Phylum Thallophyta - The Algae - Siphonocladiales - Cladophora
The Siphonocladiales are Chlorophyceae in which each branch of the ' thallus is composed of one or more coenocytes. Though in the simpler members the thallus consists of a single coenocyte and therefore may be comparable with that of the Siphonales, the group as a whole shows a progressive complexity, brought about partly by the development of septa which cut the large coenocytes into smaller ones and partly by the development of the branching system.
Asexual reproduction is by means of zoosporangia in 'which are developed zoospores, or by aplanospores. Sexual reproduction is by the fusion of isogametes.
Most of the species are marine and are more characteristic of the warmer temperate and tropical seas than of this country.Many are found in the Mediterranean while others occur particularly in the West Indies. A few genera are found in fresh water. A number of species are calcified, fossil types of which have been described.
We shall consider only one type, Cladophora, which is among the more advanced members of the group. In fact recent classifications have tended to favour separating it as an independent order, the Cladophorales, while relegating all the other families to the Siphonales, from which they have most probably arisen.
Species of this genus occur characteristically both in fresh water and also in the sea, and a few species are epiphytic or even epizoic. The species are very numerous and difficult to separate. The thallus is composed of branched,septate filaments formed of cylindrical coenocytes joined end to end. It is attached to the substratum by means of branched, septate rhizoids. Each branch grows by means of an apical segment. The separate coenocytes contain numerous nuclei which are either embedded in the meshes of the chloroplast or lie internal to it. The structure of the chloroplast is complex, and divergent views exist regarding its nature. It appears to consist of a parietal reticulum which lies in the cytoplasm, though additional chloroplast segments may develop inwards into the central vacuole. The meshes of this reticulum cover the longitudinal and transverse walls of the coenocyte and vary much in width. There are numerous pyrenoids scattered in it which can FI multiply by division. In unfavourable conditions the chlorophyll may contract around the pyrenoids in more or less spherical masses, thus giving the impression of separate discoid chloroplasts, which may explain the view expressed by some workers that the chloroplast is not a single reticulate structure but a number of small connected, discoid bodies. Fragmentation of the chloroplast has been observed not infrequently.
The wall of the coenocyte is composed of a series of thick, stratified membranes, which consist of an inner and an outer lamella, with a superficial pellicle which can be separated by treatment with acetic acid. Both lamellae show stratification which usually runs obliquely to the long axis of the coenocyte. There is little or no mucilage, which probably explains the great abundance of epiphytes which normally occur on the plants.
Branches usually arise at the upper end of a coenocyte and at first form a wide angle with the parent branch, for the septum cutting off the branch is developed nearly perpendicular to it. Sometimes two branches arise together, one on either side of the main axis.
Growth is by means of the apical segment, which, after elongating sufficiently, forms a transverse septum cutting off the greater part of the segment. At the beginning of such a division the protoplast is withdrawn from the region where the new septum will develop, the space so left being apparently filled with mucilaginous material. An annular bar of thickening is laid down here, extending transversely all round the cell. This forms the beginning of the cross wall which then gradually extends inwards towards the centre. At the same time it extends into and fuses with the inner lamella of . the longitudinal wall. The formation of septa is in no way related to nuclear division.
The attaching organs consist of a number of septate, rhizoidal branches. These may sometimes spread out as stolons which either give rise to new upright filaments or their tips may grow out into a number of short cells, which later fall apart and serve as a method of propagation. The short cells formed on the rhizoids have thick walls and are abundantly supplied with food reserves. After separation they may undergo further wall thickening and function as over-wintering organs. Many species are, however, free-floating, as can be seen in the floating tangles (" flannel weed") which are so characteristic of ponds. In the free-floating species smaller or larger parts of the vegetative branches may separate, develop thick walls, and being well supplied with food reserves may function, in some species, as the only means of reproduction.
Asexual Reproduction in Cladophora
The zoospores are small pyriform bodies with a small anterior beak and two apical flagella, or in some species four. These flagella are differentiated before the emergence of the spores. There are two granules at the point where the flagella are inserted and the chloroplast appears as a ring in the posterior part of the cell and shows a slow streaming movement. Those zoospores nearest the orifice escape first, squeezing their way through the opening with their flagella behind them, and they are followed by a steady stream of others. After about twenty minutes they settle down by their anterior ends and elongate. A septum appears and gradually the coenocyte structure of the thallus is re-established.
Sexual Reproduction in Cladophora
The formation of gametes occurs in most species and probably in all. In some species it has been definitely established that the thallii are dioecious. Any segment of the thallus is potentially capable of functioning as a gametangium, and its development is similar to that of the zoosporangium. The gametes are isogamous and differ in no marked way from the zoospores, except that they are invariably biflagellate. They are liberated in a similar manner to the zoospores, and fuse externally to give rise to a zygote which germinates immediately to form a fresh Cladophora filament.
Alteration of Generation
It has now been definitely established that in almost all species there is a strict alternation of sexual and asexual generations. These generations are morphologically alike. Meiosis occurs prior to the formation of the zoospores which therefore give rise to male and female monoploid plants, the diploid phase being established after gametic fusion, resulting in the formation of a sporophytic plant, on which the zoospores are produced. A similar type of alternation has been found in a small number of related genera.
In Cladophoraglomerata it has been shown recently that zoospore formation occurs at intervals all through the year, while gamete formation happens only in the spring and at the end of a long series of zoospore discharges. Gametes and zoospores are developed in distinct plants, but there is no reduction division in the formation of the zoospores. It appears that this species can produce a succession of diploid zoospore generations, after which meiosis occurs during the formation of the gametes. Conjugation restores the diploid condition. This condition resembles that which has been described in some members of the Siphonales.