CENTRIFUGE (sen-tri-fewj) An apparatus that is used to spin liquids in a circular motion at high rates of speed. Particles that are suspended in a liquid medium can be separated according to their density, the heavier particles collecting at the outer rim of the circle and the less dense ones collecting in layers toward the center.

CENTROMERE (sen-tro-mere) A short segment or region of the chromosome to which the spindle fiber appears to be attached when the chromosomes are separating during cell division. In stained preparations, this region is unstained. It is the last portion of the chromosome to divide during the process of chromatids formation; however, it precedes the remainder of the new chromosome during its migration to the spindle pole.

CEREALS Members of the grass family in which the seed is the most important part used for food and feed.

CHARACTER An identifiable hereditary property, such as a specific component of color, a structural detail, a color pattern, or resistance to disease.

CHLOROPHYLL (klor-oh-fill) The light-absorbing pigment in plants that gives them a green color. The absorption of light by chlorophyll is the first step in the manufacture of carbohydrates from carbon dioxide and water.

CHROMATID (kro-ma-tid) A half chromosome during early stages of cell division when it is still joined to its sister chromatid. After the half chromosomes separate, the chromatids are known as daughter chromosomes. Subdivisions of chromatids are called chromonemata. They are the forerunners of chromatids in the succeeding cell division.

CHROMATIN (kro-ma-tin) The chemical carrier of inheritance. It is the complex protein material within a cell which functions in cell multiplication to reproduce identical "daughter" cells. The material readily absorbs some dyes. In prepared microscope slides of multiplying cells, chromatin is commonly the most deeply stained portion of the protoplasm within the nucleus.

CHROMATOGRAPHY (kro-ma-tog-ra-fe) A method for the separation of compounds from one another. Separation is accomplished as a result of the movement of compounds at different rates in a solvent allowed to flow very slowly through a porous medium, such as paper.

CHROMOMERES (kro-mo-mere) Granules of protoplasm occurring along the chromosome thread. They are visible during early stages of cell division and are frequently thought of as the beads on a string that comprises the chromosome. The granules may be accumulations of nucleic acids and, in this event, actually comprise the gene. On the other hand, they may be expressions of different patterns of coiling along the chromosome thread.

CHROMOSOME (kro-mo-soam) A rodlike body contained in the nucleus of the plant cell; the bearer of the hereditary material. (Adjective: chromosomal.)

CLIMAX The final stage and condition of equilibrium of vegetation after a series of progressional stages which have developed, without serious interruption, under the influence of a given complex of environmental factors.

CLONE A group of individuals of common ancestry which have been propagated vegetatively, usually by cuttings or natural multiplication of bulbs or tubers.

COLCHICINE (kol-chi-seen) An alkaloid produced by the autumn crocus. It is commonly used to induce doubling of the number of chromosomes in the nuclei of plant cells.

COLEOPTILE (koh-lee-op-tile) A sheathlike leaf of grasses and other monocotyledons that protects the delicate growing point as it emerges from the soil.

COLUMELLA (col-yew-mell-a) An elongated floral axis that supports the carpels in certain plants.

COMA (koh-mah) A tuft of hairs attached to a seed.

COMPANION CROP A crop grown with another to secure an earlier or larger return than from one crop alone.

COMPOSITE MIXTURE Breeder seed obtained by mechanically combining seed from two or more strains. The mixture is increased through successive steps in a certified seed program and distributed as a synthetic variety.

CONIFER (konn-i-fur) A species of plant that bears its seeds in cones, such as a pine tree.

CORM Similar to a bulb, but the stem part is much thicker and broader and the scales form only a thin layer and represent a small part of the bulk. Gladiolus is an example.

CORN In American terminology, Zea mays, or Indian corn; includes sweet corn, popcorn, and field types. The word is so used in this book. In Old World terminology, "corn" may mean almost any of the Old World cereal grains.

COTYLEDON (kot-e-lee-done) Seed leaves of the embryo. Usually they are thickened for storage of reserve food. They may serve as true foliage leaves.

CROSS-FERTILIZE To fertilize the ovule or ovules of one flower with the pollen from another flower; commonly refers to the fertilizing of ovules of flowers of one plant by pollen from another plant.

