Heterosporous

Spike Moss, common name for a medium-size family of primitive vascular plants. The plants are mainly of tropical distribution, growing in moist, shaded habitats. A few occur in deserts, where they become dormant during the driest parts of the year; the resurrection plant is an example of such a species. The plants of the spike moss genus are similar to those of a related genus (see Club Moss), but in the spike moss a small outgrowth, called a ligule, is located on the upper surface of each leaf close to where it joins the stem.

The spike moss is heterosporous. That is, it reproduces through the production of two kinds of spores, produced in sacs called sporangia, that are borne by specialized leaves called sporophylls. Small sacs, or microsporangia, produce many small microspores, which germinate to form short-lived, nongreen microgametophytes that produce sperm (male sexual cells). Larger sacs, or megasporangia, produce a few larger megaspores each, and these form nongreen megagametophytes that produce eggs in specialized organs called archegonia. To reach an archgonium and fertilize an egg, a sperm cell must swim in a film of water. Several species of spike moss are cultivated in tropical greenhouses for their interesting foliage.

Scientific classification: Spike mosses make up the family Selaginellaceae. The only genus in the family is Selaginella. The resurrection plant is classified as Selaginella lepidophylla.

Resurrection Plant

Resurrection Plant, common name for a fernlike, perennial desert plant, of the family of spike mosses, that ranges from the southwestern United States down to El Salvador. The flowerless plant has small, scalelike leaves and grows to a height of 10 cm (4 in). It requires little water to survive, but when completely deprived of moisture it contracts into a ball and may be borne by the wind. When it settles near water, it unfolds and is renewed—hence the name. In their dried-up state, resurrection plants are sometimes sold as novelties. The plant is sometimes called the rose of Jericho, just as that plant is often called the resurrection plant.

Scientific classification: The resurrection plant belongs to the family Selaginellaceae. It is classified as Selaginella lepidophylla.

Rose of Jericho

Rose of Jericho, also resurrection plant, common name for a small herb of the mustard family, native to Syria. It is seldom more than 15 cm (6 in) high and has small white flowers. After the herb has flowered, the leaves fall off and the branches become incurved toward the center, so that the plant assumes an almost globular form. In this state it is often blown about by the wind in the desert. When it happens to be blown into water the branches expand again, and the pods open and spill out the seeds.

Scientific classification: The rose of Jericho belongs to the family Brassicaceae (formerly Cruciferae). It is classified as Anastatica

Club Moss

Club Moss, common name for a group of plants closely related to the ferns. Two important genera are low, sometimes creeping evergreen plants that are widely distributed throughout the United States and other temperate and tropical climates. Various tropical species grow on the trunks of trees or on the ground. Usually less than 30 cm (less than 12 in) tall, the plants resemble pine in that their leaves are needlelike. Their widespread use in Christmas wreaths has brought some species close to extinction. In reproduction, spores and asexual cells borne in an elongated cone are scattered on the ground and ripen underground into sexual reproductive organs from which new plants grow.

The common club moss, also called running pine and stag's horn moss, grows in open, dry woods and rocky places. Ground pine or ground cedar has branchlets that resemble the branches of juniper. Tree moss, fir club moss, or foxfeet grows on rocks and in bogs. Fossil species, many of gigantic size, have been found in strata of the Upper Silurian and the Devonian and Carboniferous periods.

Scientific classification: The two important genera of club mosses are Lycopodium, of the family Lycopodiaceae, and Selaginella, of the family Selaginellaceae. The common club moss is classified as Lycopodium clavatum, ground pine or ground cedar as Lycopodium complanatum, and tree moss as Lycopodium selago.

Homosporous

Spores are usually produced by the division of cells within a structure called a sporangium. In bryophytes and most ferns, horsetails, club mosses, and whisk ferns, spores give rise to the same kind of plants as the parents, which are thus called homosporous. But in a few of those just mentioned and in all seed plants, some spores grow into sexual plants (gametophytes) that produce male gametes (sex cells), while others grow into sexual plants that produce female gametes; parent plants producing such spores are called heterosporous. Spores that give rise to male gametophytes are called microspores; spores giving rise to female gametophytes are known as megaspores. In seed plants, male gametophytes are known as pollen grains, and female gametophytes are called embryo sacs. Thick-walled resting zygotes (cells formed by the union of gametes) resemble spores and are called zygospores or oospores.

