HORSETAIL FAMILY. 31 
branching providing an excellent means for the spread 
of the plant. The ordinary aérial stems are branched or 
simple, according to the species, and may be easily pulled 
apart at the joints, which are sheathed by the united 
bases of small pointed leaves. Stems and branches are 
characteristically ridged and furrowed, this and the ease 
with which the plant breaks at the joints, affording a 
definite means of identification. The Horsetails contain 
a large proportion of silica, which makes them harsh and 
rough to the touch. This has given rise to the popular 
name, Scouring-rush, for the Winter Horsetail, whose 
stout unbranched stem is sufficiently hard to scratch 
glass. The branched forms contain less silica, but still 
sufficient, it was formerly held, to account for Horsetail 
poisoning. Food manufacture is carried on by the green 
stems and branches, the scale-like leaves being useless 
in this connection. The spores are contained in cone- 
shaped structures at the tips of stems. In Equisetum 
arvense L. the stem that bears spores is brown and suc- 
culent and takes no part in food production. It grows 
and matures in early spring, living on food stored in the 
underground. parts during the previous year. In other 
species the spores are borne on the ordinary green plant. 
The cone is composed of a number of peltate or umbrella- 
shaped hexagonal structures borne on short stalks. These 
are the sporangiophores, each of which bears a number 
of sporangia or spore-sacs, suspended from its under sur- 
face. The outer coat of the green spores contained in 
them is split into four ribbons which expand and contract 
with the moisture changes of the air and give to the spore 
mass an appearance of writhing life. 
