Characeae C. Richard - Stoneworts
According to recent studies, stoneworts are more closely related to mosses and vascular plants than to most groups of algae (e.g. McCourt et al. 2004, Qui 2008). Although they are unique in many respects, the nomenclature of their morphology is essentially based on that of mosses; for example, the main axis of the thallus is referred to as a shoot in analogy to higher plants. Not least because of the ecological similarity to vascular plants, stoneworts were also included in the current revision of the Rothmaler (Korsch in: Müller et al. 2021), which is the basis for the present account.
Worldwide, the family is divided into 7 genera with approx. 450 species. According to current opinion, all 7 genera and 38 species occur in Germany.
The splitting of the genus Tolypella into Sphaerochara and Tolypella did not take place until 2023 (Schubert et al. 2024). In addition, Chara curta was included as a species in the revision of Charophytes at the European level (Schubert et al. 2024). In Germany (Korsch et al. 2013; Arbeitsgruppe Characeen Deutschlands 2016), this clade was previously considered to belong to Ch. aspera. The situation is similar to Ch. dissoluta, which was previously listed as uncertain with respect to occurrence in Germany. These new amendments took place after the manuscript was finished and could not be considered for the Rothmaler.
Many representatives of the Charophytes depend on rather clear and nutrient-poor waters, and several species show a clear preference with regard to the carbonate content. The majority of Chara species prefer carbonate-rich waters, while the opposite is true for most Nitella species. An often somewhat erratic occurrence is typical for stoneworts. In a given year, you may find a large population, only to search in vain for them at the same site the next year. However, once stoneworts have colonized a body of water, numerous reproductive units (oospores) are often found in the sediment. These remain viable for a long time, sometimes for decades (Stobbe et al. 2014), and can lead to formation of large populations surprisingly quickly if environmental conditions change or after a water body has been restored. Conversely, many Charophyte species succumb very quickly to competitive pressure of other macrophytes in the course of succession, especially in smaller water bodies. A number of stoneworts show a more or less clear seasonality, and spring and summer/autumn species can be distinguished. The water temperature plays a decisive role here.
Charophytes are mainly found in standing waters, but slow-flowing ditches are often colonized if the water quality is suitable. Faster-flowing streams or even rivers are rarely inhabited, and under exceptional circumstances certain species may grow at seepage sites if permanently moist.
Natural habitats of stoneworts include lakes, especially in northern Germany and the Alpine foothills, bodden waters of the Baltic Sea, and oxbows and the like in larger river floodplains. Human-made secondary water bodies are, however, also of great importance as habitats. Water-filled pits left over from lignite, gravel, sand and clay mining are prime examples. Old fish ponds, which are particularly characteristic of parts of Bavaria, Saxony and southern Brandenburg, deserve special attention, especially when managed extensively. Many small water bodies in our cultivated landscape are also important for Charophytes, and some species are entirely restricted to those or have their main distribution there. Small and often temporary water bodies in fields, which have received little attention in the past, can show a remarkable diversity and often host rare Charophytes, particularly so in parts of Brandenburg and Mecklenburg-Western Pomerania.
The stoneworts are macroscopically recognizable green plants. Their body is highly differentiated in comparison to other algae groups. It consists of an axis anchored by root-like structures with nodes and whorled branchlets extending from there, and ‘leaflets’ attached to these. The often strikingly colored reproductive organs are characteristic.
The Charophytes are distinguished from all other representatives of the plant kingdom by a number of special features. Their systematic position is therefore quite isolated and was disputed for a long time. Recently, they are considered to be more closely related to mosses and higher plants than to many other groups of algae. However, these peculiarities also mean that the Charophytes are relatively well defined as a group. Details of their structure are briefly described below.
