Phyllum Thallophyta - The Algae - Rhodophycae - Nemalionales
This order comprises Rhodophyceae in which no tetraspores are developed. To it belongs most of the simple members of the Red Algae, including several fresh-water species. All the genera belonging to the other orders occur exclusively in the sea. As our example we shall study the lifehistory of Batrachospermum, one of these fresh-water species.
This species is found quite commonly in fresh water, occuring most plentifully in. clear running streams, but usually under shade. Specimens grown in deep water or shade are dark violet or reddish in colour, while those living in shallow water are olive green. This variation in colour is primarily influenced by the intensity of light.
The thallus is filamentous, freely branched and gelatinous in texture. The central filament is made up of a single row of large cells, from which whorls of dichotomous lateral filaments are developed at frequent intervals. Growth is by means of a single apical cell, which cuts off cells basipetally by transverse divisions. Immediately behind the growing point these cells, which form the central filament, cut off small lateral cells, four from each central cell, and these form the initials from which the side branches arise. The lateral cells, though actively dividing, remain small themselves, while the cells of the central filament elongate considerably, so that eventually the groups of lateral cells are widely separated and appear to form nodes on the axis. From these nodes grow out the richly branched clusters of side shoots, which give the whole axis that beaded appearance to the naked eye from which the specific name of "moniliforme " is derived.
In addition to the side shoots the cells at each node give rise to filaments which grow longitudinally downwards to the next node, covering the axis cells with a loose mantle or pseudo-cortex. Here and there also one of the nodal cells is transformed into an apical cell and gives rise to a secondary long shoot which repeats the structure of the main axis. The cells of the lateral branches contain several irregular plastids, which are termed chromatophores, each possessing a single pyrenoid.
The sex organs may be produced on the same or on different plants.
The male organ is the antheridium, but the female organ is called the carpogonium and differs in some respects from the oogonium of the other groups of Algae.
The antheridia are very small, and arise in terminal clusters on the side branches, frequently near the female organs. The side branches produce short antheridial branchlets, on each of which four antheridia are borne. Each antheridium liberates a single naked, non-motile, spherical body which is called a spermatium.
The carpogonium develops on a specialized lateral branch, which is called the procarp. It arises near the base of a vegetative branch, and enerally consists of a filament of four cells. The terminal cell becomes the carpogonium and soon develops a swelling at its anterior end, which elongates very considerably and forms a filamentous structure known as the trichogyne. In some types of Red Algae the nucleus of the carpogonium divides at this stage, and one of the daughter nuclei migrates into the trichogyne, but this does not normally occur in Batrachospermum. Meanwhile the carpogonium enlarges, the chromatophore which it originally contained disappears and the female nucleus sinks to the base of the cell.
Spermatia which have been liberated from the antheridia are carried by the water to the trichogyne, to which they adhere. The wall of the trichogyne breaks down at the point of contact with the spermatium, and the nucleus of the spermatium migrates into it. It then passes down the trichogyne and fuses with the female nucleus in the carpogonium, after which the basal part of the carpogonium is isolated by a cross wall and forms the zygote. The zygote nucleus divides into two, one nucleus then migrates into a lateral protrusion which develops in the side of the zygote, and is cut off by a wall. The remaining nucleus then also divides and a second lateral protuberance is formed into which one of the daughter nuclei migrates. From these lateral cells, cut off from the zygote, filaments of cells develop which are called gonimoblasts. They branch repeatedly and small spores termed carpospores are formed at their ends.
When the carpospores are mature the spore wall bursts and the naked protoplasmic mass escapes, comes to rest, develops a new wall and germinates into a pad or sole of parenchymatous tissue, from which develop numerous shoots of a simple filamentous plant. This plant is spoken of as the Chantransia stage on account of its resemblance to another Alga of that name. Terminal cells on the lower branches of these filaments function as apical cells, and from them fresh Batrachospermum plants arise. They may also arise directly from the sole.
There are no tetraspores produced in Batrachospermum, but monospores may be developed on the Chantransia filaments. These arise singly in sporangia borne at the ends of short branches and on liberation give rise to fresh plants of the Chantransia stage.
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