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such trees usually grow, where it speedily strikes root, and separates from its parent. Trapa natans has two very unequal cotyledons: of these, the larger sends out a very long petiole, to the extremity of which are attached the radicle, the plumule, and the smaller cotyledon (Mirbel). · Cyclamen germinates like a Monocotyledon: its single cotyledon does not quit the seed till the end of germination; and its caulicle thickens into a fleshy knob, which roots from its base. The Cuscuta, which has no cotyledons, strikes root downwards, and lengthens upwards, clinging to any thing near it, and performing all the functions of a plant, without either leaves or green colour. In Monocotyledons, the cotyledon always remains within the seminal integuments, while its base lengthens and emits a plumule. In Cycas, which has two cotyledons, the seminal integuments open, and the radicle escapes.
It has already been seen, that, under certain circumstances, the vitality of seeds may be preserved for a very considerable length of time; but it is difficult to say what are the exact conditions under which this is effected. We learn from experiment that seeds will not germinate if placed in vacuo, or in an atmosphere of hydrogen, nitrogen, or carbonic acid; but no such conditions exist in nature, and, therefore, it cannot be they which have occasionally preserved vegetable vitality in the embryo plant for many years. Perhaps the following remarks, in a work lately published by the Society for the Diffusion of Useful Knowledge, may throw some light upon the subject:
“ It may, upon the whole, be inferred from the duration of seeds buried in the earth, and from other circumstances, thạt the principal conditions are, 1. uniform temperature; 2. moderate dryness; and 3. exclusion of light: and it will be found, that the success with which seeds are transported from foreign countries, in a living state, is in proportion to the care and skill with which these conditions are preserved. For example, seeds brought from India, round the Cape of Good Hope, rarely vegetate freely : in this case, the double exposure to the heat of the equator, and the subsequent arrival of the seeds in cold latitudes, are probably the causes of their
death; for seeds brought over land from India, and therefore not exposed to such fluctuations of temperature, generally succeed. Others, again, which cannot be conveyed with certainty if exposed to the air, will travel in safety for many months, if buried in clay rammed hard in boxes: in this manner only can the seeds of the Mango be brought alive from the West Indies; and it was thus the principal part of the Araucaria Pines, now in England, were transported from Chile. It may therefore be well worth consideration, whether, by some artificial contrivance, in which these principles shall be kept in view, it may not be possible to reduce to something like certainty the preservation of seeds in long voyages. Such, for instance, as by surrounding them with many layers of non-conducting matter, as case over case of wood; or by ramming every other space, in such cases, with clay in a dry state. These means seem more likely to answer their end, than the usual modes of putting seeds in bottles, packing them in charcoal, or surrounding them with coats of wax; all of which, it is well known, are absolutely prejudicial, instead of beneficial, to the seeds. In illustration of what we have recommended, we may add that seeds are well known to travel best in their own pods, or pericarps: may we not suppose that their vitality is preserved, in such instances, by the non-conducting quality of the air which the cavities of the fruit contain ?”
OF THE FOOD OF PLANTS. - MANURE.
The principal part of the food of plants is derived from the earth, and is introduced into their system through the roots. The latter are, however, incapable of absorbing anything solid; fluid and gaseous matter only can pass through their spongelets. It is, perhaps, exclusively in the form of water that the nutritive matter of the soil is received by roots; not, however, of pure water, which in fact does not exist in nature, but of water holding various solid matters in solution, the most remarkable and abundant of which are, silex, lime and many of its salts, several other earths, and oxides of iron and copper.
These substances, however, although they may each perform their allotted part in the economy of vegetation, consolidating the tissue, hardening the epidermis, or assisting in depriving a plant of organs which become unhealthy and worn out, cannot be altogether considered as nutritive matter. There are, perhaps, only three forms of matter which can properly be called nutritive; carbon, water, and nitrogen.