The reproductive organs of the stoneworts are particularly distinctive. They show considerable differentiation and there is nothing comparable in any other plant group. The antheridia are composed of almost spherically arranged, strikingly patterned shield-like cells that enclose the elongated spermatozoids. Shortly after formation, antheridia are greenish in many species, but often become orange to brownish with maturity. The oogonia are usually ovoid. They are characterized by the spiral arrangement of cells (striae) around the actual egg cell. This creates a typical surface structure. The number of coils and their appearance are species-specific. At the tip of the oogonium is the so-called coronula. In some genera (e.g. Chara) the number of cells is 5, in others (e.g. Nitella) it is 10. There are both monoecious (male and female reproductive organs are located on one individual) and dioecious Charophyte species (male and female reproductive organs are located on separate individuals). In the course of time, the initially greenish oogonia develop into permanent oospores. In many Charaspecies, the striae store calcite crystals during maturation. The light-colored, calcified remains of these cells form a shell around the actual oospore, which varies in stability depending on the species. Such oospores surrounded by a shell are known as gyrogonites. The oospores themselves vary in color from light brown to black depending on the species. They can persist in the sediment for a long time and, like pollen, provide information on the flora of water bodies over thousands of years. Germination usually only takes place with sufficient light. Due to the long survival time of the oospores, the Charophytes can build up large populations within a very short time even after decades of absence if water conditions improve. In some Nitella species, an entire fertile head is enclosed in a mucus sheath, which is usually more pronounced in males than in females.
A characteristic feature of stoneworts is their ability to take up phosphorus compounds from the sediment. This gives them a considerable competitive advantage over most other algae in waters with little dissolved phosphate. The rhizoids, similar to the roots of higher plants, ensure uptake of nutrients from the soil in addition to anchoring the shoot. Some species produce bulbils on the rhizoids, which are used for overwintering and vegetative reproduction. This is most commonly observed in the Rough Stonewort (Chara aspera) and in the Starry Stonewort (Nitellopsis obtusa). The shoots of stoneworts are characterized by the regular alternation between short nodal and long inter-nodal cells. This leads to the habit common to all Charophytes. Only the nodal cells continue with cell division, and produce the other organs. In all native species, simple or forked branchlets develop on which the reproductive organs described above are attached. Many representatives of the genus Chara also have stipulodes and, in most cases, cortical cells. The latter enclose the shoot axis and, to varying degrees, its branches. In addition to the antheridia and oogonia, Chara species also have leaflets on the branches. Both lifespan and phenology of the Charophytes are significantly influenced by site conditions. Many species are potentially long-lived, yet only if growing in deeper water. Especially some of the larger species (e.g. Bristly Stonewort - Ch. hispida, Coral Stonewort - Ch. tomentosa) tend to form large monospecific stands. These survive the winter and in the following spring are effectively protected against invasion of other species, including those of higher plants. However, this is only possible if the plants grow deep enough to be not affected by ice formation. In shallower waters, not only ice in winter but also excessively high water temperatures in summer can cause mortality of Charophytes. In such places, the population must re-establish from oospores or individual nodule cells that are still alive and able to divide. Tolypella species are generally short-lived. The small Clustered Stonewort (T. glomerata) usually germinates in early spring and often dies back in May or June. Although many stoneworts are long-lived, more or less pronounced fertile phases can still be observed. In a number of common Chara species (e.g. Common Stonewort - Ch. Vulgaris, Bristly Stonewort - Ch. hispida), vegetative development continues throughout spring, while autumn antheridia and oogonia are only developed from May or June until autumn. In others, reproduction primarily occurs in spring (e.g. Coral Stonewort - Chara tomentosa, Dark Stonewort - Nitella opaca, Slimy-Fruited Stonewort - N. capillaris, Clustered Stonewort - Tolypella glomerata). The deeper individuals settle in the water, the later is the fertile phase. In addition, a second, less pronounced fertile period can sometimes be observed in autumn. Typical summer species are much rarer among the Charophytes. An example is Nitella syncarpa, which generally only develops from August into October. Development is, however, also influenced by the presence or absence of water, and in temporary water bodies, but also in ponds that have been refilled after a dry phase, species can grow and fruit at seasons that are actually atypical for them.
Finding
Stoneworts colonize many different types of water. More detailed information on this has been provided above under ‘Occurrence’, and here we will provide more general information. Charophytes can be described as ‘fair-weather plants’, because it takes less effort to reach into the water or even go in, and pick up the wet plants when the weather is sunny and warm. However, there are also more objective reasons to look for stoneworts when there is sufficient light and one can see much deeper into the water than. Effects of clouds are already clearly noticeable from the shore, and when snorkeling even thin clouds create the impression that someone has switched off the light. Sunglasses with polarizing filters are helpful when searching from above the water surface as they considerably reduce reflections that are almost always present. Large species such as the Bristly Stonewort (Chara hispida) are easy to spot, while small Nitella species can usually only be recognized with a certain amount of experience. It should be noted that several species often occur together in one body of water. In shallow water, the Charophytes occasionally reach the surface and then even protrude slightly out of the water. More often, however, they grow at the bottom and must be searched for there. In contrast to ‘normal’ aquatic plants, torn off shoots of Charophytes quickly sink to the bottom of the water and are therefore not washed up on the shore. If, however, no Charophytes can be seen on the shore or in its immediate vicinity, this does not necessarily mean that they do not occur.
