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PLANTS, AND HOW THEY MULTIPLY.
Anther, (antheros, anthos, G.) | Phancrogamic, (plaino, gamos, G.)
Pistil, (pistillum, L.)
Pollen, (pollen, pollis, L.) Cryptogamic, (cryptos, gamos, G.) con Sepal, (sepio, L.) cealing reproduction.
Stamen, (sto, L.) Cuticle, (cutis, L.) skin or onter cover Stigma, L. and G.)
Style, (stylus, L., stylos, G.) Fecundate, (facundus, L.) to make Tunície, (tunica, L.)a natural covering. fruitful."
Viscid. (viscidus, viscus, L.) clammy, Filament, (filamenta, filum, L.)
sticky, adhesive, tenacious. (Also Ovary, (ovum, L.) the case which con viscous).
tains the ovule. Hence also ovule, Whorl, (from the same root as E. whirl the undeveloped seed.
and warble) a ring or circle of leaves Petal, (petalon, G.) a flower-lea.
arranged round a common centre,
REPRODUCTIVE ORGANS OF PLANTS. EVERY organized body, whether animal or vegetable, is the subject of perpetual change. We have seen how plants spring from seed, and how their subsequent development is provided for by those organs which collect, prepare, and assimilate their food. But these processes will not go on for ever. There comes an inevitable period of decay, when the vital powers will languish, and ultimately cease. Thus the world would soon become a dead and doleful waste, were it not that the principle of life is from time to time renewed. The individual dies, but the race does not perish. In addition, then, to the wonderful mechanism by which vegetable life is supported, we have to consider the still more remarkable functions by whose operation it is reproduced and multiplied.
In those plants which are propagated by means of seeds, whether dicotyledonous or monocotyledonous, the organs of reproduction are all included in the flower or blossom, which gradually ripens into fruit. The fruit, when fully matured, either is, or contains the seed. But acotyledonous plants are entirely destitute both of flowers and fruit. Instead of seeds, they have those seed-like bodies called spores or sporules, produced from the living plant by means of certain obscure organs, whose nature and mode of action are not well understood. In many cases it is difficult even to ascertain what are the organs by which this function is performed. That there must be, in every species, the faculty of reproduction, cannot be reasonably doubted; but it scems often impossible to determine how it operates, or even in what part of the plant it resides. All plants may therefore be divided, according to the nature of their reproductive organs, into two great classes, flowerless and flowering, or, in the language of botanists, cryptogamic and phanerogamic. Phanerogamic plants are by far the most numerous and important.
The flowers of different species present the greatest variety of external form, but they are all, in structure, more or less analogous to each other. They grow from the stalk or axis of the plant, very much in the same way as ordinary leaves, of which, indeed, all their parts may be regarded as so many different modifications. The arrangement of these parts will be best understood by referring to the accompanying illustrations. Fig. 10 shows the external coverings. In fig. 9 these are removed, so as to exhibit the internal, Fig. 9,
Fig. 11. and, as we shall see, more essential organs. A cross section of the whole flower is represented in fig. 11. The outermost covering is the calyx (a), consisting of a circle or whorl of more or less modified leaves, in which, as in a cup, the rest of the flower is inserted. The calyx is generally, but not always, green. Different from it in this respect is the next whorl, called the corolla (6). It is the ornamental part of the flower, and is in most cases gaily coloured. At first, it is wrapped up in the calyx, but gradually expands and overtops it, becoming, in its turn, a protective covering to the internal organs. These are the stamens (cd) and pistil (ef g), which are directly concerned in the production of the seed, and must therefore be present in every fertile flower, whereas the calyx and corolla are frequently wanting. The stamens form one or more whorls, surrounding the pistil, which occupies the centre; and, when there are more pistils than one, they too are arranged in whorls. It is worthy of remark that the parts of each whorl are usually found to occupy alternate positions with those of the next, as seen in fig. 11. Thus the petals, or leafy organs which compose the corolla, are disposed alternately with those of the calyx, which are called sepals. It follows that, in the same flower, the number of petals and of sepals will in general be the same. Similarly, the petals are alternate with the outermost whorl (if there be more than one) of stamens, these with the next whorl, and so on; so that the stamens will either be equal in number to the petals, or some multiple of them. In monocotyledonous plants, the number of parts in each whorl is very often three; while two, four, and five are more frequent in those which are dicotyledonous.
Such is a general description of the flower; the respective functions of its different parts must form the subject of another lesson.
REPRODUCTIVE ORGANS OF PLANTS-CONTINUED. The functions of the calyx and corolla are nutritive as well as protective. Both are intended to minister to the wants of the interior organs, which, as the direct instruments of reproduction, it is the end and aim of the whole organism to mature. The sepals of the calyx, like ordinary leaves, absorb carbonaceous food from the atmosphere; while the corolla is chiefly concerned in the production of coloured juices, and of a nutritious sugary substance which serves important purposes in the economy of the plant. When these functions are no longer necessary, the petals usually wither and fall, but the calyx often remains in connection with the fruit. It may be seen adhering to gooseberries and currants, round the point to which the stalk is attached.
A stamen generally consists of two parts, the filament (c) * and the anther (d), of which the latter only is essential. The filament is, as its name implies, a thread-like organ, at
* See flg. 9, page 104.
tached to the anther by one extremity, and by the other to the base of the flower. It serves as a pillar to support the anther, and as a conduit to convey to it the necessary nourishment. The anther is a receptacle of little cells, filled with a fine powder, called pollen, which may be regarded as the life-giving principle of the plant. When this substance has arrived at a state of maturity fitted for the performance of its functions, it is discharged by the opening or sudden rupture of its covering, and may be seen scattered, like white or yellow dust, upon any fully-developed flower.
It is essential to reproduction that some of this fecundating powder find its way to the apex of the pistil, which is called the stigma (g). Besides this stigma, the pistil includes two distinct parts: the ovary, or seed-vessel (e), containing the ovule, from which the seed is to be formed; and the style or stalk (f ), by which the ovary and stigma are connected. This last, like the filament of the stamen, may be entirely wanting. The stigma consists of a soft, spongelike cellular substance, not covered by the skin or cuticle, which envelopes almost every other part of the external surface of the plant. No sooner do the grains of pollen fall upon this delicate organ, where they are retained by a viseid fluid which it secretes, than they begin to send out small tubes in the direction of the ovary. These tubes convey to the ovule the fertilizing material contained in the pollen, and forthwith the development of the little embryo proceeds, while the rest of the flower, having performed its allotted work, gradually yields to decay.
How wonderful are these structures in the very minuteness of their complex operations ! The parts of the flower exterior to the stamens are all created in perfect subserviency to the functions of that organ; and, even of the stamens themselves, how small a portion seems to be essential! The filament supports the anther, the anther contains the pollen, and the grains of pollen enclose the fertilizing matter, of which, after all, but a small part is necessary to accomplish the purpose for which the whole flower was made !