Is spirogyra unicellular or multicellular

The color of the water (salt water) of the lagoon (on the Arabian Sea) is based on the massive occurrence of a volvocal, two-whitewashed green alga. The red color is due to the formation of a carotenoid. In culture, the algae lose this ability and then look completely normal green.

The green algae seem to be closer to the other green plants than to the other divisions of the algae. Both groups have the same photosynthetic pigments (chlorophyll a and b), the same set of carotenoids (alpha-, beta- and gamma-Carotene, lutein, zeaxanthin, violaxanthin etc.), the same reserve substance (starch) and the same structural substance of the cell wall: cellulose. It was therefore natural to examine the Chlorophyta with all available analytical methods in order to find out more about their phylogenetic background; especially about their relationship with land plants. Many of the previous efforts have failed, but in recent years, thanks to ultrastructural studies, progress has been made in this area as well. Increasing importance was attached to the structure of the flagellated cells. The analysis of the cytoskeleton and the musculoskeletal system proved to be particularly informative. It was found that the components of the flagellar apparatus are very different in structure and arrangement and that they are characteristic of certain phylogenetic lineages.

The same is true of the structures associated with mitosis and cytokinesis. During mitosis (higher organisms) the nuclear membrane dissolves. However, in a number of green algae (and many protists), the nuclear envelope remains intact to varying degrees during cell division; chromosome-like aggregates appear within the membrane-enclosed volume. The microtubules of the telophase can persist for different lengths of time.

After all, there are many different forms of cell constriction and transverse wall formation. While a phragmoplast is formed during cell division in land plants, in which the microtubules are organized perpendicular to the plane of division, a "phycoplast" was initially found in a number of green algae, which consists of microtubules arranged parallel to the equatorial plane. A whole series (approx. 8) different types of mitosis and cell division are now known. The phragmoplast is also among them and of course gives rise to speculation about the origin of the land plants.

Phycoplast and phragmoplast in various green algae. Fritschiella; Chlamydomonas; Coleochaete; D: Klebsormidium. In phycoplasts, the microtubules are parallel, in phragmoplasts perpendicular to the plane of division (according to P. H. RAVEN, R. F. EVERT, H. CURTIS, 1981; and G. L. FLOYD).

Due to DNA sequencing of selected genes (especially those of the ribosomal subunits), new phylogenetic relationships have recently emerged, which have led to a considerable change in the conventional system of green algae. However, since there are still numerous contradictions to be clarified, it is too early to present a new, binding system today. For pragmatic reasons, the Chlorophyta can be assigned to five classes, taking into account the mentioned and other ultrastructural criteria:

The classes mentioned are in part very heterogeneous groupings. There are therefore still numerous discrepancies regarding the assignment of individual genera to higher taxa or the relationships between these taxa (orders, classes). In the past one tried again and again to deduce relationships based on morphological similarities. Today, for example, you can only describe convergent groups within the "organizational levels" defined by PASCHER. The "stages" are repeated in parallel in different taxa. Probably the same applies here as was said for angiosperms: The genome contains more genetic information than is required for the existence of individuals (species). Through restructuring, therefore, similar characteristics can appear in the most varied of phylogenetically unrelated groups. There are various indications for the likelihood of this assumption.

The majority of the Chlorophyta are always or temporarily flagellated (exceptions include the Zygnematales). There are two or more equally long (isocontal) flagella present. Flagellated stages (vegetative cells, zoospores, gametes) move phototactically, many species have a stigma ("eye spot") that is usually located in a chloroplast. Eye spots also occur in non-flagellated species, e.g. in Spirotaenia condensata in front. As already explained elsewhere, the green algae usually have many and differently structured chloroplasts

Green algae are widespread. Most species are found in fresh water, others in salt and / or brackish water, in the ground, on tree trunks or as symbionts of protozoa (Paramaecium bursaria), Coelenterates (Hydra) and mushrooms (lichens: lichenes). The majority of the non-aquatic species are unicellular. Only a few - volvocale - species (see cover picture) reach such high numbers of individuals locally as the Cyanophyta, Euglenophyta or Bacillariophyceae.

