The ovary is composed of an outward skin containing cellular tissue and vascular bundles of spiral vessels, which, running upwards, converge towards the style; they vary in number, and are sometimes ramified. A single ovule, which is an unripe seed, may be produced within the ovary, or the ovary may be divided into two or more compartments, in each of which an ovule may be formed and attached to the ovary by a mass of cellular tissue, and not unfrequently by a single thread.
The style, when examined with a microscope, is a hollow tube, or canal, extending from the cavity of the ovary to the stigma. It consists of cellular tissue with vascular bundles near the circumference, which pass upwards in straight lines, and end below the summit. In certain cases the canal is open; in others it is obstructed by lax cellular tissue having many gaps. The stigma is an expansion of that lax tissue at the point where the canal opens externally. At the time of fructification, the canal of the style is occupied by soft humid elongated cells mixed with a viscous fluid, which exudes upon the surface of the stigma, rendering it moist and glutinous. When the pollen grains adhere to that humid substance, the tubular extension of their inner lining, with its contents, passes down the style and fertilizes the unripe seeds.
After fructification the anthers, stigma, and conducting tissues wither, yet, in many fruits, additions are made to the ovary and its contents by the remains of some of the other parts of the inflorescence. In the apple, which is a simple fruit, the skin of the calyx forms the skin of the apple, and the flesh or edible part is developed out of the ovary and the remainder of the calyx, while the inner layer of the ovary forms the horny cells containing the ripe seeds. In the strawberry, which is a compound fruit, the pulp is the enlarged fleshy receptacle bearing the simple fruit on its surface. In the orange, the membranous partitions of its segments are the linings of the ovary, and the pulp is formed of lax large-celled cellular tissue developed within it.[[77]] All fruits have the mark of the style; it is very evident in the apple and orange. Simple fruits are formed by a single flower; multiple fruits, like the ananas, or pine-apple, and fir cone, are formed of masses of inflorescence in a state of adhesion.
Spiral vessels are frequently found passing from the fleshy part of fruits into the seeds; they are very numerous in the seeds of the Collomia grandiflora and others, coiled up and compressed by the outer skin; but they start to their full length as soon as released. The coats of the seeds of various plants, when seen through a microscope, are beautifully marked. That of the Bignonia is surrounded by a fringe of elongated spiral vessels, the seed of the poppy has a reticulated surface, and many have wings like those of the ash tree, or down like the thistle, that they may be dispersed by the wind. The tendency of roots to strike downwards is so great, that seeds right themselves whichever way they may fall.
SECTION XIII.
MONOCOTYLEDONOUS, OR ENDOGENOUS PLANTS.
The structure, growth, and reproduction of the flower-bearing vegetation offer objects of the highest interest, though very different from those furnished by the flowerless class. The plants whose seeds have but one lobe form a transition from the lowest to the highest class of vegetables, and include those which furnish the principal articles of food to man and animals. They are all flower-bearing, and consist of numerous families of land and water plants. The most important are the palms, which serve for food in tropical countries, the grasses, which are cosmopolite and social, covering extensive tracts of country with rich verdure, and including the Cerealia, which have been cultivated from such remote antiquity, that the grasses from whence they were derived are unknown. This monocotyledonous class includes, besides the palms and cereals already mentioned, the sugar-cane, the bamboo and other canes, together with reeds, rushes, screw pines, most garden bulbs, the singular and beautiful race of Orchids, and a multitude of other ornamental and useful plants.
The seeds of this class consist of a thin skin covering a mass of white or green matter, which, when ripe, constitutes the starch and flour in the Cerealia. Within that matter lies a fleshy lobe with the infant plant imbedded in one corner. The same general structure may be traced throughout the class, though the lobe with its embryo in the surrounding matter may be covered with a hard coat, as in the cocoa-nut, or inclosed in a shell covered with a rich fruit, as in the date. The infant plant, as it lies in its seed lobe, has a stem with a leaf bud or plumule at one end, and an embryo root at the other; and, as soon as the seed begins to germinate, it is nourished by the conversion of the starch of the surrounding matter into sugar. At first the whole young plant is of cellular tissue, but, as soon as the seed leaf appears above the soil, it decomposes the carbonic acid of the atmosphere, absorbs the oxygen, and consolidates the carbon. Then cords of fibro-vascular tissue are formed within it, which converge at its base, and unite with the radicle of the young plant to form the solid centre of the underground stem, from whence the real roots descend, like slender white cords, which supply the plant with food. The seed lobe having set the current of the cell sap in motion disappears, and the aërial or above-ground stem, which now consists of pith, sap, wood, and skin or bark, grows rapidly in length to its first node, or thickened part, where the first leaf or leaves appear, for the first leaves generally appear at the top of the stem in this class, and are annually succeeded by others rising above them from the top of the elongated stem, as in the palms and grasses.
