Horsetails
Horsetail also called scouring rush, grows in moist, rich soils and almost in all parts of the world except in Australia and New Zealand. Some species produce two kinds of shoots: those with conelike clusters (strobili) of spore capsules and those lacking such structures. Some are evergreen; others send up new shoots annually from underground rootstalks. Their hollow, jointed, ridged stems contain silicate and other minerals. The leaves are reduced to sheaths that clasp and encircle the shoots.
The name horsetail arose because the branched species somewhat resemble a horse's tail. The group is now almost extinct, but one genus survives. Equisetum is the only living genus of horsetails. The name Equisetum comes from the Latin (equus = horse; and seta = bristle). The genus includes 15 species.
These plants have long underground rhizomes which give rise to aerial stems that are usually 4 to 24 inches in height. Some of the plants reach up to 30 metres in height. The stems are simple (unbranched) and carry whorls of slender branches, which are simple or have further whorls of branches. They have small leaves that are produced in whorls, each cluster being fused into a tubular sheath, except for the tips which form a serrated edge around the margin of the sheath. Most of the leaves have no chlorophyll and photosynthesis is carried out by the green stems.
The reproductive organs of horsetails are grouped together to form a terminal cone, or strobilus. Each strobilus consists of a central axis bearing whorls of mushroom-shaped sporangiophores, each with several sporangia on the lower surface. The sporangia split to release the spores. Fertilization is achieved in the same way as in the club mosses, and the horsetails also develop by an alternation of generations.