The hairs are so transparent that the gyration of the azotized liquid, called protoplasm they contain, has been distinctly traced, a motion so universal in some part of the structure of plants that according to the observations of Mr. Wenham, the difficulty is to find a plant, aquatic or terrestrial, in which it does not take place at some period of its growth. The gyration in any given cell preserves a uniform direction; in different cells the direction is different. It will persist in a detached part of a plant for several days or even several weeks. It is arrested by cold, and recommences its gyration when the temperature is raised.
It has been mentioned that in the primordial cell the solid coloured particles often form a nucleus in the centre of the viscid liquid called protoplasm, which is continually diminished by the increase of the watery vegetable sap. At length the protoplasm in the hairs is reduced to mere threads extending from the cell wall to the nucleus, so that the latter looks like a spider in the middle of its web. These threads are really streams of the viscid protoplasm flowing through the more liquid cell sap from the nucleus to the cell wall, where they turn and flow back again in another thread. When there are several currents in the same cell, the nucleus, which is the common point of departure and return, is the centre of the vital activity of the cell, though it does not always maintain a central position; in the cells of the leaves of the Vallisneria spiralis the nucleus even follows the protoplasm, which flows in a broad stream up one side of the cell and down the other, as in the Chara. In most plants the gyration is transitory, for the nucleus which always exists in young cells is dissolved as the cell advances in age, and the protoplasm is so much diminished in quantity, that its motion is imperceptible. There are exceptions, however, as in the hairs of the nettles and some other plants, where it is persistent.
The motion is in general very slow. The thinness and minuteness of the currents may be imagined, since they and the cells containing them are microscopic objects, and the solid particles carried by the liquid, which afford the means of tracing its course, are not more than between the three and the five thousandth part of an inch in diameter. M. Schleiden ascribes the motion to changes in the form of the cells produced by an internal vital action, while Professor Karsten believes, from observations he made on the rotation of liquids in the hairs of the common nettle, that it is a phenomenon of diffusion, depending upon the chemical changes taking place in the cells of the hairs independent of any contractibility, not referable to them.
The whole of the tissues that exist in a well-grown tree are not to be met with in each of the numerous woody and herbaceous plants of the first class; some may be wanting, and those that do exist may be, and generally are, much modified both in form and size. All the trees in the temperate zone, and most of those in the tropics, belong to the class of Exogens; but the annual rings of wood are less distinct in the latter, the periods of repose and activity depending upon the dry and wet seasons not being so decided as our winter and summer. The leaves of tropical plants have a thicker skin than in colder climates, to defend them from an ardent sun. The structure of herbaceous plants in all countries is lax and juicy, they have abundance of pith, large medullary rays, and zones of fibro-vascular tubes, which separate the pith from the bark. In fact, each herbaceous and ligneous family has a structure and properties peculiar to itself; but although there is almost an infinite diversity of form and character, the general type of the class may be traced in all.
Vegetable matter consists of carbon, hydrogen, oxygen, and nitrogen, yet no plant can combine these simple elements into organic substances; they imbibe them by their roots and leaves under the form of carbonic acid, water and ammonia: these they have the power of decomposing, and recombining their simple elements into new compounds. Carbon forms the hard part of plants, and enters extensively into their most delicate structure; but it is never found free. Combined with hydrogen and oxygen it not only constitutes the cell wall cellulose, which may be regarded as the skeleton of the vegetable world, but hundreds of compounds differing decidedly in their properties, yet consisting only of these three elements united with one another in different quantities and proportions. Proteine, a compound of all the four simple elements, is a mucilaginous substance, which lines the primordial cell, is homogeneous at first, and afterwards more or less granulated. It is present wherever the vital energy is in activity.
Although these four primary elements form the basis of vegetation, plants require other substances which they absorb from the ground in a state of solution, such as silex, or rather silicious salts having a base of potash or soda, the carbonates, sulphates, and phosphates of lime, the phosphate of manganese, and the oxides of manganese and iron, with various other metals and substances in a state of combination and solution. A few are universal constituents, as the earths and alkalies; in general each race of plants only absorbs such as are peculiar to itself. Soda abounds in the Algæ and is found in the Liliaceæ, Cruciferæ, and other plants that are indigenous on the sea-coast, or in brackish marshes. Potash exists in land plants, and cannot be replaced by soda, for however rich the soil may be in soda, they do not thrive in it. The ashes of land plants consequently contain the metal potassium, while most of the Algæ yield sodium; they also yield chlorine, iodine, and bromine in a state of combination. It is proved by spectrum analysis, that every plant, with the exception of the very lowest, contains a variety of metals in infinitesimal quantities, as lithium, rhodium, and others; but they are not essential to the welfare of the plants. Iron is the most frequent constituent in very small quantities; there are also occasional deposits of soda, lime, and a little manganese. All the various substances which enter the vegetable system, are combined in definite proportions into an infinite variety of organic compounds in different plants, and in different parts of the same plant, for the decomposed matter is carried by the ascending sap to every part even of the highest trees. Throughout the whole process the law of the division of labour prevails; to each part of a plant, and to each group of cells, its own duty is allotted.
The vegetable sap, consisting of water, carbonic acid, ammonia, and other substances, which enter the spongy extremities of the roots in a liquid state, rises in the form of a crude fluid through the whole loose texture of herbaceous plants, through both the wood and pith of trees under two years’ growth, and in older trees and shrubs it rises through the sap-wood of the stem into the branches, and thence into the leaves, the limit of ascent in all plants, so that in spring, all the cells are full of sap. The vascular ducts are capillary tubes, and the cellular tissue is an assemblage of closed cells or sacs, whose wall or cell-membrane is permeable by liquids; hence the imbibition of the roots and the rise of sap in the plant are essentially due to capillary attraction acting contrary to gravitation. The ascension of the liquid is inversely as the diameter of the capillary tubes and cells in the stem and branches; the quantity raised is the same at all heights, and the velocity of ascent is inversely as the height.[[79]] As soon as the leaves are expanded, they evaporate a quantity of water through their stomata during the day, so that in a tree or any plant, an enormous extent of evaporating surface aids in raising the sap by creating a vacuum in all the upper cells and vessels, by which the force of suction and the rapidity of ascent are increased. It appears that the water evaporated by the leaves is in exact proportion to that taken up by the roots to supply its place; but as soon as the young branches are formed, the buds for the following year produced, and when the leaves are full of the chlorophyll which they have consolidated during the summer, the evaporation is less, the sap ceases to rise, the spirals and vascular ducts in the medullary canal and sap-wood are left dry, and fill with air, which they convey to every part of the plant except the bark, to assist in assimilation, that is, in the formation of organic compounds.
During the whole of this process the leaves and other green parts, which are the organs of vegetable respiration, are most active. They absorb carbonic acid gas from the atmosphere by day, and exhale oxygen. For by the direct action of solar light the carbonic acid gas and ammonia in the crude sap are decomposed, part of the oxygen is set free and exhaled, and the rest, with part of the remaining elements, combine to form chlorophyll, which is a compound of starch and a little nitrogen. The oxygen inhaled by plants during the night, combined with other elements in the sap, forms oxidized vegetable compounds.
M. Kosmann, of Strasburg, observed that both the leaves of plants and their corollas give out a ponderable quantity of ozonized oxygen, much more than that which exists in the air, and that the quantity is less in the night.
All parts of plants that are not green exhale carbonic acid gas, and inhale oxygen, like animals, night and day; if prevented from inhaling oxygen they lose their vital power, are soon suffocated, and the plant dies. The expiration of oxygen by the leaves is connected with the nourishment of a plant, the inspiration of that gas is connected with its life.