Shrub

Shrub, short, woody plant, typically with several stems arising from or near the ground. Shrubs are perennial plants—that is, they live for at least three years. Although the distinction is often artificial, shrubs are generally shorter and have more stems than trees.

Shrubs grow wild in all but the coldest or driest regions of the earth. They may be evergreen (retaining foliage throughout the year) or deciduous (losing foliage each year). Shrubs are often planted to control erosion, the wearing away of topsoil by wind and rain, in exposed areas. Their variety of forms, foliage, flowers, fruits, and fragrance make them popular in gardens and as landscape plants. According to 20th century British botanist Stanley Whitehead, shrubs “have the stable, long-life qualities of forest trees without the embarrassment of the latter’s size. They match the floral beauty and grace of habit of herbaceous plants without their compelling need for support and attention.” Popular shrubs include the rhododendron, laurel, holly, dogwood, rose, and hydrangea.

Rhododendron

Rhododendron, common name for flowering plants of a genus of the heath family. The genus contains some 850 species, the majority of which are cultivated. Native to the temperate areas of the Northern Hemisphere, rhododendrons are most abundant in the Himalayas, southeast Asia, and the mountains of Malaysia. Cultivated species can now be found on all continents. Rhododendrons exhibit great variety in size, habit, and flower color. They range from small, ground-hugging shrubs to small trees, and from white to pink, dark-purple, or yellow flowers. Most species are evergreen—that is, they retain their foliage throughout the year. The deciduous species of rhododendron, those that lose all of their foliage each year, are known in horticulture as azaleas.

Scientific classification: Rhododendrons make up the genus Rhododendron of the family Ericaceae.

Heath

Heath, common name for a plant family comprising flowering plants that are widely distributed in both temperate and tropical regions of the world, and for its representative genus. The family contains about 3350 species, which are placed in more than 100 genera and include a large number of ornamentals. Important horticultural members of the family are rhododendrons and azaleas (see Rhododendron), heath, heather, madrona, mountain laurel, sweetbell, and sourwood. Various species produce such fruit as the blueberry, cranberry, and huckleberry. Briar pipes are made from burls of heath species native to the Mediterranean area.

Most members of the heath family are woody, often evergreen shrubs. Many are poisonous; indeed, many common cultivated species contain the toxin andromedotoxin, which causes vomiting and reduced blood pressure and may result in death if taken in large quantities. Azaleas, rhododendrons, sweetbells, and laurels all contain this compound.

The flowers are characterized by four or five sepals (outer floral whorls) and four or five petals (inner floral whorls) that are fused at the base to form a tube. The stamens (male floral organs) are usually twice as numerous as the petals, and the anthers (pollen-bearing structures) open by terminal pores rather than by longitudinal slits, as they do in most families of flowering plants. The ovary (female floral organ) is usually superior (borne above and free from the other floral parts).

Most members of the order to which the heath family belongs are woody plants, with small trees and shrubs predominating. Some tropical species are epiphytes (plants growing on other plants for support but not nourishment). A number of species, such as the Indian pipe, lack chlorophyll, the food-producing pigment that is found in most plants, and are parasitic, depending on other green plants for their nourishment.

Although the order is widely distributed, its members occur mainly in cooler areas. Thus, in tropical regions they are found mainly in mountainous areas. In addition, most members of the order occur in acid soils.

The seven other families of the order contain only a few species each and have limited economic importance as ornamentals. They are also used in local medicines.

Scientific classification: Heaths make up the family Ericaceae, of the order Ericales. The representative genus is Erica. The Indian pipe is classified as Monotropa uniflora in the family Pyrolaceae.

Blueberry

Blueberry, common name applied to several related shrubs (see Heath) and to their blue or black fruit. The lowbush, or swamp, blueberry and the highbush blueberry are the two most common species raised in the United States. The lowbush blueberry averages less than 1 m (less than 3 ft) in height; the highbush blueberry ranges from 1.8 to 6 m (6 to 20 ft). Although blueberries are cultivated in many regions of the world, large quantities of the shrubs grow wild. Mostly deciduous, the bushes bear sweet blue or black berries, which grow either singly or in clusters. Branches of the evergreen, also known as huckleberry greens, are used in flower arrangements and in other decorations.

Scientific classification: Blueberries belong to the family Ericaceae. The lowbush, or swamp, blueberry is classified as Vaccinium angustifolium, and the highbush blueberry as Vaccinium corymbosum.

