Fig. 824.—Seeds with pappi.
Pure water exposed to the air does not afford nourishment to the germs which fall into it, till a sufficient number of them shall have been deposited to form a food for those which come after them; but if we mix with the water any soluble vegetable or animal matter, in a short time the microscope will detect the growth of the germs that are being deposited, for where nourishment is, there only can they be developed. These germs are capable of existing for an indefinite period, either floating in the water, or blown about by the air, and have been detected hundreds of miles from land; the rigging and sails of ships far away from shore are often covered with what sailors suppose to be sand blown from the land, but which are organic substances, either vegetable or animal. According to Humboldt, the Red Sea has derived its name from the fact that at certain seasons the surface of the water has a reddish appearance, and this (as he says) he was fortunate enough to observe, which colour he found to be due to myriads of red fungi, which had formed on the surface. The seeds of some plants are furnished with minute wings or plumes, which cause them to be borne on the air or floated on the water (fig. 824), to fertilise some barren spot, perhaps a coral reef, which has at length reached the surface of the water, and which ascends no higher, for the little creatures which built it are aquatic, and cannot live exposed to the air; this coral reef now becomes a receptacle for seaweed and fungi, which float on the surface of the ocean are washed on to the reef, die, decay, and leave behind a thin layer of mould, which process being repeated again and again, forms an elevated edge to the reef, enclosing a lake, or “lagoon” as it is called, the waters of which evaporate, and the space is filled up in the same way as the edge was formed, together with the excrements of birds, etc., forming layer after layer of mould, and the surface becomes fit for the growth of larger seeds, as the cocoa-nut, banana, etc., which are drifted on to it by the waves; in this way a coral reef becomes an island fit to be inhabited by man and other animals.
It is impossible for any person not accustomed to observe the manner of the propagation of the fungi, to understand a written description, for the fructification of these plants are very varied in the manner of the development of the spores. They are not generally hurtful, but much caution should be observed in the matter of the mushroom, which may be distinguished by the pale pink and black of the under part. There are many poisonous fungi, but the greater number are harmless, though they are not intended for food. They simply clear away the decaying growths, and act as safety-valves to Nature by carrying away what is not required, to give it to the air again to be renewed into life.
The vegetable kingdom forms the link between the minerals and the animals. The vegetable derives food and nourishment from water, carbonic acid, and ammonia, which are, as we already know, made up of certain elements, and thus supply us all with food. They give out oxygen for the use of animals, and are thus, in another sense, the source of life. The growth of a plant is very interesting, and we may conclude by following it.
The seed is sown, and the cells of the “cellular tissue” become developed, passing some upwards, some downwards, to form a radicle or plumule, as explained. The latter carries up the cotyledon, which begins to decompose carbonic acid from the atmosphere, and fixing the carbon as woody fibre. The leaves are then formed and more fibres, and so on for every leaf; thus the number of woody fibres which form the trunk of a tree is in proportion to the number of leaves which that tree has borne, from which we come to the conclusion that the size of the trunk of a tree is the sum of all its branches. While all this is going on, the cellular tissue of the downward part or radicle also becomes developed and divides out into roots, on the surface and at the extremities of which are minute cellular bodies called “spongioles” (from their power of absorbing moisture), which take up the fluid of the earth which surrounds them; this moisture ascends through the vessels of the plant till it arrives at the surface of the leaves, where it is exposed to the action of light and sunshine. The ascent of the moisture of the earth was first correctly explained by Du Trochet, and is owing to a peculiar power which he discovered, and which is called “Endosmose”; this consists in the tendency which a fluid has to penetrate a membrane on the other side of which is a fluid of greater density than itself. This may be seen by the following experiment: obtain a piece of glass tubing about a foot long, having the end blown out into the form of a bell, as in fig. 825, tie a piece of bladder over the expanded end and fill it partly with syrup or gum-water, so that this shall rise in the stalk about an inch; place this in a glass of water with the bladder downwards, and the fluid will be seen slowly to rise in the stalk, so that in perhaps an hour it will rise to the top. This apparatus resembles one of the spongioles at the extremity of the fibre of a root.
Fig. 825.—Endosmose.
The rain falling through the air carries with it a certain amount of carbonic acid and ammonia, which the air always contains, and it is the whole source of the nitrogen which forms a very important part of the bodies of plants and animals. When the rain arrives at the surface of the earth, it sinks down into it and carries with it all soluble vegetable or animal matter which it meets with, together with any soluble earthy matter which may exist in the soil; this forms the sap of the tree. When it arrives at the surface of the leaf, the watery part of it combines with the carbonic acid of the air (through the influence of light), and appropriating its carbon, gives out the oxygen; this is the true respiration of plants, and is exactly the reverse of what takes place during the respiration of animals, in which case oxygen is absorbed and carbonic acid given off. The carbon thus retained by the plant combines with the elements of the water to form the solid green substance called chlorophyl, which is the basis of all the tissues of the plant; the ammonia is also decomposed, and its nitrogen combining with the oxygen and hydrogen of the water, and the carbon of the carbonic acid forms those compounds which constitute the most nourishing parts of vegetables, such as albumen, gluten, etc., and of which all the animal tissues are built up, for the production of these organic substances takes place in the vegetable only, animals simply appropriating them for their food. The sap which reaches the leaf is not all converted into chlorophyl, but also into those peculiar juices which are found in plants, some of which contain sugar, some gum, others (as the pine tribe) turpentine, and in the laurel tribe camphor, all of which are substances containing much carbon; moreover the solid wood and bark are deposited from these juices as they descend from the leaf after having been acted on by light (or the actinic power associated with it). Now, as the water, ammonia, and carbonic acid which descend with the rain are from the air, and as the vegetable is formed wholly by their absorption, it may be fairly said that the vegetable kingdom (and therefore the animal) feeds upon the air, and that the trees do not grow out of the earth, but into it.