Seasonal and long-term fluctuations in the occurrence of stoneworts must also be considered. Firstly, the temporal differences in the development of individual species should be mentioned. In a ditch near Jeseritz in the Altmark region, for example, masses of Nitella capillaris (Slimy-Fruited Stonewort) were found during a survey in May, as well as isolated specimens of Chara globularis (Fragile Stonewort) and Ch. virgata (Delicate Stonewort). On a second visit in September, Ch. vulgaris (Common Stonewort) was the most common species alongside with Ch. globularis. Only a few recently-grown specimens of N. capillaris could be observed, and Ch. virgata was completely absent.
Some general phenomena are also relevant here. Many water bodies are subject to major changes in visibility depth over the course of the year. In spring, the water is usually clearer than in summer, when phytoplankton is more abundant, but towards autumn a decrease in plankton and thus an improvement in the depth of visibility can often be observed. Filamentous green algae also develop very quickly, especially during the summer, and overgrow any Charophytes present. These are then only visible in spring.
Heavy rainfall has a significant influence on the ability to detect Charophytes. Rains often lead to massive turbidity of the water, especially in ponds connected to streams. In the vicinity of larger rivers, such as the Elbe, increased water level results may prevent access to permanently water-filled depressions. Weather events also have a significant influence on the development of Charophytes. In 2013, some previously surveyed water bodies were looked at again, and many Charophytes were less well developed than in previous years. In particular, Chara hispida (Bristly Stonewort) was present in significantly lower numbers. However, similar observations were also made for other species. In contrast, Ch. canescens (Bearded Stonewort) near Angersdorf, Ch. aculeolate (Hedgehog Stonewort) near Lunstädt and Tolypella glomerata (Clustered Stonewort) near Burgliebenau were very abundant in 2013. We can only speculate as to the causes. It is likely that the cold winter, accompanied by a long period of ice cover, caused changes in the competitive conditions at the beginning of the growing season, with which the various plant species (whether Charophytes, vascular plants or green algae) coped very differently.
In conclusion, it can be said that there is no optimum time of year for mapping stoneworts as a group, while specific species may have optima. A single visit to a body of water is often not enough to fully record the species composition. Even bodies of water in which no Charophytes were found at the time of the survey may well become colonized in later years, even if no new introduction of propagules had taken place.
Identification
A reflecting light microscope is indispensable for identifying Charophytes, especially at early stages of getting familiar with the group. With some experience, you will be able to identify many plants in the field, but even experienced experts will at least often need help of optical enlargement. Stoneworts usually develop best in full sunlight. Plants in shaded, murky or very deep waters often show untypical features. In most cases spine-cells of the cortex are then reduced, in a few cases even the entire cortex may be lacking. In addition, the internodes elongate, resulting in a slightly different habit.
Chara species can usually also be identified as sterile by means of cortication and spine-cell characteristics. The diagnostic features are often best recognized near the tip of the shoot. It is best to look at the second internode rather than the first, where the cortex cells have already stretched so far that spine-cells do not grow so densely. In addition, the differences between the raised and sunken cortical cells are already more pronounced here. The insertion type of the spine-cells is best determined near the shoot nodes, and with a little practice there is no need to make cross sections. The further you move away from the shoot tip, the fewer spines are present. Sometimes even the cortex cells detach and only the central strand remains. Such fragments can no longer be identified.
It is more difficult to identify Nitella species. Due to the lack of cortex and spine-cells, there are significantly fewer diagnostic features. For this reason, some species can only be reliably recognized if fertile. Sometimes it helps to screen many individuals and with a bit of luck, one or two fertile shoots can often be found even in apparently sterile plants. Some Nitella species enclose their gametangia in a mucus sheath. In others, a drop of water remaining in the head of condensed shoot tips, even after removal from the water, only simulates such a cover. However, clarity can be obtained with the help of a paper handkerchief. If the drop is absorbed by the handkerchief, it was only water, because real mucus sheaths remain intact even when dabbed.