They are therefore only rarely involved in water blooms. Some filamentous species (e.g. Spirogyra, Mougeotia, Enteromorpha- and Cladophora-Species), where they occur in masses, form so-called wadding, which consists of a multitude of intertwined threads. Such aggregates, mostly floating on the surface, are a characteristic of strongly eutrophic waters.

In some groups of green algae, specific, bizarre-looking cell shapes occur. They occur mainly in protozoa and are particularly noticeable in the Desmidiaceae, a Zygnematales family. Many planktonic living species (Chlorococcales) are provided with floating appendages. Most species are surrounded by more or less voluminous jellies of different chemical composition and consistency, which can serve to hold individuals together, to attach to the substrate, to exchange substances and to move. They form a preferred habitat for many bacteria. Chlorophyta cells are usually mononuclear and haploid, rarely multinucleated (polyenergid). The gametes that appear during sexual reproduction are often isogamous; Anisogamy as a derived phenomenon occurs, as does oogamy (the latter in primitive and specialized genera: Chlamydomonas,Oedogonium. Chara, etc.).

There is no unanimous opinion about the number of orders. The following list shows the most important, and thus the most clearly delimited:

The Volvocales contain unicellular and colony-forming species. Usually the cells are flagellated. However, under certain conditions, some species can temporarily pass into an uncultivated "Palmella" stage. The cells of the volvocales are usually surrounded by a multilayered wall containing proteins and carbohydrates.

The location and shape of the chloroplast is species-specific. Typical is the cup-shaped chloroplast of Chlamydomonascontaining a large pyrenoid that surrounds the nucleus. The stigma containing carotenoids is located in the front area of ​​the chloroplast, just below the plastid envelope and plasmalemma. The actual photoreceptor, however, is formed by the peripheral membranes.

By Haematococcus pluvialis colored water in a puddle (photo: K. HANDKE), Volvox aureus

The centrally located, projected chloroplast of Stephanosphaera. Among the Volvocales there are often forms with an increased carotenoid content: Chlamydomonas nivalis causes snow to turn red ("blood"), Haematococcus pluvialis a red discoloration of rain puddles. In culture, various species can turn red due to a lack of nitrogen (formation of secondary carotenoids).

The anterior isocontal flagella penetrate a typical papillae. They strike synchronously according to the "oar stroke principle": stretched out backwards, with a kink running from the base to the tip, this prevents the cells from drifting back again when the stroke is forward. At the apical pole there are also the pulsating vacuoles that fill and empty alternately.

In vegetative reproduction, the daughter cells are separated from the anterior to the posterior pole. Several divisions can follow one another within the mother cell wall, resulting in a reorientation of the axes. After flagellation, the daughter cells are released through enzymatic breakdown of the mother cell wall.

The always unicellular species from the genus Chlamydomonas belong to the much-used objects of basic plant research. Particular attention was paid to the compatibility types (= mating types) of otherwise isogamous species (+ and - strains). It should be noted that it is in the genus Chlamydomonas in addition to isogamy, there are also heterogamy and oogamy.

The molecules involved in the specificity of the mating reaction (agglutination of the flagella of two gametes ready to mate) include glycoconjugates alpha-Glycosidically bound mannose residues (L. WIESE and W. WIESE, 1975).

Species of the genus Carteria own in contrast to those of Chlamydomonas four instead of two flagella.

The colonies of Volvocales are mostly coenobia, i.e. they always contain (species-specific) a characteristic number of cells. This is determined during ontogenesis and can no longer grow after differentiation. All cells are always the direct descendants of a mother cell. Many classic objects of the botany study belong to this group: Gonium, Pandorina, Eudorina, Volvox, Stephanosphaera i.a.