Cranberry

Cranberry, common name for several species of low vines of a genus of the heath family, and for their small, sour, seedy fruit. The plants, which belong to the same genus as the blueberry, have drooping, pink flowers and small, thick, evergreen leaves. The small, or European, cranberry grows wild in marshlands of temperate and colder regions of Europe and North America. Most of the cranberry crop produced in the United States each year is canned as sauce or jelly or bottled as juice. The cowberry, or mountain cranberry, is common in both Europe and North America. It is gathered and sold in considerable quantity but is rarely cultivated. The highbush cranberry, with its clusters of white flowers followed by red berries, is a shrub of the honeysuckle family. Its fruit is sometimes used as a substitute for cranberries.

Scientific classification: Cranberries belong to the genus Vaccinium of the family Ericaceae. The small, or European, cranberry is classifed as Vaccinium oxycoccos; the large, or American, cranberry as Vaccinium macrocarpon; and the cowberry, or mountain cranberry, as Vaccinium vitis-idaea. The highbush cranberry belongs to the family Caprifoliaceae and is classified as Viburnum opulus.

Honeysuckle

Honeysuckle, common name for a plant family and for its representative genus. The family contains about 400 species of dicot flowering plants, including many ornamentals. It occurs mainly in the North Temperate Zone, particularly in China, but it also extends into tropical mountains. Its members are mostly shrubs or small trees, although some familiar members are herbaceous—for example, elderberry and some species of honeysuckle, such as Japanese honeysuckle. The leaves are opposite one another on the branch and lack stipules (leafy appendages at the leaf base); the calyx and corolla (floral whorls) have fused parts and arise from above the ovary (female flower part); and usually five stamens are borne on the inside of the corolla tube. The fruit is usually a berry.

The family contains many ornamentals, such as the honeysuckle, a genus of about 180 species of usually fragrant shrubs and vines—including the trumpet honeysuckle—more than 30 of which are cultivated; the elders; the arrowwoods; the abelias; and the weigelias. Some species of elder produce wood that has been used to make musical instruments, and the fruits and flowers of the genus are sometimes used to make wines and preserves. Japanese honeysuckle, introduced from Asia, has become a weed in eastern North America. It often kills the native vegetation by overshadowing it.

Scientific classification: Honeysuckles make up the family Caprifoliaceae. The representative genus is Lonicera. The Japanese honeysuckle is classified as Lonicera japonica and the trumpet honeysuckle as Lonicera sempervirens.

Huckleberry

Huckleberry, common name for any of a genus of shrubs (see Heath), native to temperate North America. The pale-purple or pale-red flowers, borne in racemes, have a five-part calyx, a five-cleft tubular or bell-shaped corolla, ten stamens, and a solitary pistil. The blue to black fruit contains ten bony, seedlike nutlets. Lower surfaces of huckleberry leaves are sprinkled with resinous dots.

Black huckleberry, native to woodlands and swamps of the northeastern United States and eastern Canada, is a shrub growing 30 to 91 cm (12 to 36 in) high, producing edible black fruits. Bear huckleberry, or buckberry, native to woodlands of the southeastern United States, is a slender shrub, less than 30 cm (12 in) high, producing unpalatable reddish-black fruits. Dangleberry, native to the eastern United States, is a low shrub producing dark-blue sweet fruits. Huckleberries are often cultivated in the United States for their foliage and fruit.

Scientific classification: Huckleberries constitute the genus Gaylussacia, of the family Ericaceae. The black huckleberry is classified as Gaylussacia baccata, the bear huckleberry as Gaylussacia ursina, and the dangleberry as Gaylussacia frondosa.

Laurel Plant

Laurel (plant), common name for a flowering plant family, widespread in tropical and subtropical regions, and for its representative genus. The laurel family contains between 30 and 50 genera and at least 2000 species. Several well-known and important genera of the family also occur in temperate areas, for example, camphor, cinnamon, avocado, and spice-bush. The laurel, or bay, also called bay laurel and bayberry, is a large evergreen shrub native to the Mediterranean region; it has been important since classical times and is now widely cultivated in warm temperate areas as an ornamental and for its aromatic leaves, which are used in cooking. California bay yields a valuable wood.

The order to which the family belongs contains 8 families with about 2500 species distributed mostly in tropical and subtropical regions. Many species exhibit botanically interesting structures, and some are of economic value because of the useful products derived from them.

Although a great diversity of form occurs in the order, several features are common to most of its members and distinguish the order from others. All members are woody trees, shrubs, or vines, and all produce aromatic oils in their tissues. The stamens (male floral structures) have appendages that produce nectar, and the pistil (female floral part) often consists of a single carpel (ovary receptacle).

