Pilularia.] XCII. MARSILACEA, 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 Jsoetes, 
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. 
-XCIIL EQUISETACEZ 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 striz 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 striz 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 striz, 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. 