The genus Gonium is the easiest to build. Gonium sacculiferum consists of four chlamydomonas-like cells that are surrounded by a common jelly. The coenobia of Pectoral gonium usually contain 16 cells, those of other types 8, 16 or 32.

Pandorina is the best known example of coenobia formation. Here, too, a colony usually contains 16 (-32) cells, and each of them produces a colony with 16 (-32) cells, etc. The genus name comes from Greek mythology. The messenger of the gods Pandora spread calamity, and every calamity was followed by another calamity.

We find a similar organization at Eudorina. Both genera differ due to the arrangement and the number of cells in the Coenobium. Pandorina- cells form a compact cluster,

Eudorina-Cells are arranged in the form of a hollow sphere.

Volvox forms huge colonies in which 1,000 to 50,000 cells can be united in a hollow spherical shape. The colony diameter is up to two millimeters. A. van LEEUVENHOEK saw her for the first time in January 1700. VolvoxColonies show a clear differentiation and division of labor.

The reproductive area is increasingly restricted to cells of the posterior pole. The anterior cells often have longer flagella and larger spiracles. All cells react phototactically, their movements are synchronized by the incidence of light. There is therefore a preferred direction of movement for the colony. An exchange of information takes place between the cells. The visible expression of this is a symmetrically constructed network of plasma bridges through which the cells are connected to one another.

One could therefore speak of a real multicellular organism. But the way in which it was formed is not a model for understanding the evolution of the other multicellular organisms. Their creation was different: easier and more efficient.

In order to reproduce, reproductive cells reach the interior of the colony, where they develop into daughter colonies through numerous successive divisions according to a characteristic pattern. The flagella of the daughter colony cells are initially directed towards the interior of the colony. Through an "inversion" process they get the right orientation. The release of the daughter colonies takes place after the protective covering of the mother colony has ruptured. The mode of sexual reproduction is oogamy.

SOME REMARKS ABOUT TETRASPORALES: Some authors raise this taxon to a separate order, others add it to the Volvocales. A characteristic of many species is the arrangement of the cells in groups of four within a common envelope. Tetrasporales certainly do not represent a natural group, because they are primarily a reservoir for species at a certain organizational level.

The Chlorococcales are characterized by the fact that they are usually not scourged and have solid cell walls. In this state there are also no pulsating vacuoles and spirals. Gametes and zoospores, however, have flagella and resemble the reproductive stages of the volvocales. Some species (e.g. Eremosphaera) do not form zoospores, but rather daughter cells within the mother cell wall that resemble the mother cell (autospores).

There are single-celled and colony-forming, mostly planktonic species. Multicellular species often consist of a fixed number of cells in a typical arrangement (Scenedesmus, pediastrum).

These cell associations develop from daughter cells that separate from one another (as "hemizo-spores", which are flagellated, or as non-flagellated "spores" that do not leave the mother cell wall at first) and then arrange themselves in a typical way (aggregation association). These associations grow, then rupture the mother cell wall and sometimes emerge in a specifically shaped bladder.

The cell wall of the Chlorococcales usually consists of a cellulose framework in which many other molecules are embedded. In many species it is multilayered, for example one of the layers consists of the Chlorella- and Scenedesmus-Wall made of polymers from unsaturated hydrocarbon chains, which can be derived from fatty acid biosynthesis. Structural substances are often deposited on the walls. Sexual reproduction is rare, and when it does, the gametes are isogamous. In many genera, e.g. Chlorella, gamete formation was never seen.

The most famous representative of the Chlorococcales is Chlorella. Species of this genus are standard objects of photosynthesis research. They are common in fresh and salt water as well as in the soil. The cells contain a single, remarkably complex structured mitochondrion.

To the relatives of Chlorella belongs Prototheca with their always colorless plastids. Chlorococcum resembles Chlorella morphologically, but forms mobile zoospores instead of autospores. Most species of this genus reproduce exclusively asexually, in one species isolated in the Philippines (Chlorococcum echinozygotum) isogamy has been proven. Species of the genus Trebouxia often live in symbiosis with fungi (lichens).

