Session 5 Reading

Session 5 Reading

GYMNOSPERM

Gymnosperm (Latin gymn-,"naked"; Greek sperma,"seed"), common name for any seed-bearing vascular plant without flowers. There are several types: the cycad, ginkgo, conifer, yew, and gnetophyte. Gymnosperms are woody plants, either shrubs, trees, or, rarely, vines (some gnetophytes). They differ from the other phylum of seed plants, the flowering plants (see Angiosperm), in that the seeds are not enclosed in carpels but rather are borne upon seed scales arranged in cones. The gymnosperms are the most ancient seed plants; they appear to have arisen from fern ancestors in the Devonian Period. Cycads retain the most primitive characters of the extant seed plants. Living gymnosperms are distributed worldwide, with a majority, particularly the conifers, in temperate and subarctic regions. Cycads and gnetophytes are mainly tropical to subtropical. There are about 70 genera with 600 species of living gymnosperms, far less than many families of flowering plants.

CONIFERS

Conifer, common name for a group of plants that is characterized by seed-bearing cones and that includes about 550 of the 700 known species of gymnosperms, plants with seeds that are not enclosed in an ovary. There are two orders of conifers. Conifers are known from fossils more than 290 million years old. Although more species of conifers once existed, they are still a widely distributed group and are one of the world's most important renewable resources. In late 1994, descendants of a group of conifers widespread in the Cretaceous period were discovered in the Wollemi National Park, near Sydney, Australia. Thirty-nine trees, the tallest of which reaches a height of 40 m (130 ft), were found in a remote area of the rain forest. Fossils of the ancestors of the newly discovered trees exist, but scientists had believed that the group had become extinct 50 million years ago.

Conifers, like flowering plants (see Angiosperm), reproduce by means of seeds, which contain food tissue and an embryo that will grow into a plant. The seeds are borne on the scales of female cones rather than being surrounded by carpel tissue, and the pollen is produced in separate male cones rather than in anthers. Pollination in conifers is always dependent on wind currents to blow the abundant yellow pollen from the male cones to the female cones. Conifers usually have needle-shaped or scalelike leaves, and nearly all are evergreen. They typically have straight trunks with horizontal branches varying more or less regularly in length from bottom to top, so that the trees are conical in outline. They vary in size from shrubs to giant sequoias.

Scientific classification: Conifers constitute the phylum Pinophyta. The two orders of conifers are Pinales and Taxales. The newly discovered Australian conifers belong to Auracariaceae, a family of primitive trees.

SEEDS

A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant. The formation of the seed completes the process of reproduction in seed plants (started with the development of flowers and pollination), with the embryo developed from the zygote and the seed coat from the integuments of the ovule.

Seeds have been an important development in the reproduction and spread of flowering plants, relative to more primitive plants like mosses, ferns and liverworts, which do not have seeds and use other means to propagate themselves. This can be seen by the success of seed plants (both gymnosperms and angiosperms) in dominating biological niches on land, from forests to grasslands both in hot and cold climates.

SEED GERMINATION - The term germination is applied to the process of the growth of the seed embryo after the period of dormancy. Germination does not take place unless the seed has been transported to a favorable environment by one of the dispersal methods. The primary conditions of a favorable environment are adequate water and oxygen and suitable temperature. Different species of plants germinate best in different temperatures; as a rule, extremely cold or extremely warm temperatures do not favor germination. Some seeds also require adequate exposure to light before germinating.

During germination, water diffuses through the seed coats into the embryo, which has been almost completely dry during the period of dormancy, causing a swelling of the seed; the swelling is often so great that the seed coat is ruptured. With the absorption of oxygen by the seed, energy is made available for growth. The foodstuffs stored are broken down by enzymes into simpler substances that are transported through the embryo to the various centers of growth. From the time of germination until the plant is completely independent of food stored in the seed, the plant is known as a seedling.

SEED VIABILITY - Some seeds, such as those of the willow, are viable (capable of growing into healthy organisms) for only a few days after falling from the parent tree. Other seeds are viable for years—for example, seeds of the Oriental lotus have been known to germinate 3000 years after dispersal. Each species of plant has its specific period of viability; seeds sown after the period of optimum viability may produce weak plants or may not germinate.

SEED DORMANCY - Lack of viability of seed is often confused with seed dormancy. Many seeds require a so-called resting period after falling from the parent plant before they are able to germinate into new plants. Among the members of the orchid family, the seeds complete their maturation during this resting period. In other plants, chemical changes take place during the resting period that make the seed ready for germination. Still other seeds have extremely tough seed coats that must soften or decay before water and oxygen can enter the seed to take part in the growth of the embryo, or before the growing embryo is capable of bursting through the seed coat. Plant growers who wish to shorten the period of seed dormancy in seeds with undeveloped embryos can do little; germination may be induced, however, in seeds having mature embryos by abrasion of the hard coat, by soaking in water or in such chemicals as sulfuric acid, by heating to crack the seed coat, or by alternate freezing and thawing.

SEED TESTING - In most of the United States, law requires dealers to test seeds for viability and purity before putting them on the market. Seed testing also ensures the marketing of seed that is true to type—that is, seed that does not differ from the variety of plant desired.

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