Jeffrey and myself was published, I have obtained even more 
convincing evidence of the early division of the trace, . 
