234 HISTORY OF BRITISH FERNS. 



leaves throughout the year, is commonly called Quillwort. 

 The genus differs from Pilularia, its nearest ally, and 

 \vith ■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. 



Isoe'tes lacustris, Lhinceus. 

 The European Quillwort , or Merlins Gi'ass. 

 (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 