Other widespread genera are Oocystis and Eremosphaera. The cells of many planktonic living species (genera) are provided with floating extensions.

Most of the Chlorococcales colonies can be considered coenobiae. We find typical examples of this in the genera Scenedesmus, Ankistrodesmus and Pediastrum. Scenedesmus-Coenobia usually consist of four, more rarely eight (or 16) cells arranged next to one another in a row. The ones on the outside have floating extensions. Isogamy was at Scenedesmus obliquus proven. At Ankistrodesm the cells, which taper off at the ends, are organized in groups of four.

Pediastrum forms two-dimensional coenobia, which, depending on the species or culture conditions, consist of 8, 16, 32 or 64 symmetrically arranged cells. During the propagation phase (as in Pandorina) a new colony from each cell. During sexual reproduction, wallless gametes are formed which, after mating, zygote formation and reduction division, grow into a new colony through multiple successive divisions. Fossil became Pediastrum detected in the Permian and Triassic.

One of the most striking Chlorococcales is the water network Hydrodictyon. It consists of elongated cells that are connected to one another at their ends and form a polygonal, self-contained network that can reach a diameter of up to about one meter. Such water networks occurring in stagnant or slowly flowing waters were already known to the ancient Chinese, so that one can say Hydrodictyon be the first described alga. The cells contain a reticular, wall-like chloroplast. Young cells are mononuclear, older cells are usually multinucleated. Normally, the zoospores formed in a cell are not released, rather they combine with one another (losing their flagella and forming a wall), so that a new network is created within one cell. During sexual reproduction (isogamy) a polyhedral, initially mononuclear, later multinuclear intermediate stage is formed.

This order must be narrowed down on the basis of new findings; it overlapped in older systems with the Ulotrichales. Species with filamentous, partly branched thallus are placed in this, possibly not natural, order. Some examples of Chaetophorales: Uronema, Chaetophora, Microthamnion, Aphanochaete, Draparnaldia, Fritschiella, Stigeoclonium.

The zoospores have a jelly at the whip pole that marks a polarity of the cell. During the formation of cell aggregates (after the flagella has been shed), this pole is always oriented towards the center. In most of the species, the cell threads are lengthened by dividing the terminal (= distal) cells. Draparnaldia plumosa has morphologically different main and secondary shoots (= thick and thin filaments). The cells of the secondary shoots have a carbohydrate-containing coating on their surfaces, which is believed to serve as a food source for bacteria. In contrast to thick filaments, thin filaments are always surrounded by them.

Fritschiella is a soil algae, but also occurs in aquatic environments. Depending on the habitat, it changes its growth form. Ground-dwelling species form long threads from which branch shoots protruding into the air space.

The single-row, mostly unbranched cell threads of the Oedogoniales are extremely polar. This is already expressed with every intercalary cell division and by the fact that the threads contain a basal rhizoid cell and apical tip cells. The Oedogoniales live in fresh water, they are mostly stuck multicellular cells. Your thallus is filamentous and unbranched. The main representative is the genus Oedogonium. Their cells contain a reticulated, walled chloroplast, they are mononuclear. They reproduce asexually through zoospores, sexually through oogamy. The special thing about Oedogonium is the extraordinary mode of division of the cells. Nuclear division and cell division are synchronized in time, but spatially separated from each other. As with most plant cells, the nucleus division takes place near the center of the cell. During the telophase, a septum, a preliminary stage of the later transverse wall, is created between the two daughter nuclei, which is initially without contact with the longitudinal wall. While mitosis is going on, a bead of plastic (cellulose-free) wall material forms near the apical pole - the longitudinal wall is superimposed in a ring on the inside.

