Scott and Hill.—Structure of Isoetes Hystrix. 443 
The general structure and dichotomy of the root require 
no recapitulation. 
Conclusion. 
The investigation of Isoetes Hystrix shows that this terres¬ 
trial species differs less from the aquatic forms than might 
have been expected. The xylem throughout the plant is 
somewhat better developed, but the difference is not striking ; 
the secondary wood, though a constant feature, is very variable 
in amount, and seldom plentiful. The leaves, with their 
numerous intercellular spaces, have an aquatic character, 
shown especially by the formation of the canals, with endo- 
dermal boundaries, in the xylem. This curious feature 
strongly suggests an aquatic adaptation, and it is singular to 
find it in a terrestrial plant. The whole structure indicates 
clearly that Isoetes Hystrix is in no way a primitive form, but 
rather represents an aquatic or amphibious type, which has 
become secondarily adapted to life on land. 
While we thus find aquatic characters in a terrestrial 
species, it is interesting to recall the fact that terrestrial 
characters also appear in some of the aquatic species. Thus, 
A. Braun (1863, p. 587) points out that a large part of the 
genus, including several submerged forms, possesses stomata 
on the leaves. 
We are thus naturally led to regard the genus Isoetes as 
a group that has long hovered on the limit of terrestrial and 
aquatic life, some of the forms becoming wholly submerged, 
while a few have definitely betaken themselves to dry land, 
a large proportion leading a more or less amphibious 
existence 1 . 
For this reason, among others, we cannot accept Mr. 
Wilson Smith’s conclusion (1900, p. 323) that the genus 
Isoetes represents ‘ a more primitive form of sporophyte than 
any other vascular plant.’ The group has clearly undergone 
1 In Baker’s Fern-Allies, out of the forty-nine species described, nine are classed 
as aquatic, nine as subaquatic, twenty-nine as amphibious, and only two as 
terrestrial. 
