Glossary - Seed Testing - page 323

Nucellus. The tissue of the ovary wall in which the archesporial cell arises and where megaspor ogenesis,
megagametogenesis, and ovule development occur.
Nuclear endosperm. The type of endosperm in which the early development is characterized by rapid cell
enlargement accompanied by nuclear division without cell wall formation.
Nucleic acid. A highly complex organic molecule found in the nucleus of cells; believed to be the substance
that determines heredity and governs the behavior of all cells.
Nucleus. The part of the cell bearing the chromosomes.
Nut . A dry, indehiscent, one-seeded fruit with a hard, woody shell.
Nutlet. A small, dry, indehiscent fruit composed of one-half a carpel enclosing a single seed; developed by
folding and splitting the carpel into a compound pistil.
Operculum (seed). A type of epistase (integumentary proliferation) that is deposited inside the ovule,
forming a tight-itting micropylar or chalazal plug in the mature seed; contributes to water imperme-
ability and hard seed coat dormancy.
Orthotropus ovule. The simplest type of ovule arrangement in which the ovule is erect, with the micropyle
at one end and the funiculus at the other.
Other crop seed (percentage). One of the four components of a purity test; the total percentage (by weight)
of seed of all crop species each comprising less than 5% of the seed lot.
Ottawa seed blower. A type of seed blower developed by C.W. Leggitt of the Canadian Depart ment of
Agriculture that has a slender metal blowing tube used for small seeded crops.
Ovary. The part of the pistil containing the ovule.
Ovoid . Egg-shaped.
Ovule . The structure within the ovary of the lower that becomes the seed following fertilization and
development.
Ovum . An egg cell.
Palea . One of the thin bracts of a grass loret enclosing the caryopsis that is located on the side opposite
the embryo.
Palisade layer . In seeds, this term is used interchangeably with Malpighian layer.
Panicle . An inlorescence in which the lateral branches arising from the peduncle produce lower-bearing
branches instead of single lowers (e.g., Avena sativa ) .
Paper-piercing test . A stress test for seedling vigor utilizing sand covered by ilter paper through which the
seedlings must emerge to be considered vigorous.
Pappus . A tuft of delicate ibers or bristles such as the feathery appendage on a ripe dandelion seed repre-
senting a modiied calyx.
Parietal cell . The sister cell of the megaspore mother cell originating from the division of the archesporial
cell. It is nonfunctional and usually degenerates.
Parietal placentation . A type of placentation in which the seeds are attached in the ovary near the outer
ovary wall; usually associated with vestiges of septa rather than along the ovary axis as in other types
of placentation.
Parthenocarpy . Production of fruit without seeds as in bananas and some grapes.
Pathogen . Any organism capable of causing disease by obtaining its nutrition either partially or wholly
from its diseased host.
PCR. An acronym for polymerase chain reaction , which is a technique to amplify a particular piece of DNA
to produce thousands or millions of copies of that DNA sequence. Among its applications are genetic
ingerprinting and detection of speciic genes as in genetically modiied seeds, and disease diagnosis.
Pedicel . The stalk of a loret.
Pelleted seeds . Seed that are commercially prepared for precision planting by pelleting them inside a spe-
cial preparation to make them more uniform in size. Sometimes special nutrient or growth-promoting
substances are placed in the pellets to aid in seed germination and growth.
Pepo . A fruit with a hard rind without internal separations or septa (e.g., cantaloupe, water melon, cucumber).