Phyllum Thallophyta - The Algae - Rhodophycae - Cryptonemiales - Corallina oftidnalis
The Cryptonemiales are Rhodophyceae in which there are definite unions formed between the auxiliary cells and the cells of the vegetative thallus, which not only serve for nourishment but also form the starting points of the gonimoblast filaments. The result is, in many genera, that an elaborate carposporophyte tissue develops within the tissues of the gametophyte, and the carpospores which arise from the gonimoblasts may be located at positions remote from the procarp branch. The auxiliary cells develop on special filaments before fertilization and are actively associated with the post-fertilization changes.
The plants show a diplobiontic alternation of generations and have the most elaborate carposporophytes found in the Rhodophyceae, though the type Corallina, which we shall consider in detail, is not the most highly developed genus in this respect.
Many of the Cryptonemiales have their cell walls impregnated with lime.
In some the original filamentous character of the thallus is retained, but in others the whole thallus becomes so encrusted in lime that the plant appears to be petrified and forms an amorphous mass resembling coral. Such calcareous Algae do indeed contribute appreciably to the building up of coral reefs. The genus Lithothamnion is particularly important in this respect.
As a type of this Order we shall consider the common calcareous Alga, Corallino officinalis.
Corallina oftidnalis
This species occurs very commonly on British shores between tide marks.
It is found especially in rock pools, where its small, reddish, jointed tufts are very characteristic. The colour of the plants varies according to the depth of water in which it is growing. In very deep water it is dull purple, becoming pink and finally white with increasing exposure. The plants are small, rarely more than 9 em. high, with a basal disc from which the pinnately branched segmented axes arise. These axes are made up of a series of articulated, multifilamentous segments and are densely encrusted with lime. The segments at the base of the main branches are about as long as broad, but are narrower in the upper parts.
The plants are dioecious, the male and female plants being morphologically alike. As a result of fertilization a tetrasporic plant is developed wruch again is identical in form with the sexual plants. The reproductive organs are developed in conceptacles, those bearing the male organs being pearshaped, while those producing the female and the asexual reproductive organs are both ovoid.
STRUCTURE OF THE THALLUS
The structure of the vegetative thallus is very elaborate and is an example of the fountain type of construction. The central part of the thallus is composed of parallel rows of very narrow elongated cells, with numerous lateral branches composed of small cells which are compacted to form a cortex. These cortical cells are filled with dense cytoplasm and chromoplasts, while the cells of the central region are only sparsely provided with cytoplasm.
The development of a new segment is brought about by the elongation of the axial cells at the apex of an existing segment to form three outgrowths, each consisting of a bundle of narrow cells similar to and continuous ,vith those of the central tissue. These elongated cells divide transversely, cutting off a group of long basal cells which thicken up to form the joint between series of smaller distal cells which cut off cortex. No cortex is formed on the joint
the segments and a laterally cells to form the new cells.
Growth may therefore be said to be apical, and the cells at the apices of the terminal segments are thin-walled and delicate. It is only further back that the encrustation of lime forms by deposition on the cortical cells. As there is no cortex at the joints they remain uncalcified. This deposit is due to the abstraction of Carbon dioxide from solution in the sea \vater during photosynthesis, which leads to the dissociation of the Calcium bicarbonate in solution and the deposition of insoluble Calcium carbonate on the surface of the plant. Non-calcified Algae apparently escape encrustation by the continuous sloughing of mucilage from the surface.
As a general rule there are three growing points on each terminal segment; the central one, which continues the growth of the shoot, producing fresh segments, and one formed on each side, which may develop into reproductive organs or may give rise to tvvo lateral branches.
SEXUAL REPRODUCTION
Whether the conceptacle is destined to contain antheridia, carpogonia or even tetrasporangia, its develop-
ment is similar, though as we have seen the final shape is somewhat different. Development begins at the apex of a branch. A group of cells at the apex of the growing segment are richly supplied with cytoplasm. These cells are termed the disc cells, and those lying at the periphery continue to divide and grow up around the
reproductive organs, which develop at the centre of the dIsc, leavmg only a small ostiole at the apex and enclosing these organs in a well-formed conceptac1e.
THE ANTHERIDIUM
In the formation of the antheridia each disc cell divides into two unequal parts by a wall which cuts off a small terminal cell, which is the antheridial mother cell. From this, antheridia arise in twos or threes, forming a close layer over the base of the conceptacle. These antheridial
cells now elongate, and their cytoplasm and nuclei migrate to their upper ends leaving the basal part as a long thin tail. These structures become detached from the mother cells and apparently function as spermatia, but their exact nature is uncertain.
The spermatium has a thin cellulose wall and may therefore be regarded as an antherium containing one antherozoid, the whole being shed at he same time.
THE CARPOGONIUM
Each disc cell produces a separate procarp branch. The cell first divides into two to form an upper cell, which becomes the auxiliary cell, and a lower one, which is the stalk cell. Then the auxiliary cell gives rise to two sister cells, which are produced side by side. They are not, however, produced simultaneously, and one has enlarged considerably before the other is produced. The older one enlarges and becomes the carpogonium, whose distal end becomes very greatly elongated to form a trichogyne, which protrudes through the ostiole of the conceptacle. Meanwhile the nucleus of this cell has divided into two. One remains at the base in the carpogonium, while the other migrates into the trichogyne. The sister cell soon ceases to grow and remains as a small non-functional structure beside the carpogonium. Since all the disc cells within the conceptacle form procarp branches it follows that a large number of separate carpogonia will be formed and that many trichogynes will protrude through the ostiole.
