But how is it, it may be asked, that process meets process in two contiguous filaments, and form between them a germinating spore? By what power is a plant given to understand, that a similar plant lies in its immediate neighbourhood, ready to carry on the necessary fructifying process? Certainly we can consider it nothing less than a species of the same indefinable operation, which prompts the bee to construct a cell of an hexagonal form, or a bird to build a nest in the manner peculiar to its species.

We thus find that these obscure plants form no exception to the very general, if not universal law, that each species of living being requires two distinct elements for its perpetuation. Sexual elements have been detected in most of the cryptogamic plants, and in a short time will probably be discovered in all. The power of reproduction by segmentation, or the production of numerous successions of asexual fertile generations, which, in common with many others of the humblest organisms, vegetable and animal, the confervæ possess, is in all cases limited, the species necessarily reverting to sexual admixture for its perpetuation. germs produced by the conjugation of approximated individuals, when fully ripe, burst the cells in which they are confined, and are consigned to the surrounding water, where they float about, until they meet with some substance to which their mucilage enables them to adhere; and once established in a congenial situation, they spring up into new plants, and extend themselves with amazing rapidity, in a week or two producing thousands and tens of thousands of individuals. Their lives rarely exceed a

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The

year in duration, many of them dying in the course of a few months or weeks. They complete the process of reproduction early in spring, and last during the summer, perishing in the autumn, and disappearing altogether in winter. No sooner does the ice, which had bound up the streamlet in its silent fetters, melt under the warm rays of the sun, allowing its water to flow merrily on, and flash and sparkle in the sunbeams, than every stone in its bed, though brown and naked before, is suddenly, as if by magic, invested with a green velvet coating, whose long graceful filaments float freely with the water. Every ditch and marsh, every rivulet of water, every hoof-mark and rut on the road where water has accumulated, is filled with green clouds of these mysterious plants. The purposes which they serve in these situations are sufficiently obvious. Though associated in our minds with stagnation, putrefaction, and malaria, they are the scavengers, the water-filters of nature. Like the flowers and the trees, which on dry land remove the impurities with which the animal world is continually tainting the atmosphere, they purify the waters in which they occur, by assimilating the decaying matter which they contain; while their own tissues form food and shelter to myriads of animalcules, which wander over these to them-trackless fields and endless mazes, and convert the waste pools and ditches of the wayside into scenes of busy life and enjoyment. This perfect adjustment in the economy of the animal and vegetable kingdoms, whereby the vital functions of each are maintained in the utmost efficiency, is one of the most beautiful and striking phenomena of organic nature.

The largest of the fresh-water algae is the River Lemania (Lemania fluviatilis). It is never found growing in stagnant waters; indeed, it is said to languish and die, when the streams in which it is produced have, by some cause or other, been converted into motionless pools. It loves to grow in clear swift rivers, flowing with a strong current over a rough and rocky bed, and in Alpine streamlets, on the very verge of the numerous cascades which they form during their descent from the hills. It is a matter of surprise how it can sustain the immense force and weight of the impetuous waters, without being uprooted and carried away. Examination will,

however, discover that it has been wonderfully provided with means to enable it to brave the dangers to which in such situations it is exposed. Its filaments are elastic, rigid, and bristly, from three to six inches in length, about the size of a hog's bristle, and knotted throughout at equal distances with prominent swelling joints, like those of the bamboo cane. They spring from a tough cartilaginous disk, so firmly applied to the rock as to require a very considerable force to detach it. It is impossible to convey in words, the same strong impression of fitness and perfection of contrivance, which a glance at the plant in its native haunts would produce. It appears one of the most striking examples of that compensatory adaptation of structure to requirements, which we observe more or less in all the lowest plants; in the moss, which, considering its size, adheres with more tenacity to its growing place than the oak of centuries, that strikes out its roots over half an acre of ground; and in the minute crustaceous lichen, apparently

as hard as the rock upon which it is produced, over which the devastating storms of the Alpine summit sweep for years without inflicting upon it the slightest injury.

The colour of the Lemania, when fresh, is of a fine deep olive-green; but it changes to black when dried and placed in the herbarium. The dilatations or gouty joints, are owing to the development of the sporules within the fronds; and these may be squeezed out by being compressed between the fingers. The force with which they naturally break through the tough and cartilaginous skin of the frond, in order to form independent individuals, is not the least curious circumstance in the economy of this strange plant. Bory, to whom we are indebted for the name, informs us that the recent filaments of the Lemania, owing to some unascertained gas shut up in the knots, when applied to the flame of a candle explode and extinguish it, while a remarkable movement of retraction is felt by the fingers which hold them.

The confervæ generally grow in single branchless filaments, forming a loose fleecy stratum; but sometimes they are aggregated together into singular forms. There is one species known as the water-net or water flannel, (Hydrodictyon utriculatum), which looks more like a piece of green baize manufactured by man, than a production of nature. It forms a beautiful tubular purse or net, with regular polygonal meshes, varying from half a line to half an inch in diameter, grey on the one side, and green on the other. The filaments which compose these meshes are sometimes slender as a horse hair, and sometimes as coarse as a hog's bristle, feeling harsh to the touch when handled.

There is no granular fructification

within the filaments, consequently the plant is propagated viviparously, each of the articulations giving birth to new filaments, which add new meshes to the net, and, in this singular manner, a single individual often weaves a green net-work covering over the whole surface of a pond. It is not attached to any aquatic plants, but floats freely in the water. It is rare in Scotland and Ireland, but is of common occurrence in ponds and ditches in the middle and south of England.

Another curious conferva, which departs widely from the normal form, is the Moor Ball or Globe Conferva. It is found occasionally in lakes in North Wales, in Cumberland, and in the Highlands of Scotland. The filaments radiating from a central point form dense round pale-green balls, as if composed of faded silk thread, sometimes four inches in diameter, and having a strong resemblance to the hair balls that are found in the stomachs of goats. They are sometimes employed as pen-wipers in the places where they are found. These balls float freely at a small depth in the water, and are often washed ashore by the waves, where they accumulate in dense masses, and are again covered over with a parasitic confervoid growth.

In ditches by the waysides, may often be seen large dark-green intensely slimy masses of rigid filaments as thick as horse hair. This is the Zygnema (Fig. 18), one of the largest and most curious of the confervæ. Under the microscope, the filaments are found joined parallel to each other by transverse tubes, and marked by articulations longer than broad. They are interesting especially as exhibiting the spiral arrangement in their