The members of the order are believed to resemble the earliest flowering plants in their woody nature, floral characteristics, and details of anatomy and pollen structure. In addition, the order has a long fossil history, with specimens similar to plants of several living genera having been found in rocks more than 65 million years old. Members of the order to which laurels belong, however, are considered more advanced than another primitive group of woody flowering plants (see Magnolia), which have stamens and carpels that more closely resemble leaves.

Of the remaining 7 families in the order, one, the Monimia family, has about 450 species, and the rest have a total of fewer than 100, with 3 having only one species each. They are mostly of restricted distribution and little economic importance.

Unrelated plants also called laurel include the mountain laurel and several other members of its genus, along with several members of the cherry genus.

Scientific classification: Laurel is the common name for the family Lauraceae of the order Laurales. The laurel, or bay, is classified as Laurus nobilis and the California bay as Umbilicus californica.

Tansy

Tansy, common name for a genus of about 70 north temperate species of flowering plants of the family of composite flowers. A few are native to the United States. The common tansy is an aromatic perennial herb growing to about 1 m (about 3 ft) high, with deeply divided leaves and yellow flowering heads. Native to Europe and now naturalized in North America, it is grown as a garden ornamental; it also has been used in cooking and to make a tonic.

Scientific classification: Tansies make up the genus Tanacetum of the family Asteraceae (formerly Compositae). The common tansy is classified as Tanacetum vulgare.

Composite Flowers

Composite Flowers, common name for one of the largest families of flowering plants, with more than 20,000 species. The common name refers to the clustering of the flowers into compact heads so that an entire cluster resembles a single flower, as in the familiar aster or dandelion.

The composite family is worldwide in distribution except for Antarctica, where two species of grasses are the only flowering plants known. Composites are particularly well adapted to semiarid parts of tropical and subtropical regions, to arctic and alpine tundra, and to temperate regions. They are poorly represented only in tropical rain forests.

Although the composite family contains nearly 10 percent of all the flowering plants, its direct economic importance is relatively small. Lettuce is the most important crop; other notable food plants are artichoke, endive, salsify, chicory, and tarragon. Sunflower and safflower are important sources of vegetable oils, and some members such as guayule have been studied as potential sources of rubber. Weedy members of the family include dandelion, thistle, cocklebur, and ragweed, which is a major source of the airborne pollen that affects hay fever sufferers. Horticulturally important members of the composite family include marigold, dahlia, zinnia, daisy, cosmos, chrysanthemum, tansy, and aster.

Cinnamon

Cinnamon, common name for several related trees of the laurel family, and for a spice made from the dried bark of the trees. The best-known species is native to Sri Lanka; it is cultivated in many other tropical countries in Southeast Asia and elsewhere, but that grown in Sri Lanka is superior in taste. The tree, which usually grows up to 12 m (40 ft) high, is cultivated to grow four to five stems. When the bark begins to turn brown, the stems, which may be about 2.5 m (about 8 ft) tall and 5 cm (2 in) in diameter at the base, are harvested and new ones are trained to grow in their place. After the tree is stripped of leaves and twigs, the inner and outer bark is removed. As the bark dries, it forms rolls (quills), the smaller of which are inserted into the larger, and when fully dry, these are tied in bundles for shipment.

Cinnamon is yellowish brown and has a distinctive fragrant aroma and a sweetish, pungent taste. It has been used since early times as a spice in many culinary preparations, and it is also used in some medicines. The aromatic qualities of cinnamon result from a volatile oil, oil of cinnamon, that may be extracted from the bark by distillation. The oil varies in color from yellow to cherry red; the yellow is used in scenting soap and flavoring candy.

Scientific classification: Cinnamon trees belong to the family Lauraceae. The best-known species is Cinnamomum zeylanicum.

Avocado

Avocado, also alligator pear, common name for a tree (see Laurel), native to tropical America, and for the fruit of this tree. The fruit is a greenish, thick-skinned drupe, similar in size and shape to a large pear. When ripe, the flesh has the consistency of firm butter and a faint nutlike flavor. It has a high fat content, containing 10 to 20 percent oil, and is rich in protein. In the United States avocado is popular as a salad vegetable, and in the tropics it is often used in soup. The tree is extensively cultivated in the southern United States and California.

Scientific classification: The avocado tree belongs to the family Lauraceae. It is classified as Persea americana.

