414 Scott and Hill.—Structure of Isoetes Hystrix . 
of which, according to Baker’s arrangement, there are only 
two (I. Hystrix and I. Duriaei) among the forty-nine species 
described in his ‘ Fern-Allies.’ Although so much work has 
already been done on Isoetes , such a re-investigation does not 
appear superfluous, in view of the widely different views 
which prevail as to the affinities of the genus, and of the 
ever-increasing interest which attaches to the surviving 
Pteridophyta, as we become better acquainted with their 
history in past ages. 
Our own observations relate entirely to the structure and 
development of the vegetative organs, for it is these points 
which appear most to need further elucidation, while the 
reproductive and embryological phenomena have been 
thoroughly investigated by our predecessors. 
A terrestrial species, such as I. Hystrix , is clearly the most 
favourable for anatomical investigation, as the tissues are 
more characteristically developed than in the reduced, 
aquatic forms. In a genus, such as Isoetes , however, with 
so great a preponderance of aquatic or sub-aquatic repre¬ 
sentatives, we must regard it as an open question whether 
these, or the exceptional terrestrial forms, are the more 
primitive. In any case there can be no doubt that the terrestrial 
species share, to a considerable extent, in the reduction 
which characterizes the genus as a whole. (See below, p. 443.) 
As regards the plan of the paper, after a few general 
descriptive remarks, the structure of the stem will be con¬ 
sidered, then that of the leaves, and finally that of the roots. 
In conclusion, the systematic position of the genus and its 
relation to other Pteridophyta, recent and fossil, will be 
briefly discussed. 
With reference to method, it need only be said that we 
have relied for the most part, though not exclusively, on 
serial microtome-sections, prepared by the usual paraffin 
process, and double-stained with safranin and haematoxylin. 
The small size of the plant and the softness of most of its 
tissues render it peculiarly amenable to microtome-treatment, 
and the study of complete series of sections, extending 