Preparing herbarium specimens
In principle, algae are processed in a similar way to higher plants. They can be stored in plastic bags or boxes (without water!) for several hours or even days in the refrigerator without being damaged. Of outmost importance are labels clearly giving details of where they were found, the date they were collected and who collected them. Special care is needed due to the often delicate growth and, in the case of the Chara species, their fragility when dry. It is best to place the plants on an A4 sheet of paper or card (approx. 160-200 g/m², normal copy paper has 80 g/m²) while they are still damp. This sheet is then placed between absorbent paper to dry, and is “pressed” (kept flat) with heavy objects (e.g. books) or ribbons. To ensure that the plants only adhere to the support sheet and not to the drying paper, it has proven useful to cover plant with a sheet of commercially available baking paper during the drying process. This can be used several times. As a rule, it is sufficient to change the layers of absorbent paper twice, keeping the sheets the plants. As Charophytes have no protection against evaporation, the plants are usually completely dry after just one day. Once dried, specimens, especially of Chara species, must be handled more carefully than other plants. This also refers to postal transport: sending them in a simple letter, which runs through sorting machines at the post office, usually results in only fragments arriving. Identification is then difficult.
The database contains all taxa keyed by Korsch in Müller et al. (2021) as well as Chara curta and Ch. dissoluta. Systematics and nomenclature follow the treatment of the Characeae at European level (Schubert et al. 2024).
In contrast to the other taxa in the database, live photos of the plants are also presented for the Characeae. In addition, photographs of morphological details that are important for identification were only taken of selected specimens.
We would like to thank Silke Oldorff, Volker Krautkrämer, Robert Schmidt, Lorenz Seebauer and Klaus van de Weyer for providing the excellent photographs of living plants. We would also like to thank the Datz Foundation for the financial support that made it possible to produce the high-resolution scans and detail photographs.
Arbeitsgruppe Characeen Deutschlands Hrsg.; 2016. Armleuchteralgen. − Die Characeen Deutschlands. − Berlin, Heidelberg, 618 S.
Korsch, H. 2013. Die Armleuchteralgen (Characeae) Sachsen-Anhalts. – Berichte des Landesamtes für Umweltschutz Sachsen-Anhalt 1/2013: 85 S.
Korsch, H. 2018. The worldwide range of the Charophyte species native to Germany. − Rostocker Meeresbiologische Beiträge 28: 45−96. (Full text)
Korsch, H., Doege, A. Raabe, U. & van de Weyer, K. 2013. Rote Liste der Armleuchteralgen (Charophyceae) Deutschlands. 3. Fassung, Stand: Dezember 2012. – Haussknechtia Beiheft 17: 1–33. (Full text)
McCourt, R. M., Delwiche, C. F. & Karol, K. G. 2004. Charophyte algae and land plant origins. – Trends Ecol. Evol. (London) 19: 661–666. (Full text)
Müller, F., Ritz, C. M., Welk, E. & Wesche, K. (Hrsg). 2021. Rothmaler Exkursionsflora von Deutschland. Grundband. Berlin, Heidelberg, 944 S.
Qui, Y.-L. 2008: Phylogeny and evolution of charophytic algae and land plants. – J. Syst. Evol. (Beijing) 46: 287–306. (Full text)
Schubert, H., Blindow, I., Nat, E., Korsch, H., Gregor, T., Denys, L., Stewart, N., van de Weyer, K. & Casanova, M. T. (Hrsg). 2024. Charophytes of Europe. Cham, Switzerland, 1144 S.
Stobbe, A., Gregor, T. & Röpke, A. 2014. Long-lived banks of oospores in lake sediments from the Trans-Urals (Russia) indicated by germination in over 300 years old radiocarbon dated sediments. − Aquatic Botany 119: 84−90. (Full text)
Korsch, H. 2025. Characeae C. Richard. In: Dressler, S., Gregor, T., Hellwig, F. H., Korsch, H., Wesche, K., Wesenberg, J. & Ritz, C. M. Bestimmungskritische Taxa der deutschen Flora. Herbarium Senckenbergianum Frankfurt/Main, Görlitz & Herbarium Haussknecht Jena. [online] https://bestikri.senckenberg.de
