Lycopodiales. 
161 
general replacement by the abnormal kind is a later modification. 
On the other hand the secondary growth may have been primitively 
abnormal in the Isoetaceae, and the normal one may have arisen 
in connection with the demands for more secondary thickening in 
a terrestrial species. 
The genus shows no other variations of great phylogenetic 
interest, so that we may proceed to consider the affinity of the 
Isoetaceae with the Lepidodendraceae. Dr. Scott’s comparison of 
Pleuromeia with the latter order has already been mentioned; in 
the final chapter of the first edition of his “ Studies ” we find the 
following interesting passage: “The curious Triassic genus, 
Pleuromeia, .has much in common with Sigillaria and in some 
points appears to approach Isoetes. The four Stigmarian axes which 
radiate from the base of the Sigillarian stem are in Pleuromeia 
replaced by four rounded lobes, separated by furrows and marked 
with the bases of roots. A further reduction might conceivably 
have led to the characteristic furrowed stem of [socles. However, 
neither the inner structure of Pleuromeia, nor the nature of its 
reproductive organs, is known, so the question must remain sub 
judice” (13). 
The arguments favouring the reduction of Isoetes from the 
Lepidodendraceae will now he stated and examined. Secondary 
growth in thickness of the stem of Isoetes is a very significant 
character, for we are obliged to regard it as a survival in reduced 
forms, since the stunted nature of the plant and the probability 
that the terrestrial species are descended from an amphibious form 
preclude the notion that it might he a recently acquired character. 
If the normal kind of cambium is primitive Isoetes may well have 
been descended from a typical Lepidodeudrou; if the abnormal 
secondary growth is primitive it is noteworthy that, although Dr. 
Scott and Mr. Hill found no exact parallel to this method of 
growth in any other plant, they expressed the opinion that among 
Lycopods it resembled most closely the type of secondary growth 
found in Lepidodeudrou fuligiuosum. The absence of a pith 
may be due to reduction, but if Isoetes is descended from the 
Lepidodendraceae it may well have originated from primitive, proto- 
stelic forms with centripetal primary xylem, and though the stele 
was probably larger in the ancestors of Isoetes it may never have 
been more complex. The presence of a ligule cannot be held to 
weigh heavily in favour of an affinity with the Lepidodendraceae, 
for it is also characteristic of the herbaceous Miadesmia and 