Holly

Holly, common name for a family of plants, containing about 300 species, and for its typical genus. The holly family consists of trees and shrubs usually having separate staminate and separate pistillate flowers that are small in size, four- to eight-parted in structure, and white or greenish in color. The fruit are usually red drupes, containing two to eight one-seeded stones. English holly is a small tree with spiny evergreen leaves and bright-red fruit. The common American holly has duller, less spiny leaves. It is native to the eastern United States, where it has been harvested so extensively that several states enforce protective laws. Both species of holly are widely cultivated for ornamental purposes. A South American species, the Paraguay tea, has stimulant properties and is used to make an aromatic beverage, maté.

Scientific classification: Hollies make up the family Aquifoliaceae. English holly is classified as Ilex aquifolium, the common American holly as Ilex opaca, and the Paraguay tea as Ilex paraguariensis.

Maté

Maté, also yerba maté, a tealike beverage used extensively in South America, especially in Brazil. It is made from the dried and roughly ground leaves and shoots of certain species of holly, especially Ilex paraguariensis. Its stimulating effect is derived from its content of theine (see Caffeine). Maté is also known as Paraguay tea.

Dogwood

Dogwood, common name for a family of flowering plants distributed mainly in the temperate areas of the northern hemisphere, with a few species occurring in tropical South America and Africa. Of the 14 genera in the family, only the dogwood genus is native to North America. Members of the family are mostly trees or shrubs with simple, opposite leaves. Well-known exceptions, however, are the bunch berry, a perennial herb; and the pagoda dogwood, which has alternate leaves. Dogwood flowers are small and are produced in branched terminal clusters that are sometimes surrounded by showy white bracts. Thus, the so-called petals of the familiar flowering dogwood are actually bracts.

The main economic importance of dogwoods stem from their valuable ornamental species, grown for their attractive flowers, flower bracts, fruits, twigs and stems, and colorful autumn leaves. In addition to the species already mentioned, most of which are commercially grown, others commonly found in cultivation include the Cornelian cherry, the red-osier dogwood, the Japanese dogwood, and certain sour gums. The fruit of the Cornelian cherry, a native of Europe, is used in France to make an alcoholic beverage, vin de courneille, and is also used in preserves. Oil extracted from the fruit of the blood-twig dogwood is used in France for making soap. The wood of several species is used in furniture.

The order to which the dogwoods belong contains 4 families and about 150 species, about 100 of which are in the same family as dogwoods. The sour gum family contains three genera. The two other families both contain one genus. Members of the order vary greatly in flower structure, making the order difficult to characterize. The flowers are usually small, however, with the four or five sepals commonly reduced in size and forming a tube that is fused to the ovary (female flower part). A nectar-producing disk is usually present on the upper part of the ovary. Four or five petals are commonly found; they are not fused to one another. Occasionally, however, the petals are absent.

Scientific classification: Dogwoods make up the family Cornaceae, in the order Cornales. The bunch berry is classified as Cornus canadensis, the pagoda dogwood as Cornus alternifolia, the flowering dogwood as Cornus florida, the Pacific dogwood as Cornus nuttalli, the Cornelian cherry as Cornus mas, the red-osier dogwood as Cornus stolonifera, the Japanese dogwood as Cornus kousa, and the blood-twig dogwood as Cornus sanguinea. The sour gums commonly found in cultivation are classified in the genus Nyssa, in the family Nyssaceae. The two other families in the order Cornales are Alangiaceae and Garryaceae.

Hydrangea

Hydrangea, common name for certain deciduous and evergreen shrubs and woody vines. Hydrangeas are native to Asia and the Americas. The flowers are borne in clusters, the outermost flowers of which consist of a colored, membranous envelope, but the inner flowers of which have an eight- to ten-ribbed, tubular envelope, four or five ovate petals, eight to ten slender stamens, and a two-celled pistil. The fruit is a many-seeded capsule. The wild hydrangea of the eastern United States is an erect shrub that grows up to 3 m (up to 10 ft) high and bears white flowers in round clusters. The showier hydrangeas are cultivated Asian species, such as a typical cultivar of the hortensia, which grows up to 3.6 m (up to 12 ft) high; has broad, thick, shining leaves; and produces white, blue, or pink, flowers in round or flat clusters. The mock orange, a separate genus of the same family, also produces showy white flowers.

Scientific classification: The genus Hydrangea belongs to the family Saxifragaceae. The wild hydrangea is classified as Hydrangea arborescens and the typical cultivar of the hortensia as Hydrangea macrophylla cultivar hortensia.

