i6o Lady Isabel Browne. 
to be present (22); the fructification, judging from Graf Solms- 
Laubach’s description, appears to have resembled that of the Lepido- 
dendraceae in so far that it was strobiloid (22). Professor Bower, 
however, regards the strobilus as undifferentiated from the main 
axis that bears it (4). Unfortunately the interpretation of the scales 
composing the fructification of Pleuromeia is very doubtful. In 
one case at least the scale seems to have borne on its back a small 
body, which Graf Solms-Laubach states one might feel inclined to 
term, as as did Spieker, an earlier investigator, a sporangium or an 
ovule of the type found in Araucaria (22). Such an interpretation 
cannot be harmonized with the fructification of any of the known 
Palaeozoic Lycopodiales—or indeed with any known member of 
the phylum,—especially as regards the insertion of the sporangium, 
if such be the structure borne on the scale. The details of the 
fructification are, however, so ill-preserved that it is quite likely 
that they have been misinterpeted ; mention has therefore been 
made of the genus here, though its position is admittedly a very 
doubtful one. 
The next order, that of the Isoetaceae, contains but the single 
genus Isoetes. By far the greater number of species are amphibious 
or aquatic. Dr. Scott and Mr. Hill investigated Isoetes hystrix, 
one of the two terrestrial species, and as a result of their 
researches it appears likely that the common ancestor of all the 
species of the genus was amphibious (18). Presumably, therefore, 
this habit was retained in the greater number of species, a certain 
number becoming modified in connection with an aquatic or sub- 
aquatic habit, and two adapting themselves to a terrestrial 
existence. The plant of Isoetes is much stunted and bears closely 
set ligulate leaves, most of which bear a single sporangium on their 
upper surface. The stem has little primary xylem (which is 
apparently centripetal) and shows secondary growth in thickness ; 
this is generally described as abnormal, for the cambium produces 
a small amount of xylem and phloem on its inner side and 
secondary cortical tissues externally. But Dr. Scott and Mr. Hill 
have found that in Isoetes hystrix the cambium is sometimes at first 
normal in position with regard to the elements to which it gives 
rise ; this method of secondary growth, viz., the formation of xylem 
internally and phloem externally, is, however, soon replaced by the 
abnormal one. As this species has, owing to its terrestrial habit, 
better developed xylem than many others, it is possible that this) 
normal form of secondary growth is primitive and that its very 
