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petiole to a leaf, or a filament to an anther.” The pistil, or ovarium, is frequently composed of several carpels, (61) each having its separate ovary, style, and stigma. 61. Carpel. The pistil, anatomically considered, is in reality a modified leaf, or whorl of leaves, and a carpel “is formed by a folded leaf, the upper surface of which is turned inwards and the lower outwards; and within which are developed one or a greater number of buds, which are the ovules.”t 62. The Ovule, as has just been seen, is contained within the carpel, and becomes the germ of the new plant; it is either naked or enclosed in a covering, sometimes sessile, sometimes stalked : in its most complete state it consists of a nucleus, surrounded by two coats or integuments. 63. The Fruit is the mature state of the pistil or carpels.
* Lindley, El. Bot. pp. 47,49, 50.
t Lindley's Elements of Botany, p. 50.
Professor Lindley has made the subject of the carpels so clear in his “Ladies' Botany ” that it may be well to add his explanation to what is given above. “Next to the stamens, and occupying the very centre of the flower,” (the common Ranunculus, or Buttercup, is the one he takes as his example,) “are a number of little green grains, which look almost like green scales; they are collected in a heap, and are seated upon a small elevated receptacle; we call them carpels. They are too small to be seen readily without a magnifying glass; but if they are examined in that way, you will remark that each is roundish at the bottom, and gradually contracted into a kind of short bent horn at the top; the rounded part is the ovary, the horn is the style; and the tip of the style, which is rather more shining and somewhat wider than the style itself, is named the stigma: so that a carpel consists of ovary, style, and stigma. At first sight you may take the carpels to be solid, and, if you already know something of botany, you may fancy them to be young seeds; but in both opinions, you would be mistaken. The ovary of each carpel is hollow and contains a young seed called an ovule, or little egg; so that the carpel, instead of being the seed, is the part that contains the seed.” (Letter I. p. 7.)
64. The ovary of the pistil becomes what botanists call the Pericarp of the fruit; it has a great variety of names, dependent on the number of carpels, their situation, the quality of their texture, &c. 65. The Seed is the perfected ovule; it is covered with an integument, which is sometimes curiously spread out so as to form wings, and contains the embryo lying in it as the embryo chick is in the egg, and often similarly surrounded by the albumen which affords its nourishment. 66. Spores. The principal organs of reproduction in those plants, called Acrogens or Flowerless, which are destitute of stamens and pistils, are called spores; these are cells which are seen by a microscope to be analogous to a grain of pollen; the cases containing them are termed thecae or sporangia. Sori are clusters of thecae, and the Indusium is a portion of the epidermis which encloses them. 67. The reproduction of plants is of two kinds, that by seed and that by division, which is either natural or artificial, and will afterwards be noticed. When the flower is fully developed,—a period which arrives in different kinds of plants at very different times, in some for instance, in the first, in others in the second year of their existence,—a process occurs by which that contact between the pollen (59) and the stigma (60) takes place, which in all the phanerogamic plants is absolutely essential to the reproduction of the species by seed. This contact or impregnation is thus effected. “The pollen emits a tube of extreme delicacy, which pierces the stigma and style, and passing downwards into the ovary,” thus reaches the ovule. The result of this process is the gradual development of the embryo
which becomes the fruit: or, in other words, the pistil, after this impregnation, arrives at maturity, and the ovary of the pistil becomes the pericarp of the fruit. This main fact remains in all cases unaltered, though in consequence of the non-development or obliteration of some of the parts, the identity of the fruit with the original pistil is sometimes difficult to recognize. Various names have been applied to fruits according to their form, nature, &c.—All, however, are receptacles for the seed, which is the perfect state of the ovule, as the fruit generally, is of the pistil. The provisions for ensuring this necessary contact between the pollen and the stigma, are among some of the most curious in nature. The stamens of many plants, by a spontaneous movement, approach the pistil at the season when fructification should commence. The action of water on the pollen, which would be injurious to it, is in some cases avoided by the corolla closing on the approach of rain, and in aquatic plants the organs of fructification are defended from wet, by being produced in a cavity filled with air, or by the flowers being raised above the surface of the water. The Vallisneria, whose flowers are diaeceous (that is, the pistil is situated on one plant, and the stamens on another) is a very remarkable instance of the method by which the contact of the two organs is effected. It grows in the waters of the south of Europe, strongly embedded in the mud by its roots. The pistils are situated in flowers which are on long peduncles, spirally rolled up at first, but which uncurl till they reach the surface. The flowers which bear the stamens have, on the contrary, a very short peduncle, but the buds form little bladders, on which they float, detached from their stems, around the pistilliferous flowers, they then expand, emit their pollen, and die. 68. The seed itself consists, as has been stated above (65), of an embryo, and of the albumen, &c., which nourish and protect it. This embryo, “the organized body that lies within the seed, and for the purpose of protecting and nourishing which the seed was created,” “consists of the cotyledons, the radicle, the plumule, and the collar.”* The cotyledons are those undeveloped leaves which are seen to push their way above the ground when a plant first makes its appearance: they vary in number, but most usually there are either one or two of them. If a plant have but one cotyledon, it is said to be Monocotyledonous, which is the case with all the Endogenous tribes; if there be two or more, the plant is called Dicotyledonous, in which latter division all the Exogenous tribes are found. The Cryptogamia are all Acotyledonous—that is, without cotyledons. 69. The ascending portion of the embryo plant is called the plumule, and is sometimes hardly distinguishable from the cotyledons; the descending portion is named the radicle, and forms the future root, &c.: the collar is the line of separation between them. “When the seed is called into action, germination takes place. The juices of the plant, which before were insipid, immediately afterwards abound with sugar,” as in the conversion of barley into malt, “which process consists in promoting the germination of the seed by moderate heat and moisture, and checking it by the higher temperature of the kiln as soon as the largest possible quantity of saccharine matter is formed. When the seed has germinated, and sugar is produced, the period of growth commences.” This growth is in the first instance caused by the absorption and decomposition of water, whose oxygen combines with the superfluous carbon of the seed, and is expelled in the form of carbonic acid gas. When the absorption of oxygen has removed a sufficient quantity of carbon from the seed, “the young plant begins to absorb food, and to grow by the processes of assimilation and respiration already described;” and as soon as the seed is once active it receives, by a special provision of nature, a larger proportional share of the sap than any other part of the plant. Probably the heat produced by the consumption of its carbon is also essential to the welfare of the newly formed plant, and may give the necessary stimulus which brings its organs into action. 70. The fact that darkness is essential to germination has long been known—“an embryo, exposed to constant light, would not germinate at all, and hence the care taken by nature to provide a covering to all embryos in the form of the integument of the seed, or of a pericarp.” Mr. Hunt has recently turned his attention to this subject, and he remarks thus on it: “It is not at present in our power to explain in anything like a satisfactory manner the way in which the luminous rays act in preventing germination. The changes which take place in the seed during the process have been investigated by Saussure: oxygen gas is consumed, and carbonic acid gas evolved ; and the volume of the latter is exactly equal to the volume of the former. The grain weighs less after germination than it did before ; the loss of weight varying from one-third to one-fifth. This loss of course depends on the combination of its carbon with the oxygen absorbed, which is evolved as carbonic acid. According to