Pilularia.] XCII. MARSILACE^E. 551 



stock, globular. Sori 2 to 4, vertically adnate, their indusia dividing 

 the involucre into 2 to 4 cells, and each consisting of numerous spore- 

 cases, the lower ones few and larger, the upper ones numerous, minute, 

 and powdery. 



Besides the European species, which is also in the southern hemi- 

 sphere, there is a distinct North American one. 



1. P. globulifera, Linn. (fig. 1267). Creeping P. — The slender root- 

 stock often creeps to a considerable length, rooting at every node. 

 Barren fronds filiform, of a bright green, like the leaves of Isoetes, 

 varying from 1 to 3 inches in length. Involucres like little pills, nearly 

 2 lines diameter, covered with short hairs. 



In the shallow edges of pools and lakes, in Europe north of the Alps. 

 Widely distributed over England and Scotland, and in some places not 

 uncommon, but often overlooked, very rare in Ireland. Fr. summer and 

 autumn. 



XOIII. EQUISETACEiE. THE EQUISETUM FAMILY. 



A family consisting of a single genus, distinguished from all 

 others as well by the articulate and whorled stems, only re- 

 sembling some of the larger fossil plants now extinct, as by 

 the fructification. 



I. EQUISETUM. EQUISETUM. 



Leafless herbs, with a perennial, usually creeping rootstock, and erect, 

 rush- like, hollow, and jointed stems, marked with longitudinal striae or 

 furrows, with a sheath at each joint which encloses the base of the next 

 internode, and is bordered with short or elongated teeth, usually as 

 many as the striae of the stem. These stems are either simple or have 

 at each node, from the base of the sheath, a whorl of jointed branches, 

 similar to the stem, but with fewer striae, and always simple, except in 

 E. sylvaticum. Fructification an ovoid or oblong terminal spike, con- 

 sisting of several whorls of peltate, shield-shaped, shortly-stalked scales 

 (usually brown or black), under each of which are several (about 6 or 7) 

 spore-cases, filled with minute spores and opening down the inner side. 

 Under the microscope there will be seen to be attached to each spore 

 at its base 4 thread-like filaments, club-shaped at the top, rolled spirally 

 round the spore when moist, uncoiling elastically when dry. 



The species are not numerous, although widely diffused over the tem- 

 perate and colder regions of the northern hemisphere, extending more 

 sparingly into tropical countries. Some of them accommodate them- 

 selves to a great variety of stations and become very variable. To 

 determine them it is not only necessary to have the fruiting stem, but 

 also to observe whether the plant bears or not barren fronds at the same 

 time, and whether these are similar or dissimilar to the fruiting ones. 

 Accidental variations must also be guarded against. The side branches 

 sometimes bear spikes, or shoots similar to these side branches may 

 arise from the stock, and if gathered alone, without observing the more 

 ordinary state of the stems, may become very puzzling. 



