The sun spots which are situated in that region of the sun which lies below the photosphere consist of a central darkness or umbra, surrounded by a penumbra which is less dark. Professor Wilson, of Glasgow, proved that the spots are cavities, of which the umbra or darkest part forms the bottom, and the penumbra the sloping sides, by observing that the umbra encroaches on that side of the penumbra which is next to the visual centre of the sun. Hence the umbra of a spot is at a lower level than the penumbra; and since luminous ridges and sometimes detached portions of luminous matter cross over the spots, it is concluded that the whole phenomenon is below the surface. The spots have an apparent motion from east to west, due to the rotation of the sun; and Mr. Carrington discovered that they have a proper motion also from east to west, those nearest the solar equator moving fastest. They are confined to the equatorial regions.
No reason has yet been assigned for the periodicity of the spots, which go through a cycle of maxima and minima every ten years nearly. They are singularly connected with terrestrial magnetism; the maximum of the spots coincides with the period of the greatest disturbance of terrestrial magnetism. The spots seem to be influenced by the planet Venus in such a manner that when a spot comes round by rotation to the ecliptical neighbourhood of this planet, it has a tendency to dissolve; and, on the other hand, as the sun’s surface recedes from the planet it has a tendency to break out into spots.[[28]]
PART II.
VEGETABLE ORGANISMS.
SECTION I.
MICROSCOPIC STRUCTURE OF THE VEGETABLE WORLD.
The study of the indefinitely small in the vegetable and animal creation, is as interesting as the relation between the powers of nature and the particles of matter.
The intimate organic structure of the vegetable world consists of a great variety of different textures indeterminable by the naked eye, and for the most part requiring a very high magnifying power to discriminate. But ultimate analysis has shown that vegetables are chemical combinations of a few very simple substances. Carbon and the three elementary gases constitute the bases of all. No part contains fewer than three of these universal elements, hence the great uniformity observed in the chemical structure of vegetables. The elements unite according to the same laws within the living plant as in the inorganic creation, and the chemical laws acting upon them are the same. For, as already mentioned, M. Berthelot having combined carbon and hydrogen into acetylene, which no plant is capable of doing, he assumed it as a base from which he deduced, by the common laws of synthetic chemistry, hundreds of substances precisely similar to those produced by vegetables. Although it may be inferred from this that chemical action is the same within the vegetable as it is in the inorganic world, yet it is accomplished within the plant under the control of the occult principle of plant-life. No mere physical powers are capable of forming directly out of inorganic elements, the living organism whose passage through the cycle of germination, growth, reproduction and decay, serves so pre-eminently to distinguish between it and inert matter. Plants, indeed, borrow materials from the inorganic, and powers from the physical world, to mould them into living structures, but both are returned at death to the great storehouse of nature.
All other circumstances being the same, the vigour and richness of vegetation are proportionate to the quantity of light and heat received. The functions of light and heat are different, but their combined and continued action is indispensable for the perfect development of vegetation. Light enables plants to decompose, change into living matter, and consolidate, the inorganic elements of carbonic acid gas, water, and ammonia, which are absorbed by the leaves and roots, from the atmosphere and the earth; the quantity of carbon consolidated being exactly in proportion to the intensity of the illumination, which accounts for the darker green tint of the tropical forests. Light acting in its chemical character is a deoxidizing principle, by which the numerous neutral compounds common to vegetables are formed. It is the principal agent in preparing the food of plants, and in all the combinations and decompositions the law of definite quantitative proportions is maintained. It is during these chemical changes that the specific heat of plants is slowly evolved, which, though generally feeble, is sometimes very sensible, especially when the flowers and fruit are forming, on account of the increase of chemical energy at that time. To the same cause, the phosphorescence of certain flowering plants and a few fungi, is supposed to be due.
The action of heat is manifested through the whole course of vegetable life, but its manifestations take various forms suited to the period and circumstances of growth. Upon it depends the formation of protein and nitrogenous substances, which abound in the seeds, buds, the points of the roots, and all those organs of plants which are either in a state of activity, or are destined to future development. The heat received, acting throughout the entire organism of a plant, may augment its structure to an indefinite extent, and thus supply new instruments for the chemical agency of light, and the production of new organic compounds. The whole energy of vegetable life is manifested in this production, and, in effecting it, each organ is not only drawing materials, but power, from the universe around it. The organizing power of plants bears a relation of equivalence to the light and heat which act upon them. The same annual plant from germination to the maturation of its seed receives about the same amount of light and heat, whatever be the latitude, its rate of growth being in an inverse ratio to the amount it receives in any given time. For one of the same species, the more rapid the growth, the shorter the life.
The living medium which possesses the marvellous property of being roused into energy by the action of light and heat, and which either forms the whole or the greatest part of every plant, is in its simplest form a minute globe consisting of two colourless transparent concentric cells in the closest contact, yet differing essentially in character and properties. The external one, which is the strongest, is formed of one or more concentric globular layers of cellulose, a substance nearly allied to starch, being a chemical compound of carbon, hydrogen, and oxygen in the proportions of 12, 10, and 10, respectively.[[29]] It forms the universal framework or skeleton of the vegetable world, but it has no share whatever in the vital functions of vegetation. It only serves as a protection to the globular cell within it, which is called the primordial cell because it is first formed, and because it pre-eminently constitutes the living part, since the whole phenomena of growth and reproduction depend upon it. In its earliest stage the primordial cell is a globular mass of an azotized colourless organizable liquid, called protoplasm, the life blood of vegetation, containing albuminous matter and dextrine or starch-gum. It is sufficiently viscid to maintain its globular form, but its surface becomes slightly consolidated into a delicate soft film. The viscid albuminous liquid within it is mixed with highly coloured semi-transparent particles containing starch; besides cavities or vacuoles full of a watery vegetable sap of highly refractive power are imbedded in it. By degrees the coloured particles become more and more condensed within a globule of mucus, which constitutes the nucleus of the primordial cell. The watery sap in the cavities increases so much as ultimately to fill nearly the whole of the cell at the expense of the viscid protoplasm, which then merely forms a lining to the cell either coloured or hyaline. The primordial cell then secretes and envelopes itself with the strong protecting coats of cellulose already described. On account of its high colour, which is chiefly green, the whole contents of the primordial cell are called the endochrome. The minute globular nucleus contains a liquid of high refractive power, and is coated with a delicate film. Its structure, which is best seen in the hairs and young parts of plants, is not always the same, nor is it always in the centre of the primordial cell, being sometimes attached to the internal cell wall.[[30]] On the minute but complicated organ, the primordial cell, vegetable life depends.
It will be shown afterwards that the primordial cell sometimes constitutes the whole plant, with or without its cellular coat. By its continual bisection when so coated, linear plants, such as the confervæ, are formed and lengthened ([fig. 3]). When bisection is about to take place, the cell increases in length; the nucleus, which always plays an important part in cell formation, spontaneously divides into halves; at the same time the cell wall becomes constricted in the middle and gradually folds between them, and divides the original cell into two new ones, in which the nuclei become perfect and assume their normal position. The terminal cell may undergo the same process, so that the plant may be lengthened indefinitely.