284 
HISTORY OF BRITISH FERNS. 
leaves throughout the year, is commonly called Quillwort. 
The genus differs from Pilularia^ its nearest ally, and 
with which it is associated in the order of Pepperworts, in 
having its spore-cases enveloped by the dilated bases of its 
hollow leaves ; some of the spore-cases containing large, 
and some much smaller pollen-like spores. It may also be 
known by its hollow leaves being composed of four rows 
of elongated cells, which give it a bluntly quadrangular 
section ; but this peculiar construction of the stems is not 
always to be observed, except in fresh specimens, the pres- 
sure to which they are subjected in the process of drying 
breaking up the partitions of the cells, so that the stem 
appears to be composed of one series of large elongated 
cells. There is but one species, the /. lacustris, a stem- 
less quill-leaved submerged plant, which gives the appear- 
ance of a green turf to the bottom of the water where it 
occurs. 
Isoetes lacustris, Linnwus. 
The European Quillwort, or Merlins Grass. 
(Plate XIX. fig. 1.) 
This is a very curious plant, growing at the bottom of 
our mountain lakes, and having so much the appearance of 
