Thallophyta

Plant Characteristics

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Read about Thallophyta in the Standard Cyclopedia of Horticulture

Plants characterized rather indefinitely by the absence of an archegonium around the egg, and the absence of the type of antheridium found among the higher plants. The plant body is rarely differentiated into organs simulating stem and leaves, and no true vascular tissue is found in the group. Formerly the Thallophyta were divided into the Algae, Fungi, and Lichens; but this, though a good classification on physiological grounds, does not indicate actual relationship so well as the modern division into fifteen classes founded on structure, as follows: CLASS I. BACTERIA Unicellular or filamentous organisms without green color, possibly "degenerated" from the Cyanophyceae, with no true nucleus, the cell-wall often gelatinous: reproduction wholly asexual by division into two equal portions and subsequent separation (fission); or by asexual spores, one of which may be produced in each cell. Bacteria are probably the smallest known organisms, some being not over .00003 inch in diameter. In form, the cells are either oblong, spherical or spiral, and may be separate or united in groups or chains, and may be either motile by means of cilia or non-motile. Bacteria, while showing little structural diversity, have become highly specialized physiologically, and it is on this basis that the species are usually distinguished. Many cause disease among animals and human beings, while others cause disease among plants. Nitrifying bacteria in the soil are of vital importance to higher plants. Bacteria and fungi are the causes of decay. CLASS II. CYANOPHYCEAE (Blue-green Algae) Unicellular or filamentous algae of blue-green color; true nuclei wanting: cell-wall often gelatinous: reproduction wholly asexual by fission or by asexual spores borne as in the bacteria. The blue-green algae inhabit water, damp soil, damp rocks, or damp tree trunks, where they often form filamentous or gelatinous, dark green patches. The aquatic forms prefer water containing much organic matter and hence are abundant in sewers. Certain species inhabit flower-pots in greenhouses, and brick walls. CLASS III. FLAGELLATA (Flagellates) Simple unicellular aquatic organisms intermediate between the Thallophyta and Protozoa. During a portion of their life they possess no cell-wall, and often show amoeboid movements. The cells contain a nucleus, pulsating vacuole, and chlorophyll; and one or more cilia are present. Some reduced forms are colorless and saprophytic. Reproduction is wholly asexual by fission and thick-walled resting spores. Found in waters of ponds and streams. CLASS IV. MYXOMYCETES (Slime Molds) A very distinct and independent group, formerly often classified in the animal kingdom. The plants consist of naked masses of protoplasm called plasmodia, which contain many nuclei but no chlorophyll. These are found in forests and damp, shady places. When ready to fruit, the plasmodia move toward the light and away from the water, hence ascend grass stems, stumps and logs, where they transform into elaborately constructed sporangia. The asexual spores, each enclosed by a cell-wall, are distributed by the wind, germinate, produce a ciliated bit of naked protoplasm which swims in the soil moisture, multiply by division and at length fuse with neighboring protoplasts to form the plasmodium, which latter may be sometimes a foot in breadth. During unfavorable weather, the plasmodia are often transformed into sclerotia. Plasmodiophora brassicae, which is the cause of the club-root of cabbage, is the only Myxomycete of great economic importance. CLASS V. PERIDINEAE A small group mostly inhabiting the sea, more rarely fresh water. They are unicellular, free-swimming organisms with nucleus, vacuole, chromatophores, and cilia. The cell is usually surrounded by a cellulose, sculptured, or pitted and transversely furrowed, wall. Reproduction is by cell-division and swarm-spores Sexual reproduction has recently been discovered. The Peridineae often form an important part of the plankton in the sea. CLASS VI. CONJUGATAE Green filamentous or unicellular fresh-water algae: cell-wall and nuclei present: reproduction by division of the plant body, and by sexual spores, which latter result from the union of two body cells by means of a connecting tube (conjugation). Plants of the sub-group Desmidiaceae are not filamentous, but often star-shaped, lunate, or geminate in outline. The Zygnemaceae are filamentous with star-shaped (Zygnema), spiral (Spirogyra), or plate-like chloroplastids. The Conjugatae are of little economic importance. CLASS VII. DIATOMEAE (Diatoms) Unicellular algae of very peculiar and interesting habit. The wall consists of two silicious valves, one of which fits over the other like the lid of a box. These valves are frequently very beautifully sculptured. Through division, new cells and new walls are formed, which are always smaller than before, until finally as a limit a sexual spore is produced which reestablishes the size of the cell. Diatoms inhabit stagnant water, wet rocks, and the sea. They are either free-floating or pedicelled and attached. The silicious walls will resist burning. Diatoms contain little, if any, chlorophyll, and are mostly saprophytic. A large part of the oceanic plankton