ISOETES. 205 



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 sporules. It may also be known 

 by its hollow leaves being composed of four rows of elon- 

 gated 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 pressure 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 I. lacustris, a stemless quill-leaved 

 submerged plant, which gives the appearance of a green 

 turf to the bottom of the water where it occurs. 



ISOETES LACUSTRIS, Linnaeus. The European Quillwort, 

 or Merlin's Grass. (Plate XIX. fig. 1.) 



This is a very curious plant, growing at the bottom of 

 our mountain lakes, and having, as has been remarked, 

 the appearance of a submerged grass, so that the unexpe- 

 rienced eye would probably pass it by unnoticed. It has a 

 fleshy tuber, of a nearly globular form, white, and of compact 

 texture internally, but spongy and of a dark brown colour 

 externally. In the centre is a small nearly pellucid part, 

 which appears to be the growing point, since it is from this 



