Stonewort

Stonewort, common name for about 200 species of algae belonging to the phylum Charophyta of the plant kingdom. Stoneworts are so called because the plant surface is usually covered by a thick, brittle, limy crust of calcium carbonate. They are closely related to green algae and have sometimes been regarded as a class (Charophyceae) within that phylum.

Stoneworts grow submerged in hard (alkaline) water and often form dense mats on the bottom of ponds and brackish waters. They are much larger than typical green algae and superficially resemble higher plants in their rootlike rhizoids, leaflike branches at regular intervals, and upright cylindrical axes surrounded by a sheath of cells. Asexual reproduction usually involves the shedding of reproductive branches. Sexual reproduction is by the formation, at leaf nodes, of a large egg in a female sexual structure called an oogonium, and of sperm in filaments constituting the male sexual organ, or antheridium.

Deciduous Plants

Deciduous Plants, plants that shed their leaves annually at the approach of a season of cold or drought. The term is also applied to plants that drop leaves, fruits, or flowers at maturity, in contrast to those that retain their foliage or flowers. Plants that keep their foliage throughout the year are called evergreens.

Evergreen

Evergreen, name applied to any plant that retains its foliage through two or more consecutive seasons. In contrast to deciduous plants, which shed their leaves seasonally, evergreens retain their foliage anywhere from 1 to 18 years. Many common varieties, such as spruce, fir, and other members of the pine family, have needle-shaped foliage, but most evergreens are broad-leaved. Some shrubs and trees are both deciduous and evergreen, depending on where they are grown. Many species are valuable for their timber, while mistletoe and other evergreen foliage is popular as Christmas decoration.

Grasses

Grasses, common name for a large family of flowering plants that is economically and ecologically the most important in the world. The grass family contains about 635 genera and 9000 species, making it the fourth largest family after the legume, orchid, and composite families (see Composite Flowers).

IMPORTANCE

All the world’s cereal crops are grasses, and thus the grass family is economically very important. The world’s 5 top crops produce more tonnage than the next 25 combined, and 4 of the top 5 are the cereals rice, wheat, corn, and barley. Human well-being depends on these few grasses, so even small crop failures of any one of them can produce widespread hunger and economic disruption. In addition, the family provides most of the world’s sugar (see Sugarcane). Another member of the family, bamboo, is an important construction material as well as a food source; it also has been used in papermaking. Citronella, used both in perfumery and as an insect repellent, is an oil distilled from the leaves of certain grasses.

Grasses are the primary source of food for domestic and wild grazing animals, which feed on pastures and grasslands and which are fed hay and silage harvested from them. The total land area devoted to these kinds of croplands is greater than the land area for all other kinds of croplands combined.

Another economically significant use of grasses is for the lawns maintained in many parts of the world. Perennial grasses are well adapted for use in lawns because their basal meristems (growing points) are not lost with mowing. Bermuda grass and zoysia are better adapted to the warm conditions of the southern states. Often special grasses are used where particular conditions of soil or exposure make the more common species unsuitable.

OTHER GRASSLIKE PLANTS

Plants of two other groups, the sedges and the rushes, are superficially similar to grasses and are often confused with them. The three groups are easily distinguished, however, by examining their flowers, stems, and leaves. Rushes have flowers with six-part perianths (floral whorls) and from three- to many-seeded fruits, whereas the flowers of sedges and grasses have no perianth and have single-seeded fruits. Sedges usually occur in moist habitats and have solid stems that are triangular and bear three rows of leaves. Grasses usually occur in dry habitats and have round, hollow stems with two rows of leaves.

Scientific classification: Grasses make up the family Poaceae (formerly Gramineae). Citronella is distilled from grasses of the genus Cymbopogon. Kentucky bluegrass is classified as Poa pratensis. Bent grasses are classified in the genus Agrostis. Fescue grasses are classified in the genus Festuca. Bermuda grass is classified as Cynodon dactylon. Zoysia grasses are classified in the genus Zoysia. Buffalo grass is classified as Buchloë dactyloïdes, carpet grass as Axonopus affinis, redtop as Agrostis gigantea, and annual bluegrass as Poa annua.

Halophyte

Halophyte, term for salt-resistant plants, coined by the U.S. geologist Oscar E. Meinzer (1876-1948). Found in salt marshes and mangrove swamps, many halophytes could thrive in well-watered lands except for their inability to compete with the other plants found there. Generally fleshy and with gray-green foliage, many halophytes deal with the salt in their environment by eliminating it through special salt-secreting glands in their leaves. Others store the salt in leaves and stems that are shed at the end of the growing season, while the remaining halophytes keep salt from entering their cells by means of semipermeable membranes in their roots. One typical genus is that of the saltbush, Atriplex.