Besides the spiral vessels that are attached to the interior walls of the vascular ducts, there are groups of independent spiral vessels of great beauty and elasticity, of which the seeds of the wild clary afford a remarkable instance. They consist of cylindrical tubes with conical extremities twisted into a right or left-handed screw, which can be unrolled without breaking. They are found in the leaves of almost all plants, in the petals and stamens of flowers, in the stalks of all fruits, even in the minutest seeds; large parallel bundles of them imbedded in hexagonal cellular tissue may be seen in the veins of the kernel of the hazel nut, and they constitute the medullary sheath which surrounds the pith in trees. They are all hollow, and capable of conducting liquids.
Fig. 5. Longitudinal section of stem of Italian reed:—a, pith; b, fibro-vascular bundles; c, cuticle.
The laticiferous vessels or vasa propria, those which contain the proper juices of plants whether milky or coloured, are exceedingly varied in their forms and arrangement in different plants, and in different parts of the same plant. In the leaves they generally form a delicate capillary network, in the bark they constitute a system of long vascular ducts forming an elongated irregular network pervious to the proper juices throughout; sometimes they are formed of cells joined end to end, and frequently they are thin branching flexuous tubes, meandering through the passages or interstices of the cellular tissue, and occasionally filling the lacunæ.[[31]]
Every one of the preceding tissues may be found in many of the highest class of vegetables—those which are distinguished by having seeds with two lobes or seed leaves, such as our common trees, shrubs, and most of the herbaceous plants. Palms, the cereals, grasses, canes, and all plants having seeds with but one lobe, which form the second class, consist of cellular tissue mixed with fibro-vascular bundles; whilst in the third or flowerless spore-bearing class, there is a general tendency to a more and more simple structure, from the tree fern to the lichens and algæ, which last consist of cellular tissue alone, and contain the lowest germs of vegetable life.
In seeds the miniature plant is enclosed between the two lobes, as in peas and beans, or in a cavity of a lobe, as in a grain of wheat or barley; and all the parts of the embryo are merely developed into the perfect plant during the progress of vegetation. A spore, on the contrary, which is the seed of a Cryptogam, or flowerless class of plants, is a most minute globular cell, full of granular matter, in which no embryo has yet been discovered, so that the parts of the future plant are supposed to be formed during the progress of vegetation, instead of being developed. Seeds, and spores also, sometimes produce new varieties, while buds and offsets only transmit the parent plant, with all its peculiarities. In the higher classes, the organs of nutrition and reproduction are always separate; in the lowest grades of vegetable life they are often the same. Seeds bear no proportion to spores either in size or number; the latter are often so extremely small that they are invisible to the unaided eye, and are not to be counted even by thousands. It appears that beings, whether animal or vegetable, are prolific in the inverse ratio of their size. The incredible multitudes of the lowest grades of vegetable life, the rapidity of their growth, the shortness of their existence, and their enormous fruitfulness, make them powerful agents in preparing soil for the higher classes which are nourished by their decay. But no sooner do even the monarchs of the forest fall than the work of destruction begins; the light and heat which in their chemical form brought them to maturity, now in their physical character accelerate their decay; the moss and the lichen resume their empire, and live at the expense of the dying and the dead, a cycle which perpetuates the green mantle of the earth.
Notwithstanding the important part these inferior beings perform in the economy of nature, they were imperfectly known till they became a test for the power of the microscope. Then indeed not only were the most wonderful organisms discovered in the ostensible tribes of the Cryptogamia, but a new and unseen creation was brought under mortal eye, so varied, astonishing, and inexhaustible, that no limit can be assigned to it. This invisible creation teems in the earth, in the air, and in the waters, innumerable as the sand on the seashore. These beings have a beauty of their own, and are adorned and finished with as much care as the creatures of a higher order. The deeper the research, the more does the inexpressible perfection of God’s works appear, whether in the majesty of the heavens, or in the infinitesimal beings on the earth.
SECTION II.
ALGÆ.
The principal objects in the study of plant-life are the organs by means of which they obtain and assimilate substances that are essential for their nourishment and growth, and those by which the perpetuity of their race is maintained and their type transmitted from age to age. In the lowest group of plants, represented by the Algæ, which come first into consideration, the two properties are combined; in the highest they are distinctly different, but the progress from one to the other may be traced through an ascending series of vegetable structure. In the simple grades of vegetables, the primordial cell frequently constitutes the whole plant; it appears first, and then envelopes itself with a coat either of cellulose or of a gelatinous substance.
Many instances of this are to be found amongst the Algæ, which are all aquatic plants, and are found growing either attached to other bodies, or floating independently, and live, some species in fresh water, and others in the sea and its estuaries. The Algæ absorb carbonic acid and give out oxygen, under the influence of sun-light, exactly as do the flowering plants; and the quantity of oxygen disengaged by them is said to be enormous.