Pilularia. | XOII, MARSILEACEZ. 549 
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, Er, summer and 
autumn, 
XCIII. EQUISETACEA, 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. 
Leafiess 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 strie 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 Z. sylvaticum. 
Fructification an ovoid or oblong terminal spike, consisting 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 micro- 
scope 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 themselves 
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. 
Fruiting stems, in spring, simple, thick, with long loose sheaths, 
and withering before the barren ones appear. 
Sheaths of the fruiting stems more than an inch long, with 
numerous subulate teeth . 1. BE. Telmateia. 
Sheaths under an inch, distant from each other, with about 8 
or 10 lanceolate teeth 2. FE. arvense. 
Fruiting stems appearing in or lasting till summer, at the same 
time as the barren ones, and nearly similar to them. 
Spikes very obtuse. 
Sheaths with 3 to 5 large teeth. Lower pasrehes recurved 
and again branched . 3. E. sylvaticum. 
Sheaths with 14 to 20 long subulate teeth, "Branches all un- 
divided 4, E. pratense. 
Sheaths with 6 to 20 ‘minute teeth. Branches few. 
Stem smooth, not serored, 10-20 striate. Sheaths cylindric 
appressed : : : ; : : . 5. HB. limosum, 
