550 THE OLUBMOSS FAMILY. [Isoetcs. 



within the enlarged base of the leaves, those of the inner leaves filled 

 with minute powdery grannies, those of the outer leaves containing 

 larger grains, at first cohering in fours. 



A small genus, widely spread over the greater part of the globe. 



1. I. lacustris, Linn. (fig. 1265). European Q. A perennial, of a 

 bright green, forming dense tufts under the water. Leaves narrow- 

 linear, thick, and nearly terete or 4 -angled, much like those of several 

 Monocotyledons, varying from 2 to 6 inches long, their enlarged bases 

 giving the plant often a bulbous appearance. 



In mountain pools, and shallow lakes, in central and northern Europe, 

 northern and Arctic Asia, and North America. In Britain, in the moun- 

 tainous parts of Scotland, northern England, Wales, and Ireland. Fr. 

 summer and autumn. [/. Morci, Moore, is a variety with leaves 18 inches 

 long, found in Wicklow.] Modern botanists distinguish as 7. echinospora, 

 Durieu, a form found in our mountain lakes, often growing with the 

 common one, but said to be only where the soil is peaty. It differs 

 chiefly in the larger spores covered with acute tubercles instead of being 

 granulate only or smooth on the surface. A more distinct form referred 

 to /. ITystrix, Durieu (fig. 1266), occurs in moist sandy hollows on 

 Laucresse Common in Guernsey. The rootstock is covered, outside the 

 tuft of leaves, with a number of small, imbricate, toothed or jagged 

 brown scales, which are the persistent remains of old leaves, and which 

 are never observed in the common under-water forms. It remains to 

 be seen how far this difference may be owing to situation. 



XCII. MARSILEACE^E. THE MARSILEA FAMILY. 



No true leaves. Fronds, as in Filices, proceeding from the 

 rootstock and rolled inwards at the top, barren ones either 

 reduced to a narrow-linear stipes, or in an exotic genus bearing 

 4 digitate leaflets ; fertile ones sessile or on a short stipes, 

 bearing a globular or ovoid utricle, usually called an involucre, 

 and formerly considered as analogous to the spore-cases of 

 Lycopodiacece, but which is really the recurved fertile lamina 

 with the margins united. Ifcal spore-cases of two kinds, larger 

 and smaller, as in Selaginacece, but arranged, as in Filices, inside 

 the involucre, that is, on the under surface of the recurved 

 frond, in sori enclosed in membranous indusia, dividing the 

 involucre into as many cells. 



The Order was formerly supposed to be closely connected with Lyco- 

 podiacece, in which the only British genus was included in our first 

 editions, but its still nearer relation to Filices has been well pointed out 

 chiefly by German botanists. " It contains only one genus besides the 

 British one. 



I. PILULABIA. PILLWORT. 



Rootstock creeping under water, with subulate, barren fronds, almost 

 solitary at the nodes, Involucres (or fertile fronds) almost sessile on the 