In this position they are exposed to the sea water and readily catch any spermatia which come into contact with them. Though the point has not yet been definitely proved it is assumed that after liberation from the male conceptacle the spermatium loses its wall, and at the time it comes into contact with the trichogyne it is a naked spherical protoplasmic mass. The nucleus in the trichogyne disorganizes, and the male nucleus migrates down the trichogyne and fuses with the carpogonial nucleus.
At this stage the auxiliary cell of this procarp branch fuses with the auxiliary cells of the neighbouring procarp branches until finally a single large central cell is produced from the union of auxiliary cells. The diploid zygote nucleus now migrates into the central cell, which thus contains many monoploid nuclei from the auxiliary cells which have contributed to its formation
and the diploid nuclei from the fertilized carpogonia. Since many trichogynes may be involved and many simultaneous fusions have occurred there may be as many as a hundred diploid nuclei from different carpogonia.
The monoploid nuclei disintegrate, while the diploid nuclei migrate to the periphery of the central cell, where each divides into two. A lobe appears on the central cell at each of these points, and one daughter nucleus enters it and the tip of the lobe is cut off as a cell. The other nucleus remains in the central cell. This process may be repeated so that chains of cells are cut off in basipetal succession, the number of chains being ultimately dependent on the number of carpogonia which were fertilized. These cells enlarge and become spherical, and are finally constricted off as separate carpospores. They separate and escape through the ostiole of the female conceptacle.
The carp os pores germinate within twenty-four hours and give rise to new plants identical in structure 'vvith the parents, but the nuclei contain the diploid instead of the monoploid chromosome number. These are the plants which will ultimately produce tetraspores and are therefore sporophytic plants.
ASEXUAL REPRODUCTION
The disc cells of the developing asexual conceptacles divide into two, the lower half forming a stalk cell, while the upper becomes the tetraspore mother cell. This latter cell grows and assumes a clavate form. Its nucleus enlarges and undergoes two divisions during which meiosis occurs. This reduction division takes place at about the time when the conceptacle has been completely developed. Wall formation follows meiosis. These walls are laid down one above the other so that a row of four tetras pores is produced. Such a method of septation is said to be zonate.
The tetraspores are liberated through the ostiole of the conceptacle and float about freely in the water. They become attached to a suitable substratum, such as a rock face, and germinate, giving rise to sexual plants again.
The life-cycle of Corallina therefore shows an alternation of generations between gametophytes, carposporophytes and tetrasporophytes as in Polysiphonia, Chondrus and Ceramium, though there is only a relatively simple carposporophyte, compared with other members of the Cryptonemiales.
ALTERNATION OF GENERATIONS IN THE RHODOPHYCEAE
In the Algae the life history may include only one type of vegetative plant, or there may be two separate kinds of vegetative plants, the one producing gametes and the other producing non-sexual spores or zoospores. In the latter case the plants may be morphologically identical or they may be different in form, sometimes so much so that they were once considered to belong to distinct genera, as in Cutleria.
A life-history of the first category, in which there is only one type of vegetative plant, is said to be haplobiontic. Examples of this are Chlamydomonas, Spirogyra, Fucus and Batrachospermum. A life history of the second category which includes two vegetatiye plants, whether they are alike or not, is called diplobiontic. Examples of this are Cladophora: Coleochaete, Dictyota, Laminaria, Cutleria and Polysiphonia.
In a diplobiontic life-history, if the two plants are morphologically identical, the alternation between them is said to be homomorphic. This applies to Cladophom, Dictyota and Tetraspor- Polysiphonia.
angium In the opposite case, where the plants are dissimilar, the alternation is called heteromorphic. This applies to Coleochaete, Laminaria and Cutle1'ia among those mentioned above.
The use of these terms is quite independent of the cytological life-cycle, and it matters not whether the plants concerned are monoploid or diploid.
For example, as we have seen above, the life-histories of both Bat1'achospermum and Fucus are haplobiontic, but the vegetative phase of the former is monoploid, while that of the latter is diploid. On the other hand in a diplobiontic life-cycle it is inevitable that one vegetative plant of the cycle must be monoploid while the other must be diploid.
It follows therefore that, in considering the type of alternation of generations exhibited by an Alga; the criterion is not whether the plant is monoploid or diploid but \yhether one or two separate plants are necessary to complete the life-cycle.
A plant in which the vegetative phase is monoploid is termed a monoplont or haplont, while one in which the vegetative phase is diploid is termed a diplont. Further, if there are two vegetative phases, \yhether similar or dissimilar, the species is called haplo-diplont. These terms refer solely to the cytological condition and are not dependent upon the type of alternation of generations exhibited by the species.
The Rhodophyceae present a further and peculiar condition, brought about by the interpolation of a special post-fertilization carposporophyte tissue which produces the carpospores. This tissue may be quite small or it may be extensive, but it is always produced in organic connection with the gametophyte. In forms like Batrachospermum the carposporophyte is the only diploid structure, and thus we have an alternation of two generations, the monoploid gametophyte and the diploid carposporophyte, \yhich are never separated from each other. In such a life-cycle meiosis occurs in the formation of the carpospores. In types like Polysiphonia, Chondrus, Ceramium and Corallina the carpospores give rise to independent diploid tetrasporic plants, so that here we have in effect an alternation of three generations, two of which, namely, the gametophyte and tetrasporophyte, are quite independent. In this case the gametophytic plants are monoploid, while both the carposporophytic and tetrasporic plants are diploid. Meiosis, howeyer, does not occur in the carpospores, but is postponed until the formation of the tetraspores.
