HORSETAIL FAMILY. 31 



branching providing an excellent means for the spread 

 of the plant. The ordinary aerial 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. 



