i 62 
Lady Isabel Browne. 
Selaginellacese. A remarkable similarity between the two orders is 
the presence of mucilage-containing cavities in the leaf-bases of 
Isoetes and parichnos in the Lepidodendraceae. In Isoetes these 
arise by the degeneration of a strand of parenchyma and in their 
origin as well as in their position on either side of the vascular 
bundle they appear to be strictly comparable to the parichnos. 
They are, indeed called parichnos by Mr. T. G. Hill, their 
discoverer (7). This point is not perhaps of great importance, 
since though parichnos are best known in the Lepidodendraceae 
they occur also in the Lycopodiaceae. The relatively large size of 
the leaves may be due in part to the extreme stuntedness of the 
stem, but this cannot account for their actual size, which finds its 
nearest parallel, within the phylum, in the Lepidodendraceae. The 
occasional mesarchy of the foliar bundles, especially towards the 
base of the leaf, also recalls Lepidodendron, but this character is 
also found in herbaceous Lycopods and may, in this case, be due to 
the size of the leaves. The morphology of the Stigmarias, or 
underground organs of the Lepidodendraceae, and of their appen¬ 
dages has been much discussed. It is still an open question 
whether the main axes of Stigmaria are really represented by the 
lobes of the Isoetaceous stem, but such an exceptional character 
as the downward growth of the base of the stem may indicate that 
the lobes were once, in the phylogeny, capable of distinct growth in 
a contrary direction to the normal upward growth of the stem. If 
Isoetes arose from the Lepidodendraceae, the Stigmarian axes must 
have been reduced, in the course of its evolution, almost to 
disappearance. But in any case, in a phylum such as the Lycopo- 
diales, where root and stem are admittedly not sharply differentiated 
from one another, the comparison between the roots of Isoetes and 
the appendages borne by Stigmaria, a comparison that has been 
strongly emphasized by Dr. Scott in a course of lectures delivered 
in 1904, seems a legitimate one. Both are monarch dichotomously 
branching subterranean organs. The agreement between them is 
striking, for monarch roots are rare, though they occur within the 
phylum in Selaginella. The radial elongation of the sporangium of 
Isoetes is certainly suggestive of Lepidostrobus and more particularly 
of the allied Sigillariostrobus, for the degree of elongation of the 
sporangium corresponds more nearly to that found in the latter. 
Though this character carries a good deal of weight, it should be 
remembered that radial elongation was not characteristic of all 
Lepidodendraceous sporangia, for those of Bothrodendron and of 