Following mitosis, the upper daughter nucleus and the septum are displaced towards the apical. Then the cell wall tears open in a ring at a preformed point outside the bulge. At its lower end it grows together with the septum. Through the deposition of additional cell wall material (cellulose), the septum and bulge differentiate into finished transverse and longitudinal walls of the new daughter cell. The remnants of the mother cell wall remaining above the predetermined breaking point form a "cap", which is the typical feature of this mode of cell division. This process can be repeated several times at the apical pole of all intercalary cells, so that some cells have a whole series of these caps.

The species are diocesan, some are characterized by a pronounced sexual dimorphism; then the male plants are reduced to "dwarf males".

The Ulvophyceae are primarily marine organisms. Zoospores, if any, are built symmetrically. They have two, four or more flagella. A class feature is the clearly pronounced generation change with haploid gametophytes and diploid sporophytes. Permanent spores are only formed in exceptional cases. The Ulvophyceae belong to the following orders:

The "Ulotrichales" formerly classified as order have proven to be heterogeneous. The Ulothrix similar genera Uronema and Klebsormidium belong to other classes. The Codiolales are more precisely defined. Characteristic of their life cycle is the large, photosynthetic zygote, the "Codiolum"Stage that can grow for a while. Meiosis takes place in him; bifurcated zoospores become free, so that the stages that develop from them are haploid.

The order includes single-celled and multicellular species whose thallus consists of unbranched or branched threads or is leaf-shaped (flat). Almost all species live benthically. They are anchored to substrates by rhizoid cells that cannot divide. Most species and genera (e.g. Acrosiphonia) are marine, few live in fresh water.

A progression from isogamy to oogamy can be seen in some, but not all, tribal lines (J.D. PICKETT-HEAPS, University of Colorado, Boulter, 1975).

What is striking is the genre that is now to be brought into this order Ulothrixof which the species Ulothrix zonata is best known and most common (in freshwater). Your thallus consists of unbranched threads (= + and - gametophyte). Their growth is intercalary, and four-flagellated zoospores are formed during sexual reproduction. Isogamets are bifurcated. They only arise under long-day conditions. The zygote, on the other hand, only germinates under short-day conditions. After meiosis, it breaks down into four to eight zoospores, from which half + and half - gametophytes grow. The marine kind Ulothrix acorhiza reproduces exclusively asexually. The genus Monostroma is single-layer and leads morphologically to the next order.

Ulva lactuca (Sea salad) and others UlvaSpecies are characterized by a flat thallus consisting of two cell layers. The species is a textbook example of an isomorphic heterophasic generation change. The gametes are sexually separated. At Enteromorpha the thallus is tubular, at Prasiola stipitata again flat. The species occur in fresh, brackish and salt water.

This order has a number of independent characteristics, which differ from the orders listed above as well as from the following classes, so that the separation into a new class ("Cladophorophyceae") may appear justified. This includes multicellular species with filamentous and (mostly) branched thallus. The striking feature is the multinucleated nature of the cells, because nucleus and cell division are not synchronized here. The threads elongate through tip growth, i.e. through division of apically located parietal cells. The cell wall is rich in protein and contains a modified form of cellulose (cellulose I) as a structural component. The microfibrils in superimposed layers are each offset by 90 degrees, creating a microscopically clearly visible layering. The pyrenoid-containing chloroplasts are interconnected to form a peripheral network. The haploid and diploid phases, if any, are isomorphic and rarely heteromorphic. The best known genus is Cladophora. In the marine area is Cladophora vagabunda predominant in fresh water Cladophora glomerata. The latter reproduces exclusively vegetatively.

In this class all algae of the "siphonal organization level" are summarized (without Dasycladophyceae).

This subheading includes unicellular species with a characteristically shaped (siphonal), multiply branched, multinucleated thallus. Apart from a few species (Bryopsis-, Derbesia and CodiumSpecies) they can only be found in tropical and sub-tropical zones. Some species are characterized by xanthophylls that only occur here: siphonein and siphonoxanthin.