Fennel

Fennel, common name for a perennial plant (see Parsley), characterized chiefly by its aromatic leaves and seeds. Fennel grows from about 61 to 122 cm (about 24 to 48 in) in height and has small yellow flowers. The common fennel is cultivated in both Europe and America. Its leaves and seeds are used for flavoring. Another variety, called variously Florence fennel, sweet fennel, Italian fennel, or Cretan fennel, is cultivated in southern Europe. The bases of the leafstalks of Florence fennel are greatly enlarged and form a bulbous structure, which is bleached by earthing and then eaten raw or cooked. The seed of Florence fennel is used widely in Italy as an aromatic condiment and as a medicine.

Scientific classification: Fennel belongs to the family Apiaceae (formerly Umbelliferae). It is classified as Foeniculum vulgare. The common fennel is classified as Foeniculum vulgare variety azoricum, and the Florence fennel as Foeniculum vulgare variety dulce.

Gametophytes

Spore, term applied to a specially modified asexual reproductive cell produced by many fungi and plants and by some protozoans. The spore is resistant to heat, drought, and other adverse conditions, remaining in a resting state until the environment is favorable for development or germination. Many bacteria (see Prokaryote) concentrate their cytoplasm and encapsulate under unfavorable conditions; these resting stages are generally called spores, but they are not reproductive cells and are therefore not comparable to the spores of other organisms.

Spores are usually produced by the division of cells within a structure called a sporangium. In bryophytes and most ferns, horsetails, club mosses, and whisk ferns, spores give rise to the same kind of plants as the parents, which are thus called homosporous. But in a few of those just mentioned and in all seed plants, some spores grow into sexual plants (gametophytes) that produce male gametes (sex cells), while others grow into sexual plants that produce female gametes; parent plants producing such spores are called heterosporous. Spores that give rise to male gametophytes are called microspores; spores giving rise to female gametophytes are known as megaspores. In seed plants, male gametophytes are known as pollen grains, and female gametophytes are called embryo sacs. Thick-walled resting zygotes (cells formed by the union of gametes) resemble spores and are called zygospores or oospores.

Brown Algae

Brown Algae, about 1500 species of almost exclusively marine, brown-colored algae, known as seaweeds, that make up the brown algae phylum in the protist kingdom. They are found mainly in the tidal zones of temperate to polar seas, but some exist in the deep ocean. Brown algae are the largest of the algae; well-known forms include the giant kelp and the free-floating sargassum weed. Their brown color is derived from the presence of the pigment fucoxanthin, which along with other xanthophyll pigments, masks the green color of the chlorophyll in the algal cells.

The brown algae are multicellular and have differentiated structures that, in some species, bear a superficial resemblance to the roots, stalks, and leaves of true plants. These structures, however, are quite different internally. The cell walls of the algae are made of a cellulose similar to that found in red algae; the outsides of the walls are covered by a gelatinous pectic compound called algin. The plants undergo an alternation of generations; the diploid phase (two sets of genes in a cell) is microscopic and brief, and the haploid phase (one set of genes in a cell) is macroscopic and comparatively long-lived. Brown algae such as kelp are harvested for use as an emulsion stabilizer, an ingredient of ice cream; as a fertilizer; as a vitamin-containing food source; and for iodine.

Scientific classification: Brown algae make up the phylum Phaeophyta in the kingdom Protista. Kelp are classified in the order Laminariales. Sargassum weed is classified in the genus Sargassum.

Desmid

Desmid, common name for numerous single-celled freshwater green algae. The cell is divided into two symmetrical halves, held together by an isthmus containing the nucleus. Each half of the cell contains a single chloroplast. Desmids sometimes grow in colonies, where individuals remain attached after cell division. Many species are capable of self-locomotion by gliding. They are phototropic; the direction of their movement is determined by their response to light.

Desmids reproduce both sexually and asexually. Asexually, the desmids reproduce by a simple division across the isthmus; the one cell becomes two. In sexual reproduction, the desmids come together in pairs and their cell contents fuse. From this fused body come two new desmids. Although this form of sexual reproduction does not increase the population, it does maintain genetic variation within the population.

Desmids of one common genus are crescent shaped and have a small vacuole at each end of the cell. These vacuoles contain minute crystals of gypsum, which are in constant Brownian motion.

Scientific classification: Desmids make up the family Desmidiaceae. Crescent-shaped desmids with vacuoles containing gypsum crystals make up the genus Closterium.