Equisetum, contains 24 pieces, 10 of which occur within the Gray's Manual range. Equisetum is often called the "scouring rush," because the rough stems have been used for scouring kettles. The roughness is due to silica. Species, like E. hiemale, which contain much silica, must be treated with hydrofluoric acid before the older parts can be cut in paraffin.
Vegetative Structure - The roots are very small, but have large cells and easily yield good preparations. If a handful of Equisetum fluviatile or E. hiemale growing in water be pulled up, scores of roottips may be secured in a few minutes. Fix in chromo-acetic acid with a little osmic acid. In case of such small objects it is a good plan to add a few drops of eosin to the alcohol during the process of dehydrating, in order that the material may be seen more easily. The slight staining does no damage, even if more critical stains are to be used after the sections are cut. Longitudinal sections of the roots may also be obtained by cutting transverse sections of the nodes.
The growing points of stems may be cut with ease in paraffin. E. arvense is particularly favorable on account of the numerous apical cells which may be found in a single preparation.
The "fertile" stem of Equisetum arvense is so free from silica that it can be cut in paraffin without any difficulty. The adult vegetative stem of E. arvense, and all stems which contain so much silica, must be treated with hydrofluoric acid before imbedding in paraffin. However, nearly all of these stems can be cut freehand, before fixing, without removing the silica. Fix freehand sections in 95 per cent alcohol. Material for paraffin sections should be fixed in formalin, alcohol, acetic acid; or in hot alcohol, corrosive sublimate acetic. Safranin and anilin blue, with or without a little orange, is a good combination.
The Strobilus - E. arvense affords the most favorable material for a study of the development of sporangia, since the strobilus contains almost no silica and, even in its latest stages, is easily cut in paraffin. In this species, the young strobili, in the Chicago region, can be distinguished from vegetative buds in July; sporogenous tissue is well advanced by the middle of August; and the reduction divisions occur late in August or early in September.
The spores are not shed until the following April. If you know a patch of this species which "fruits" every year, dig up the horizontal underground stem in July. The tip of the main axis is almost sure to be a strobilus. Dissect away the scale leaves and fix the strobilus in chromo-acetic acid with a little osmic acid. August and September stages are easy to recognize. If strobili are brought into the laboratory in December or January, they shed their spores within a week.
The spores of Equisetum are excellent for illustrating hygroscopic movements. Shake out the spores from the strobili and let them dry thoroughly. They can be kept dry for years. When wanted for demonstration, put a few on a slide, moisten a little, and watch the movements under the microscope. Strobili of other species, like E. fluviatile and E. hiemale, contain a large amount of silica and, consequently, only the younger stages cut well in paraffin. Hydrofluoric acid damages the cell contents more or less. In species like these, all stages in the development are found in a single season.
The Gametophytes - The spores of Equisetum germinate as soon as they are shed, but, like all spores with a considerable amount of chlorophyll, they do not long retain the power of germination. A comparatively small percentage will germinate a week after shedding, and after a month, there may be no germination at all. There is no difficulty in growing prothallia to maturity and securing stages in the embryo, if fungi or blue-green algae do not appear and ruin the cultures.
The prothallia fix well in chromo-acetic acid. The younger stages may be stained in iron-alum, haematoxylin and mounted in Venetian turpentine. The older stages, even of E. arvense, are too large for such mounts. E. laevigatum has prothallia a centimeter in diameter.
For the development of antheridia, the blepharoplast, and the development of the sperm, fix in Flemming's weaker solution and stain in iron-haematoxylin. The sperm of Equisetum is the largest in Pteridophytes.
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