Equisetums, or Horsetails (Pteridophyta)
There are about twenty-five species of equisetum, constituting the only genus of the unique family Equisetaceæ. Among these E. arvense (Fig. [299]) is common on clayey and sandy soils.
In this species the work of nutrition and that of spore production are performed by separate shoots from an underground rhizome. The fertile branches appear early in spring. The stem, which is 3 to 6 inches high, consists of a number of cylindrical, furrowed internodes, each sheathed at the base by a circle of scale leaves. The shoots are of a pale yellow colour. They contain no chlorophyll, and are nourished by the food stored in the rhizome (Fig. [299]).
The spores are formed on specially developed fertile leaves or sporophylls which are collected into a spike or cone at the end of the stalk (a, Fig. [299]). A single sporophyll is shown at b. It consists of a short stalk expanded into a broad, mushroom-like head. Several large sporangia are borne on its under side. The spores formed in the sporangia are very interesting and beautiful objects when examined under the microscope (× about 200). They are spherical, green bodies, each surrounded by two spiral bands attached to the spore at their intersection, s. These bands exhibit hygroscopic movements by means of which the spores become entangled, and are held together. This is of advantage to the plant, as we shall see. All the spores are alike, but some of the prothallia grow to a greater size than the others. The large prothallia produce only archegonia while the smaller ones produce antheridia. Both these organs are much like those of the ferns, and fertilization is accomplished in the same way. Since the prothallia are usually diœcious, the special advantage of the spiral bands, holding the spores together so that both kinds of prothallia may be in close proximity, will be easily understood. As in the fern, the fertilized egg-cell develops into an equisetum plant.
Fig. 299.—Equisetum arvense.
st, sterile shoot; f, fertile shoot showing the spike at a; b, sporophyll, with sporangia; s, spore.
The sterile shoots (st, Fig. [299]) appear much later in the season. They give rise to repeated whorls of angular or furrowed branches. The leaves are very much reduced scales, situated at the internodes. The stems are provided with chlorophyll and act as assimilating tissue, nourishing the rhizome and the fertile shoots. Nutriment is also stored in special tubers developed on the rhizome.
Other species of equisetum have only one kind of shoot—a tall, hard, leafless, green shoot with the spike at its summit. Equisetum stems are full of silex, and they are sometimes used for scouring floors and utensils; hence the common name “scouring rush.”