EQ'UISE'TUM (Neo-Lat., from Lat. equine tum, mamas, cquiseta, from equus, horse sesta, bristle), HORSE-TAIL Rusts, Or SCOURING RUSH. The only living genus of the group Equi• setales or Equisetaceie. This group is one of the three great divisions of the Pteridophyta, the other two being the ferns (Mikales) and the club-mosses (Lycopodiales). The genus Equisetum is represented in the living flora by about twenty five species, which are the lingering remnants of an extensive race that was a conspicuous feature of the flora of Carboniferous and Mesozoic times. The living forms are mostly small and incon spicuous, but they are very characteristic in ap pearance. The stem is slender and conspicuously jointed, the joints separating easily; it is also grooved and fluted by small longitudinal ridges, and there is such an abundant deposit of silica in the epidermis that the plants feel rough. At each joint there is a sheath of minute leaves, the individual leaves sometimes being indicated only by minute teeth. Since these leaves contain no chlorophyll, and evidently do not function as foliage leaves, the chlorophyll work is carried on by the green stem, which is either simple or pro fusely branched, one of the distinguishing features of the group is that they have distinct, •pore-bearing leaves (sporephylls), and that these are arranged so as to fern] a cluster or •strobilus: Lath sporuphyll in the strobilus consists of a stalk-like portion bearing a peltate expansion.
Beneath this shield-like expansion hang the spore •ases (sporangia), usually ranging from five to ten in number. 'fhe spores produced are all alike, so that the group is not one of those in which heterospory Iq.v.1 occurs at present, al though it is suspected that some of the ancient members of the group were heterosporous. The spores have a very interesting structure. In ad dition to the two eoats common to spores, there is a third outer one consisting of two intersect ing spiral bands which are attached to the spore only at their point of intersection. On drying, the spiral bands loosen and become uncoiled, and when moistened they close again around the spore. By means of these movements they serve to hook together the spores, and in this way the close proximity of germinating spores is secured. The significance of this proximity lies in the fact that the sexual plants (gametophytes) which the spores produce are unisexual—that is, one plant produces the male organs (antheridia). and an other produces the female organs (arehcgonia), a condition called diceeism (q.v.).