Soil in its natural state is filled with the remains of organic bodies, which decompose, and yield nitrogen, or become converted into carbonic acid. In proportion to the abundance of these is soil fertile. Nitrogen, and the carbonic acid incessantly forming below the surface of the earth, enter freely into the roots; combining with water and such other principles as may already have been formed there, they ascend the stem, the carbonic acid decomposing to a certain extent as it passes along, and giving, apparently, its oxygen to the spiral vessels, which convey it into other parts of the system; when it reaches the leaves, it liberates its oxygen completely, and leaves its carbon
to unite with the tissue of vegetation, or to enter into new combinations with water, atmospheric air, or other elements that it finds itself in contact with : whence proceed the gummy, amylaceous, resinous, oily, and other products peculiar to the vegetable kingdom. Upon this subject it has been observed by a modern writer, “that, if the roots of a plant are placed in a close vessel, in distilled water, from which carbonic acid has been carefully expelled, the plant may increase a little in size, in consequence of the decomposition of the water, and the combination of its elements with the vegetable system; but it is only when carbonic acid is added, that the plant acquires its natural vigour and rate of growth. But, if a plant is placed in solid carbon, and you water it with distilled water, it might as well be planted in powdered glass, until the carbon begins to combine with the oxygen of the air, and to form carbonic acid. Sir Humphry Davy placed a plant of Mint in water mixed with carbon in a state of impalpable powder, and he found that not a particle could enter the roots. If we look to the effects of manures, we shall find that in most cases, except when their object is to alter the state of the soil mechanically, or to act as stimulants, as is probably the case with sulphate of iron, their energy is in proportion to their capability of forming carbonic acid. Yeast, for instance, which is one of the most active manures we have, is so from possessing, beyond all other substances, the power of exciting fermentation, and thus of causing the formation of carbonic acid among the vegetable matter which lies buried in the soil.
6 While, however, all experiments combine to prove that carbonic acid is the most essential of the elements upon which plants are nourished, it is necessary that the student should be aware that other species of matter are constantly taken into the system, and probably, therefore, contribute to their nutrition.
“ Water is one of these. Although we know that a very large proportion of all the water absorbed by a plant is lost again by evaporation, yet the experiments of Théodore de Saussure have shown that a portion of it is actually solidified. He found that when plants are grown in a close vessel, in an
artificial atmosphere, containing a little carbonic acid, the weight which the plant acquired in a given time was augmented, not only by the quantity of carbon produced by the decomposition of carbonic acid, but to a much more considerable extent, which could only be ascribed to its having fixed a considerable quantity of water; thus plants of the Periwinkle, which, in a vessel without carbonic acid, had gained 14 grain from water, acquired 5%, when they were at the same time able to procure carbon. The same excellent observer has computed that, if we calculate with the utmost care all the weight which a plant can gain, by fixing carbon, by depositing earthy, saline, alkaline, and metallic matter which it borrows from the soil, by respiring oxygen, or from the soluble matter of soil, we shall not be able to account for more than a twentieth part of the real weight of such a plant. The other nineteen twentieths must, therefore, be fixed water. Whatever errors there may be in calculations of this nature, there cannot be much doubt that they are correct to so considerable an extent, as to oblige us to admit that water forms a considerable part of the solid tissue of plants; so that it would appear that, like minerals, plants have a water of crystallisation independently of their water of vegetation.” It has already (p. 360.) been shown, that Messrs. Edwards and Colin have proved experimentally that plants decompose water by their vital force, fixing the hydrogen and parting with the oxygen, which combines with carbon, forming carbonic acid.
As it has been supposed that all the oxygen given off by plants is produced by the decomposition of carbonic acid, it has been inferred that, if the water which is consumed by plants is ever decomposed, it is in the formation of the various secretions which contain more oxygen (acids), or more hydrogen (oils), than water : but, as the greater part of vegetable substances, such as gum, sugar, fæcula, &c., contain oxygen and hydrogen in the same proportions as water, it has been thought that the greater part is undecomposed and simply fixed; but the experiments of Edwards and Colin, above referred to, prove the contrary.
It was formerly thought that nitrogen, or azote, has no