is composed of Diatoms. CLASS VIII. HETEROCONTAE or CONFERVAE A small group of green algae, inhabiting wet soil or water, but of little, if any, economic importance. The zoospores have unequal cilia; and the chloroplastids are yellowish green and oil-producing. Asexual resting spores also occur. Conjugating zoôspore-like gametes are found in some genera. Botrydium and Conferva are examples of this class. CLASS IX. CHLOROPHYCEAE A large and important group of fresh-water, or rarely marine, algae. Plant body unicellular, filamentous, or even thalloid: the cells contain chloroplastids and produce starch: reproduction sometimes vegetative, but also by asexual zoospores; sexual reproduction consists of the fusion of two zoospore-like gametes, or the fusion of one such gamete and a specialized non-motile egg. The latter condition is characteristic of the higher forms in nearly all the sub-groups of the Chlorophyceae. The plant body in the Order Siphonales is peculiar in that it consists of a continuous tube without cross-walls. Some common genera in this class are Volyox, Chlamydomonas, Pandorina, Protococcus, Pediastrum, Scenedesmus, Hydrodictyon (Water-net), Ulothrix, Ulva (Sea-lettuce), Oedogonium, Cladophora, Caulerpa, and Vaucheria. CLASS X. CHARACEAE (Stoneworts) Attached plants (1 inch to 1 yard in length) of fresh or brackish water, consisting of a slender stem, which bears at each node a whorl of branches, usually again bearing whorled branchlets. The internodes consist of one immense multinucleated cell often as much as 3 inches long, which is naked or inclosed in a sheath of smaller cells. The branches are similarly constructed though the cells are correspondingly smaller. Asexual spore-reproduction is absent. Sexual reproduction is by means of an egg-cell inclosed in a jacket of spiral wall-cells, and of sperm-cells inclosed in an antheridium which has a multicellular wall. These sexual organs are borne at the nodes of the branchlete. The fertilized egg and its investment becomes a thick-walled resting structure. Many species of Chara and Nitella, the only two genera, have the power to deposit lime from solution, and thus become incrusted with that substance, hence the popular name. In this way the Characeae have played a part in the filling up of calcareous lakes and the production of new land. They are mostly inhabitants of calcareous waters. CLASS XI. PHAEOPHYCEAE (Brown Seaweeds) A large group of salt-water algae, well known in all waters of the globe, but most abundant in the colder regions. Plant body attached, usually thalloid and branched, but very diverse; in some cases filamentous, in others disk-shaped or globular. The larger forms of Laminaria are sometimes 200 feet long. The chromatophores of the Phaeophyceae contain a brown pigment which gives to these plants a brown or yellowish color instead of green. The thallus is often very tough and cartilaginous, to resist the waves. Zoospores are often produced. In sexual reproduction, the gametes are either similar and motile, rarely non-motile, or more often the sperm is motile while the egg is much larger and non-motile. Details of structure in respect to reproduction, however, are very great. The thallus of various species of Phaeophyceae yields iodine and soda. Some species (e.g., Laminaria saccharina) yield mannite and are used in the Orient for food. The dried stalks of L. digitata and L. Cloustoni have been used in surgery. Fucus and other genera are used as manure. One species, Sargassum bacciferum, has accumulated in great quantities in the Atlantic Ocean between the Bermuda Islands and the Spanish coast, in the so-called “Sargasso Sea." CLASS XII. RHODOPHYCEAE (Red Seaweeds) Mostly marine algae, a few only inhabiting fresh water, widely distributed, but most abundant in the tropics and temperate region at lower depths. The thallus is very diverse, filamentous, branched, often thalloid, attached by holdfasts, and red, violet, or purple in color, rarely green. True starch is not found. Asexual spore-reproduction is frequent. These spores are non-motile and produced in fours (tetraspores). Sexual reproduction is by dissimilar gametes, the antheridium becoming without change a single non-motile sperm-cell. The egg-cell is prolonged upward into a slender tube (trichogyne). The fertilized egg by division gives rise to a globular mass of short filaments (cystocarp) which produce asexual spores. These spores in turn give rise to the mature plant. The cystocarp and its spores, thus following fertilization, suggest the alternation of generations found in the mosses and liverworts and all higher plants. About 300 species of Rhodophyceae have been described. Carragheen, or Irish moss, used in jellies and puddings, is the dried thallus of Chondrus crispus and Gigartina mamillosa of northwestern Europe. Agar-agar, used in the preparation of culture media in bacteriology and mycology, is obtained from various species of this group. CLASS XIII. PHYCOMYCETES. A large group of parasitic or saprophytic organisms (fungi), without chlorophyll: thallus (mycelium) of much-branched filaments (hyphae); usually without cross-walls (non-septate), as in the algal group Siphoneae: asexual reproduction by motile or non-motile spores which are usually borne in sporangia, and by conidia which are cells abstricted from the tips of specialized hyphae: sexual