At Bryopsis the thallus is regularly single or multiple single-row branched to pinnate; it can grow up to 10 centimeters in most species, in a species that occurs off the Japanese coast Bryopsis maximum he reached a size of 40 centimeters. In the dioecious species Bryopsis plumosa male and female gametophytes are separate; the male gametes are smaller than the female (heterogamy, anisogamy). The zygote grows into a (single-celled) branched thread. The type and timing of meiosis is still unclear. A "giant nucleus" appears as an intermediate stage, which breaks down into a multitude of small nuclei (meiotic?, Mitotic?). The gametophyte either develops directly from the sporophyte or a zoosporangian stage is interposed. The closely related species Bryopsis hypnoides is monoecious. Cellulose and xylan occur in the gametophyte cell wall, while the sporophyte wall consists primarily of mannan.

The sporophyte of Derbesia marina resembles Bryopsis. It is siphonal, branched, single-row. Occasionally, older sections of the thallus are separated by transverse walls. The associated gametophyte is designed quite differently and under the name Halicystis ovalis known. The approximately 1 cm large vesicles are spherical and sit firmly with rhizoid-like branches. Again, we are dealing with a typical example of a heteromorphic, heterophasic generation change. The gametes are of different sizes (anisogamous). As in Bryopsis the thalli of both phases differ in their cell wall composition.

Some species occur in the North Sea. The macroscopically conspicuous, several decimeters large, often forked or spherical thalli of this order consist of multi-branched siphonal cell threads that are woven into a network. At Codium the surface consists of a layer of club-shaped protuberances which, like palisade cells in the leaves of higher plants, are rich in plastids. At the base they sometimes develop the smaller, club-shaped gametangia in which meiosis is supposed to take place. The wall consists of one beta-1,4 -manner structure, which is incorporated in arabinogalactan.

At Caulerpa On the one hand, rhizoid bundles sit on long runners, on the other hand, leaf-like, upright parts arise from there. These can be up to several decimeters in size. The strength of the structure is increased by inserted crossbeams made of wall material, their framework substance beta-1,4-xylan is what beta-1,3 -glucan is associated. It is noteworthy that leukoplasts also occur here in addition to chloroplasts. The plants are diploid gametophytes.

The siphonal structure of the thalli, the chemical composition of the cell walls, but above all the differences between the gametophyte wall and that of the cysts (sporophyte wall) speak for the close relationship with the Bryopsidophyceae. In contrast to the Caulerpales, the cells are mononuclear for a long time. The thallus is built with radial symmetry. Reproduction takes place via cysts, from which isogametes are released.

Belongs to the Dasycladiales Acetabularia, a standard object of basic biological research. This genus is represented with several species on the sea coasts of warmer zones. Acetabularia mediterranea is common in the Mediterranean, Acetabularia crenulata in the Caribbean. The thallus is usually calcified in natural locations. In the laboratory, on the other hand, culture conditions are selected that prevent calcification. A cell is divided into the three sections rhizoid, stem and cap.

The cell nucleus (primary nucleus) lies in the rhizoid. It contains, among other things, lamp brush chromosomes (H. SPRING et al., 1975) and is believed to be polyploid. Before cyst and gamete formation begins, it migrates through the stem in the direction of the hat with meiotic and mitotic division into 10-15,000 secondary nuclei. The products created along the way are distributed over its septa.

Zygnematales, also called conjugates, are characterized by a variant of their reproductive behavior, conjugation. Flagellated gametes are never formed. The protoplasts of the conjugation partners temporarily assume an amoeboid shape. The zygote resulting from the gamete fusion (fusion) surrounds itself with a thick wall and is usually to be regarded as a permanent stage (hypnozygote). After a long period of maturation, often in an annual cycle, haploid cells are released from which, depending on the species, unicellular or filamentous gametophytes develop. The Zygnematales are usually divided into three families: Zygnemataceae, Mesotaeniaceae, Closteriaceae and Desmidiaceae.