The stem or axis of a palm is a cylinder with a graceful plume of leaves at its summit, and a cone of white roots at its base. A section of the stem perpendicular to its axis exhibits a mass of cellular tissue with a number of dark-coloured spots irregularly scattered throughout the whole with the exception of an envelope of dense cellular tissue, which forms a kind of bark. The dark spots are the sections of bundles consisting of two or three very wide and large vascular ducts enclosed in extremely fine woody fibre and spiral vessels closely pressed together. These bundles after having formed the ribs and footstalks of the leaves, enter the upper part of the stem, approach towards its centre, bend down for a short distance, then turn towards the exterior, interlace with the bundles of the previous year, which have followed a similar course, and make a circle of rough marks on the surface of the stem. The stem then increases in length, and a new plume of leaves crowns its top; the same process is repeated; and, as this goes on indefinitely, the lower part of the stem is rough, and sometimes even rugged, as that of the Palmyra palm. In consequence of this manner of growth, a perpendicular section of a stem shows a series of curves intersecting each other and originating in points gradually ascending as the palm grows in height. These curves proceed from every point of the circumference of the stem, present their convexities to the centre, and bend round, and enter the leaves, which are long, lanceolate, and often pinnate with veins running through them longitudinally. In some palms the greater part of the centre of the trunk is altogether cellular tissue, but each species has an arrangement of its own. Some palms, as the date palm, are diœcious, one plant bearing male, the other female flowers. In these cases, the pollen is carried from one tree to the other by the wind or by insects, and, as much is lost in the transit, there is always more produced than is required to fertilize the female flowers.
The Graminaceæ are very numerous, but the general structure is virtually the same, whether it be a simple herbaceous grass, a sugar cane, or a bamboo. The stem of the grasses is jointed, and furnished with long lanceolate leaves, springing alternately to the right and left of each successive joint, and the parts of the stem between the joints are embraced and nearly surrounded, as by a sheath, with the expanded bases of the leaves; for, in this class of plants, one growth always springs from the interior of that which precedes it. In youth the stems of the grasses are solid, and the bundles of woody tissue which surround the pith are parallel to one another and to the axis of the stem; but, at the joints, they are turned aside and are condensed into a node or joint, where they enter into a new arrangement, and pass into the lanceolate leaf, to form its longitudinal ribs. The compression of the fibro-vascular tissue is so great as to form a septum across the interior of the stem as the node. This is the case in all the grasses; for the pith, which is of moist green cellular tissue when they are young, soon disappears, so that a full grown grass is a hollow stem articulated at intervals by solid joints, from whence the long narrow leaves spring alternately, and as the bases of the leaves are continuous with the stem, they do not fall off when they wither.
In the majority of cases, the inflorescence of the Graminaceæ consists of a pistil and three anthers; but in the Anthoxanthum odoratum, or sweet-scented vernal grass, and some others, there are but two anthers, while some few species have more than three. In general, the flowers are hermaphrodite; but in certain genera they are monœcious, as in Zea and Zizania; and in others they are polygamous, as in Andropogon and Sorghum. The grasses are, for the most part, low and herbaceous; but the Arundo Donax of Southern Europe, and the sugar cane and bamboo of the tropics are lofty strong-growing plants, the latter reaching as much as 50 or 60 feet in height. The stems and leaves of the grass family are strengthened by silex; many of them are so entirely coated with it, that their leaves are as sharp as the edge of a knife. It gives hardness to the beards of wheat and barley, and is often found in concrete masses, called tabasheer, in the joints of bamboos, which in the Indian jungles have been set on fire by the friction of their silicious coats during a gale of wind. As no solid matter can enter the roots of a plant, the silex must be absorbed in a state of solution; and, as it coats the surface, which is full of pores, the liquid is removed by perspiration, and the silex is consolidated. The whole of this family abounds in sugar, and its farinaceous products are too well known to require any notice.