reproduction diverse, either by the conjugation of similar gametes, or by the conjugation of a specialized antheridial branch (male) and an enlarged oogonial branch (female) which contains the egg; free sperm-cells are rare. The order Oomycetes, with differentiated gametes, contains the following important fungi: Saprolegnia (water-mold), a whitish, aquatic mold growing on decaying plants, insects, or living fishes; Olpidium brassicae, parasitic in cells at the base of the stem of young cabbage plants causing their death; Phytophthora infestans (potato disease); Plasmopara viticola, downy or false mildew of the grape; Albugo candida, white rust of Cruciferae; Pythium de Baryanum, causing damping off of seedlings. Order Zygomycetes, with similar gametes, contains Mucor mucedo, white mold of bread, fruits, etc.; Rhizopus nigricans, a mold on bread, fruit, etc.; Empusa muscae, parasitic on houseflies, causing their death and producing a white halo about them on the surface where they die. CLASS XIV. EUMYCETES A very large and important group of saprophytic or parasitic organisms (fungi) without chlorophyll: thallus (mycelium) composed of fine tubular threads, which are septate: sexual organs usually obscure or apparently wanting: asexual reproduction by spores or by conidia, a modified form of which is termed basidia. The conidia and basidia do not always represent homologous organs. The group is divided into Ascomycetes and Basidiomycetes. The Ascomycetes are characterized by a group of usually 8 spores inclosed in a unicellular sac (ascus), which is produced immediately after the imperfect sexual fertilization. The asci are borne in spherical bodies (perithecia) or in open cups (apothecia). The Perisporiaceae, Discomycetes, Pyrenomycetes, and Tuberaceae are orders within this sub-class. Among the many important economic fungi belonging here are the following: Erysipheae (Downy Mildews); Aspergillus and Penicillium (Fruit Mold, Blue Mold); Morchella (Morel), edible; Nectria (Currant Cane Rust and Tree Canker); Claviceps purpurea (Ergot), parastic in the ovaries of grains; Taphrina (including Exoascus), causing witches' broom, leaf curl of peach, plum pockets, etc.; Saccharomyces (Yeast), causing fermentation in saccharine solutions. The Basidiomycetes are characterized by the production of four spores on a special hyphal tip or thread (basidium). Each spore is raised on a minute slender stalk (sterigma). These spores, in some cases, if not in all, follow immediately after a nuclear fusion, which probably represents a reduced sexual act. In this group are the Ustilagineae (Smuts), infesting the ovaries of grains, etc.; the Uredineae (Rusts), which infest a wide variety of cultivated and wild plants, and among which may be mentioned the wheat rust; the Hymenomycetes (Mushrooms, Toadstools, and Bracket Fungi), which are saprophytic or inhabit timber; and the Gasteromycetes (Puff-balls), which are saprophytic. The rusts exhibit alternation of generations to a most remarkable degree, the different generations often inhabiting different host plants and possessing a wholly different appearance, as well as a wholly different method of spore-formation. Thp Hymenomycetes are saprophytic, except the genus Exobasidium which inhabits the living foliage of various plants, the genus Armillaria which infests living tree-trunks, and many genera of the Polyporaceae (Bracket Fungi) which also attack the wood of living trees. The last-mentioned fungi, including Armillaria, inhabit the trunks and branches of forest trees, causing their death. CLASS XV. LICHENES (Lichens) Green, gray or highly colored plants of very diverse habit and habitat, either thalloid, fruticose or crustaceous, and growing on the soil, bark of trees, rocks, or rarely on foliage: propagation by division of the thallus or by the separation of special minute powdery parts (soredia): spore-reproduction by ascospores borne in perithecia or apothecia, rarely by basidiospores. The lichen thallus is not a single organism, but is probably a symbiotic structure, comprised fundamentally of fungus hyphae between which many unicellular green algae are distributed, usually in a définite fashion. The fungi belong to the Ascomycetes in the great majority of cases, rarely to the Basidiomycetes. The algae may belong to the Chlorophyceac, in which case they are unicellular, or to the Cyanophyceae, in which case they are either unicellular or in chains. Because the symbiotic structure behaves as a unit, it has been decided to continue to treat the lichens as a class by themselves, rather than to consider the algal and fungal components independently in their respective groups. Except as soil-producers, lichens are of little economic importance: Cetraria islandica furnishes Iceland moss; Sticta pulmonaria was once used in medicine; Cladonia rangiferina furnishes the main food of the reindeer in Lapland, and, possibly, of other arctic animals; Roccella tinctoria of Africa and the East Indies is the source of the chemical indicator, litmus and of the dye orchil or orseille. The above text is from the Standard Cyclopedia of Horticulture. It may be out of date, but still contains valuable and interesting information which can be incorporated into the remainder of the article. Click on "Collapse" in the header to hide this text.

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