ecomii - a better way
April 23, 2014  |  Login
Seed

A fertilized ovule containing an embryo which forms a new plant upon germination. Seed-bearing characterizes the higher plants—the gymnosperms (conifers and allies) and the angiosperms (flowering plants). Gymnosperm (naked) seeds arise on the surface of a structure, as on a seed scale of a pine cone. Angiosperm (covered) seeds develop within a fruit, as the peas in a pod. Flower Fruit

Structure

One or two tissue envelopes, or integuments, form the seed coat which encloses the seed except for a tiny pore, the micropyle . The micropyle is near the funiculus (seed stalk) in angiosperm seeds. The hilum is the scar left when the seed is detached from the funiculus. Some seeds have a raphe, a ridge near the hilum opposite the micropyle, and a bulbous strophiole. Others such as nutmeg possess arils, outgrowths of the funiculus, or a fleshy caruncle developed from the seed coat near the hilum, as in the castor bean. The embryo consists of an axis and attached cotyledons (seed leaves). The part of the axis above the cotyledons is the epicotyl (plumule); that below, the hypocotyl, the lower end of which bears a more or less developed primordium of the root (radicle). The epicotyl, essentially a terminal bud, possesses an apical meristem (growing point) and, sometimes, leaf primordia. The seedling stem develops from the epicotyl. An apical meristem of the radicle produces the primary root of the seedling, and transition between root and stem occurs in the hypocotyl. Root (botany) Stem

Two to many cotyledons occur in different gymnosperms. The angiosperms are divided into two major groups according to number of cotyledons: the monocotyledons and the dicotyledons. Mature gymnosperm seeds contain an endosperm (albumen or nutritive tissue) which surrounds the embryo. In some mature dicotyledon seeds the endosperm persists, the cotyledons are flat and leaflike, and the epicotyl is simply an apical meristem. In other seeds, such as the bean, the growing embryo absorbs the endosperm, and food reserve for germination is stored in fleshy cotyledons. The endosperm persists in common monocotyledons, for example, corn and wheat; and the cotyledon, known as the scutellum, functions as an absorbing organ during germination. Grain embryos also possess a coleoptile and a coleorhiza sheathing the epicotyl and the radicle, respectively. The apical meristems of lateral seed roots also may be differentiated in the embryonic axis near the scutellum of some grains.

Many so-called seeds consist of hardened parts of the fruit enclosing the true seed which has a thin, papery seed coat. Among these are the achenes, as in the sunflower, dandelion, and strawberry, and the pits of stone fruits such as the cherry, peach, and raspberry. Many common nuts also have this structure. Mechanisms for seed dispersal include parts of both fruit and seed. Population dispersal

Economic importance

Propagation of plants by seed and technological use of seed and seed products are among the most important activities of modern society. Specializations of seed structure and composition provide rich sources for industrial exploitation apart from direct use as food. Common products include starches and glutens from grains, hemicelluloses from guar and locust beans, and proteins and oils from soybeans and cotton seed. Drugs, enzymes, vitamins, spices, and condiments are obtained from embryos, endosperms, and entire seeds, often including the fruit coat. Most of the oils of palm, olive, and pine seeds are in the endosperm. Safflower seed oil is obtained mainly from the embryo, whereas both the seed coat and embryo of cotton seed are rich in oils.Plant anatomy Reproduction (plant)

Physiology

Physical and biochemical processes of seed growth and germination are controlled by genetic and environmental factors. Conditions of light, temperature, moisture, and oxygen affect the timing and ability of a seed to mature and germinate. Seed development (embryogenesis) is concerned with the synthesis and storage of carbohydrate, protein, and oil to supply nutrients to the germinating seedling prior to soil emergence. Seed development occurs in several stages: rapid cell division, seed fill, and desiccation. The timing of each stage is species-specific and environmentally influenced.

Dormancy

Seed dormancy is the inability of a living seed to germinate under favorable conditions of temperature, moisture, and oxygen. Dormancy does not occur in all seeds, but typically occurs in plant species from temperate and colder habitats. This process allows for a delay in seed germination until environmental conditions are adequate for seedling survival. At least three types of seed dormancy are recognized: primary, secondary (induced), and enforced. Primary dormancy occurs during seed maturation, and the seed does not germinate readily upon being shed. Secondary and enforced dormancy occur after the seed is shed and may be caused by adverse environmental factors such as high or low temperature, absence of oxygen or light, low soil moisture, and presence of chemical inhibitors. Seeds with secondary dormancy will not germinate spontaneously when environmental conditions improve, and need additional environmental stimuli. Seeds with enforced dormancy germinate readily upon removal of the environmental limitation. Regulation of dormancy may be partly controlled by hormones.Dormancy

Dormancy is terminated in a large number of species when an imbibed seed is illuminated with white light. Biochemical control of this process is related to the functioning of a single pigment, phytochrome, frequently located in the seed coat or embryonic axis. Phytochrome imparts to the seed the ability to interpret light quality, such as that under an existing vegetative canopy, and to distinguish light from dark with respect to its position in the soil. Phytochrome also is affected by temperature and is involved in the seasonal control of the ending of dormancy. Hormones that promote germination of dormant seeds include gibberellins, cytokinins, ethylene, and auxins.

Germination

Germination is the process whereby a viable seed takes up water and the radicle (primary root) or hypocotyl emerges from the seed under species-specific conditions of moisture, oxygen, and temperature. Dormant seeds must undergo additional environmental stimuli to germinate. The germinating seed undergoes cell expansion, as well as increases in respiration, protein synthesis, and other metabolic activities prior to emergence of the growing seedling.

 Back to all terms
From McGraw-Hill Concise Encyclopedia of Environmental Science. The Content is a copyrighted work of McGraw-Hill and McGraw-Hill reserves all rights in and to the Content. The Work is © 2008 by The McGraw-Hill Companies, Inc.
 
 
 
 
ecomii featured poll

Vote for your Favorite Charity

 

 

 
 
ecomii resources
 
ecomii Tips Newsletter 

Sign up today to receive a weekly tip for living greener

 
Get in Touch

Got suggestions? Want to write for us? See something we could improve? Let us know!