Chapter XIX. 



Scouring-rushes and Horse-tails. 



The peculiar family of plants known as scouring-rushes or 

 horse-tails was very much better developed during the age when 

 coal was being deposited than it is to-day. Most of its species 

 are extinct, but there remain, widely distributed over the world, 

 some forty different varieties, of which ten occur in Minnesota. 

 They are not very closely related either to the ferns or to the 

 club-mosses, although they clearly belong in their general vicin- 

 ity. The unbranched forms are known as scouring-rushes on 

 account of the usual deposit of silica in their outer layers. 

 This mineral is useful for scouring tinware, and rushes are 

 actually thus employed by some housewives in the country. 

 The branched forms are known as horse-tails from their peculiar 

 aspect as they stand in fields, in the woods or along the road- 

 side or railway tracks. 



Each variety of scouring-rush or horse-tail is distinguished 

 by an underground rootstock which shows much the same struc- 

 ture as the above-ground portions. Sometimes on the root- 

 stocks tuber-like propagative swellings are formed. Both the 

 erect and subterranean branches are divided into very distinct 

 joints which may be separated from each other like sections of 

 stove-pipe, hence the plants are also called joint-rushes. In 

 some species the plant produces only one kind of erect stem 

 and at the tip of this, or more rarely at the tips of lateral 

 branches, firm and solid cones are borne, each made up of little 

 shield-shaped leaves with central stalks. The leaves are ar- 

 ranged in circles about the axis, not in spirals as in the cones 

 of club-mosses. On the under side of each of the shield-shaped 

 leaves a ring of spore-cases is developed, commonly about eight 

 in a group. The cones bear the leaves so close together that 

 from their mutual pressure they assume a more or less hexagonal 

 outline. 



