164 
Lady Isabel Browne. 
developed xylem, a part of the cylinder is purely cauline ; but even 
if Professor Farmer’s view of the nature of the stele of Isoetes 
lacustris were made to apply as a generalization to the rest of the 
genus it would not invalidate their origin from the Lepidodendraceas, 
since if the latter supposition be correct extreme reduction of the 
stem must have taken place, and this must obviously have affected 
the size of the stele considerably; the leaves, probably, were also 
reduced, though not nearly to such an extent as the stem, and their 
bundles, therefore, carry away a comparatively larger portion of the 
stele of the stem, so that the purely cauline portion of the latter 
might well become obsolete. 
This leads us to the second and more serious objection to the 
hypothesis under discussion : the extreme reduction which it 
supposes and the absence of any intermediate forms, except the 
doubtful Pleuroineici. The anatomy of the latter is unknown, 
though judging from the figure given by Potonie its external 
appearance is somewhat similar to that of a Sigillaria (12). 
Further, though Dr. Scott states that the nature of the reproductive 
organs of Pleuroineici is not known, such doubtful indications as the 
fructifications afford agree neither with the Lepidodendraceae nor 
Isoetes. Pleuroineici, then, somewhat resembles Sigillaria in 
appearance and is intermediate in age between the latter genus and 
Isoetes, but it has not been shown to be intermediate between them 
in any essential character. The reduction is then very great, for, 
whereas most Lepidodendraceae were trees, bearing numerous 
well-developed leaves, Dr. Campbell considers that in Isoetes all the 
leaves were sporophylls, except those imperfect ones separating the 
sporophylls of successive years (5). As he calls these sterile leaves 
imperfect he presumably regards them as sporophylls sterilized by 
unfavourable conditions. Their position would agree with such a 
hypothesis, but if this interpretation is correct the vegetative leaves 
of the Lepidodendraceae have been reduced until they became 
obsolete. The amphibious mode of life, hypothecated for the 
common ancestor of all the species of the genus, and the aquatic 
habit of many recent species may, however, account for a consi¬ 
derable amount of reduction. 
The third objection to the origin of Isoetes from the Lepido¬ 
dendraceae is the fact that it is nou-strobiloid while most of the 
latter appear to have possessed very definite strobili. In Pinako- 
dendron, however, the sporophylls were not differentiated from the 
leaves, and were not aggregated into cones ; should this genus 