The Zygnemataceae include the filamentous ones that are common in freshwater Spirogyra, Mougeotia and ZygnemaSpecies. The genera differ in the structure of their chloroplasts. Spirogyra contains a helical, at Mougeotia it is plate-shaped and rotatable about its longitudinal axis. Under strong light conditions it orientates itself with the edge side to the light source, under weak light conditions with the broad side. Zygnema- and ZygnemopsisSpecies have star-shaped chloroplasts. To initiate the conjugation, bridges are formed between cells of adjacent threads, through which the protoplast of the cell of one thread changes over to the neighboring one.

The Mesotaeniaceae are unicellular. The cells are surrounded by a uniform, self-contained cell wall. Asexual reproduction occurs through the transverse division of the mostly elongated cells. Before conjugation, each of the cells involved in the pairing forms protuberances (papillae) that fuse to form a common bridge-like connection within which the protoplasts fuse and form a zygote. The best known genera are Mesotaenium, Spirotaenia, Cylindrocystis, Netrium. Closely related to the Mesotaeniaceae are the Closteriaceae.

The Desmidiaceae differ from the other two families primarily in the mechanism of their vegetative reproduction and the resulting peculiarities in the architecture of the cell wall. The cells consist of two half-cells, one of which is younger than the other. Between them lies the isthmus, narrowed by a more or less deep constriction (= sinus). The wall is not continuous, rather it consists of two overlapping half-shells.

After the actual cell division through an iris diaphragm retracted septum in the isthmus, each half-cell everts a new daughter half-cell. As these grow and differentiate, the two old half-cells move apart. The daughter cells separate after they have almost reached their full size. The stages of development have been studied in a number of species and documented by impressive series of images.

Growing daughter half-cells are surrounded by a primary wall, which is replaced in the course of further growth by a secondary wall placed underneath. After its completion, the primary wall is either knocked off (as e.g. with Cosmarium), or it dissolves successively (e.g. with Cosmocladium saxonicum). In the latter type, different glycoconjugates are exposed on the surfaces of the primary and secondary walls. In addition to special movement jellies, various conjugation jellies (copulation jellies) have been detected that form during papilla formation, i.e. during early stages of the conjugation process.

The walls of the Desmidiacea are often reinforced with pads, decorated with ornaments and broken up by regularly arranged pores. These contain specifically structured gelatinous plugs (pore complexes), which can already be identified by light microscopy by relevant colorations (e.g. with methylene blue), but also by electron microscopy or by using fluorescence-marked lectins

With a few exceptions (Staurastrum i.a.), the species of this family are not among the common algae, but because of their symmetrical cell shapes they are among the most conspicuous algae. They prefer biotopes with a low pH value (pH 4-5).

Pleurotaenium kayei (Photo: M. ENGELS). This picture as well as all other Zygnematales pictures in botany on-line were made from cultures from the collection of conjugate cultures (SVCK) at the Institute for General Botany at the University of Hamburg.

Species of genera Micrasterias, Cosmarium, Euastrum, Staurastrum, Pleurotaenium

i.a. are unicellular, those of the genera Hyalotheca, Desmidium, Spharozosma including thread. Divisions are made in the same way as with the unicellular species. We are therefore dealing with intercalary growth.

The formation of the septum partly shows echoes of the formation of cell walls in a phragmoplast. The latter, however, is characteristic of cell division in the following class. Modern authors therefore occasionally summarize the Zygnematophyceae with her or classify her.

Charyophyceae in the traditional sense are the candelabrum algae with Chara and Nitella as representative representatives. The cell and nucleus division resembles that of the higher plants; the outer shape of these algae also points in this direction. Nevertheless it has been known for a long time that the candelabrum algae cannot be considered an ancestor of the land plants because they can be regarded as too specialized in too many other characteristics.

