The whole substance of the stems of the Equisetums is so completely penetrated with silex, that a silicious skeleton remains after the herbaceous part is destroyed, and in some species as much as thirteen per cent. of the whole plant, and fifty per cent. of its ashes are found to be pure silex. On account of the crystallization of the silex, the Horsetails form some of the most beautiful of microscopic objects. If a fragment of the cuticle is magnified and viewed by polarized light, the colours are seen to be intense and vivid, and the arrangement of the silicious particles so elaborate and symmetrical as to resemble necklaces of diamonds and coloured gems. The rows of crystals run in lines parallel to the axis of the stem, the greater number being so disposed, but the rest are grouped so as to form ovals joined together by a chain of particles forming a sort of curvilinear quadrangle, these rows of oval combinations being arranged in pairs. The effect is brilliant when they are seen in polarized light. According to Sir David Brewster every particle has a regular axis of double refraction; but Professor Bailey, of the United States of America, states that the effect under polarized light is not produced by the silicious particles, but by the organized tissues, since, after these have been destroyed, the silex shows no double refraction.
The vascular tissue of the Equisetums shows them to be of a higher class than the Ferns. The recent forms vary much in size, but even the Equisetum giganteum of Brazil, which is eighteen feet high and three-quarters of an inch in diameter, is incomparably less than the Calamites and other fossil Equiseta, which appear in the coal measures and new red sandstone. Recent species are found in Iceland and the high northern latitudes of America, in the tropics, and in most parts of the world, except Australia and New Zealand.
SECTION X.
MARSILEACEÆ, OR RHIZOSPERMÆ.
Fig. 71. Pilularia minuta:—a, mature plant, natural size; b, receptacle, slightly magnified.
The Marsileaceæ, a natural tribe of small perennial aquatic herbs, have a filiform creeping rhizome with alternate erect leaves, curled in vernation like those of the ferns. The sporangia are enclosed in oval or spherical leathery capsules, or receptacles, which contain two dissimilar forms of reproductive organs—sporangia and antheridia, and are sessile, or nearly so, on the rhizome at the base of the leaves, whence the general name of Rhizospermæ. The Pilularia globulifera, or Pillwort, the only British species of its genus, may be taken as a type. Its rhizome creeps over sand or mud, at the margins of lakes and pools, where it is always submerged, or in sandy or gravelly places, which are only occasionally overflowed. At regular intervals the rhizome sends off a tuft of roots and a tuft of leaves opposite to them; the leaves are smooth, erect, and very slender, in deep water almost hair like, varying from four to five inches in height. The solitary globular receptacles, brown, hairy, and about the size of a small peppercorn, spring from the axils of the leaves, supported on so short a stem that they appear to be sessile. [Fig. 71] shows an Algerian species of its natural size. The receptacles are divided by cross partitions into two or more cells, and separate at maturity into four equal valves. Each cell has a sort of placenta to which the sporangium and antheridia are attached; and its upper half is lined with three minute, sessile, obovate, yellowish bodies, which are the antheridia; the other half is occupied by a larger, roundish, or oblong sessile sporangium, containing one spore, which has a firm outer coat, tapering to a point, and leaving a cavity at the top of the nucleus. According to Hofmeister, this cavity becomes filled with cellular tissue, constituting a conical prothallus confluent with the nucleus. A single archegonium is formed in the centre, the orifice of which corresponds with the apex of the prothallus. The antheridia contain numerous granules, from which long spiral delicate spermatozoids are ultimately developed.[[76]] The embryos of the young plants push forth their radicle in one direction to fix them to the soil, and a frond or leaf in the opposite direction. Thus these minute plants, approaching so nearly to a monocotyledon, exhibit a high organization, only inferior to that of the Lycopods.
The group contains the genera Marsilea, which has leaves made up of cuneiform lobes, and resembling those of some leguminous plant; and Salvinia and Azolla, both of which consist of small floating plants, which mostly occur in tropical or subtropical countries.
SECTION XI.
LYCOPODIACEÆ, OR CLUB MOSSES.
The Club Mosses are mostly perennial plants, with slender creeping stems, often several feet or yards in length, occasionally erect, and clothed by small, sessile, closely set, often imbricated leaves without veins. They have in some instances a habit resembling that of Conifers. The stems consist of a mass of thick-walled, often dotted cells, enclosing one or many bunches of scalariform tissue, which sends off branches to every leaf and bud. The scalariform tissue is accompanied by fine, elongated, and sometimes rather coarser cells, which are occasionally reticulated. The stem approaches to that of ferns, but the bundles of vascular tissue are confined to the centre. The branches of the stems are often bifurcate, and terminate in one or a pair of cone-like spikes, which are either sessile or stalked. The sporangia are sessile in the axils of the imbricated leaves or bracts that cover the cones. Two kinds grow on the same plant, one of them bivalved, containing a powdery substance, whose particles a high magnifying power shows to be globular spermatozoids: these do not germinate; and the other three-valved, enclosing comparatively large, nearly spherical granules, marked with three prominent ridges, radiating from one extremity. These certainly germinate, forming by cell-division a prothallus of hexagonal cellular tissue, adherent to and confluent with the spores, as in Marsileaceæ, or penetrating their cavity, but without the protrusion of threads, as in ferns or mosses. On this prothallus, archegonia are soon produced, the embryo being formed from a cell at its base; this sends down roots on one side, and a minute stem with the two primary leaves of the young plant at the other. The plant thus bears a close resemblance to a young plant of the dicotyledons, and exhibits, with regard to fructification, the highest organization of which the Cryptogams are capable, although in their tissues the Lycopods are inferior to the Equisetaceæ and Marsileaceæ.
There are four genera of this order, very widely dispersed, most abundant and larger in the hot moist parts of India and the Indian islands, but large tracts are covered with them within the arctic circles and in temperate latitudes. There are 200 species of the genus Lycopodium, or Wolfs-claw, and of these, six only are indigenous in Britain; they grow in very exposed situations, as in the case of L. clavatum, which lives on upland heaths and pastures, and which has a procumbent stem, creeping for many feet or yards, sending out branches in all directions, with a pair of cones at their extremities, and strong roots at intervals to fix it to the ground. Lycopodium inundatum forms large patches on the marshes in the south of England. New Zealand has many more species of this genus than Britain, and some of the noblest specimens. The dried spores of the L. clavatum are so inflammable, that they have been used on the stage to produce the effect of lightning. Some Club Mosses yield a blue dye, a colour which is now obtained of a better quality from coal tar. Others possess cathartic properties, but although they have been used as medicine, they are very dangerous from the violence of their effects.