CROSSOVER UNIT A measurement of the degree of linkage between two genes. It is expressed in percentages. Crossing over is the interchange of corresponding segments between two homologous chromosomes before the formation of sex cells. When the terminal portion of a segmental interchange occurs between linked genes, the association between them is broken. The frequency of such occurrence is a function of nearness of the genes. The crossover units separating two genes is equivalent to the percentage of sex cells in which the association has been broken through segmental interchange.

CROSS-POLLINATE To apply pollen of one flower to the stigma of another; commonly refers to the pollinating of the flowers of one plant by pollen from another plant.

CRUCIFEROUS (croos-if-er-us) Pertaining to plants in the family Cruciferae. These are tap-rooted plants with four sepals and four petals arranged in crosses. They include such crop plants as radish, turnip, mustard, rape, and the many cabbagelike crops.

CRYPTOXANTHIN (crip -toe -zan -thin) C₄₀H₅₆O, a yellow carotenoid pigment in plant parts similar to xanthophyll. It functions as a provitamin A to some extent.

CUTINIZE (kew-tin-ize) To impregnate a cell or a cell wall with cutin a complex fatty or waxy substance which makes the cell more or less impervious to air and moisture.

CYTOPLASM (sie-toe-plasm) The contents of a cell outside the nucleus or chromosome-bearing portion. In reproduction, the male parent normally contributes only chromosomes, whereas both nuclear and cytoplasmic constituents from the female parent become a part of the offspring.

CYTOPLASMIC MALE STERILITY A type of male sterility conditioned by the cytoplasm rather than by nuclear genes and transmitted only through the female parent.

DAMPING-OFF A disease of seeds or young seedlings caused by fungi. The disease is most evident in young seedlings that topple over and die just after they emerge from the soil (postemergence damping-off). Two other types of damping-off are often mistaken for poor seed rather than disease: Germination failure, in which a seed is invaded in the early stages of germination and fails to sprout; and preemergence damping-off, in which the young seedling is attacked before it pushes its way through the surface of the soil.

DAYLENGTH The number of hours of light in each 24-hour cycle.

DECORTICATION Removal of the pith and bark from fibrous other tissues. Generally, this is a mechanical process.

DEFOLIANT (dee-foe-lee-ant) A chemical or method of treatment that causes only the leaves of a plant to fall off or abscise. The fruits remain attached.

DEGENERATION The progressive decrease in vigor of successive generations of plants, usually caused by unfavorable growing conditions or diseases. Virus diseases cause great loss of vigor.

DEHISCENCE (dee-hiss-cents) The bursting open at maturity of a pod or capsule along a definite line or lines.

DESICCATE (DESICCATOR) (des-ik-kate, des-ik-kat-er) To dry thoroughly; to remove moisture from an object definitely below the normal level. A desiccator is a laboratory apparatus for thoroughly drying substances. It commonly consists of a glass container with an airtight lid. The drying agent, a desiccant, is placed in the container with the material to be desiccated. The desiccant absorbs water and water vapor very strongly and literally takes most of the water away from the material being dried.

DETASSEL To remove the tassel or pollen-producing organ at the top of a corn plant before pollen is released.

DIAPAUSE (die-a-pause) In insects, a state during which growth and development is temporarily arrested. Exposure to low temperature is frequently but not always required to permit the resumption of these processes.

DICOT (dye-kot) A short synonym of dicotyledonous plant, the term refers to plants which have two seed leaves in the seed. Dicot stems always have definite wood and bark layers, and the leaf veins are branched.

DIFFERENTIATION OF CELLS (dif-er-en-shiay-shun) The development of specialized kinds of cells from nonspecialized cells in a growing tissue.

DIFFUSIBLE (dih-fuze-ih-bul) Able to spread through a system.

DIMER ACID (die-mer) A chemical compound in which two fatty acid molecules are combined to form a single molecule.

DIOECIOUS (die-eesh-us) Having stamens and pistils on different plants. The plants are unisexual; therefore both sexes must be grown near each other to produce fruit, as in American holly.

DIPLOID (dip-loid) A plant with two sets of chromosomes.

DIPLOSPORY (dip-plo-spo-ree) The formation of a diploid egg cell as a result of failure of the egg mother cell to undergo normal reduction division.

DISPERSAL (diss-per-sal) Spreading or scattering.

DISTAL END The part of a fruit, leaf, tuber, or root farthest from its connection with the plant bearing it.