Choleochaete represents a modern group of green algae (the Charophyceae) whose ancestors were closely related to the earliest land plants. Because the fossil record of the algae-plant transition is sparse, comparative study of modern charophytes and simple seedless plants can illuminate the evolutionary process that generated a terrestrial flora. Courtesy: Linda E. Graham - © Botany for the Next Millenium

Most interesting, however, is the genus Coleochaete (Order: Coleochaetales), which was previously always placed with the Ulotrichales, but is characterized by a nucleus and cell division mechanism that is similar to that of land plants. In addition, in the peroxysomes of Coleochaete the enzyme glycolate oxidase was detected, which is also typical for higher plants. Finally, in geological formations where the first land plants were found, Coleochaete- Similar fossils have been identified (L. E. GRAHAM, 1984). The next few years will surely bring more evidence to light that will determine whether Coleochaete-like forms come close to the preliminary stages of land plants or whether other candidates have to be sought for this. According to other views, one should Coleochaete assign them to their own order and then put them together with the Klebsormidiales to form the "Klebsormidiophyceae".

They have a highly organized thallus, which is divided into nodes (nodes) and internodes. Short shoots ("leaves"), whose cells are only able to divide to a limited extent, sit in a lively arrangement at the nodes. In the "leaf axils", long shoots that continue to grow indefinitely can branch off. Chara-Species occur on sandy soils at the bottom of clear standing water. They are anchored in the ground by rhizoids (objects of study in geotropism research). Chara prefers an alkaline environment, but it can also be found in brackish water; However, it is sensitive to phosphate and is therefore rarely found in eutrophic waters, one reason why it has become very rare in our latitudes.

The growth of the axes takes place with single-edged vertex cells; the differentiation into the individual sections is based on a strict sequence of cell division. The cell separated from the apex cell initially divides inequally into a nodal and an internodal cell. Parenchymatic cell complexes develop from the nodial cell, from the periphery of which the apex cells of the short shoots arise. The internode cell grows enormously under nuclear divisions. It forms a large central vacuole and finally contains a large number of chloroplasts and up to 1000 cell nuclei in its plasmatic wall covering. Fully grown internodes can be very large (up to 10 cm), and their length corresponds to a single cell. The cell plasma is two-phase.

The outer chloroplast-containing phase is stationary, the inner one in constant motion. The "rotating flow", which is easy to observe, pulls upwards in a spiral on one cell side and downwards on the other. Cell division to cell division The changing orientation of the nuclear spindle leads to the formation of regular patterns. "Sprouts" and rhizoids are capable of regeneration to a considerable extent. The cells of the Charophyceae can be very large. The length of the internodes corresponds to the length of individual cells. Both CharaSpecies, they are surrounded by shorter, parallel arranged cortical cells.

Chara- and NitellaSpecies have therefore been used as test objects for the study of intracellular movements. Nitella is also suitable as a model for electrophysiological studies of plant cells. The cells are large enough to insert a glass electrode into them and derive the membrane potential.

The internodes of the Chara-Species are barked. Parallel threads completely cover the central cell. This connection is based on the initials of the nodes and grows with the elongation of the central node cell. The cell threads covering the surface follow the same strict differentiation scheme as the cells of the main axis. This also applies to the rhizoids. Achen and rhizoids are capable of regeneration to a considerable extent. With the genus Nitella the connection is missing.

In addition to the characteristic structure of the vegetative parts, Charophyceae are characterized by complex reproductive organs. There are monocial and diocesan Chara-Species. the mode of fertilization is oogamy. Unusual for algae are the single egg cells, which are surrounded by helically arranged sterile envelope cells. The so-called spermatogenic threads from which the male gametes develop are also surrounded by specialized, plate-shaped, sterile cells that join together to form a hollow sphere. The two-pointed spermatozoids are twisted like a screw and are reminiscent of the peat moss.

Charophyceenthalli are often calcified and therefore also fossil-forming. The structure of the oogonia shell turned out to be an important distinguishing feature. Petrified forms have existed since the Devonian.