Agriculture Reference
In-Depth Information
Perfect lower . A lower having both staminate (male) and pistillate (female) organs.
Perianth . A collective term for all the petals of a lower.
Pericarp . The ovary wall. It may be thin and fused with the seed coat as in corn, leshy as in berries, or hard
and dry as in pods of legumes.
Perisperm A type of endospermlike storage tissue in a mature seed that develops from the nucellus of the
parent plant—thus it has the 2n chromosome number. Examples of species with well-developed peri-
sperm tissue include beet and pigweed.
Petal . A unit of the inner perianth whorl or corolla.
Petiole . The stem of a leaf.
Physiological dormancy . Seed dormancy caused by internal physiological conditions that pre vent germi-
nation. Often referred to as epicotyl or embryo dormancy.
Phytochrome . The bluish photoreversible protein pigment responsible for the photoperiodic control of
lowering and seed germination. It exists in two forms in plants, the biologically active P FR (receptive
to far-red light) and the biologically inactive P R (receptive to red light).
Pipered embryo type . A type of embryo classiication in which the second wall of the zygote (fertilized
egg) is longitudinal, or nearly so.
Pistil . The female, or seed-bearing organ of the lower. It is composed of the ovary, style, and stigma.
Placentation . The method of attachment of the seeds within the ovary.
Plastid . Small cytoplasmic organelles containing pigments (e.g., chloroplasts, which give the green color
to plant leaves).
Plumule . The major leaf bud of the seed or seedling. That part of the embryonic plant axis above the coty-
ledons. Also known as epicotyl.
Pod . A fruit that is dry and nonleshy when ripe, and splits open at maturity to release its seeds.
Polar nuclei . Two nuclei of the female gametophyte (sex cell) that unite with one of the sperm cells to form
the endosperm of a developing seed.
Pollen . The small, almost microscopic, yellow bodies that are borne within the anthers of lowers and con-
tain the male generative (sex) cells. The mature microgametophyte.
Pollen tube . A microscopic tube that grows down the stigma from the pollen grain through which the sperm
cells are deposited into the embryo sac.
Pollination . The process by which pollen is transferred from the anther where it is produced to the stigma
of a lower.
Polyembryony . The condition in which an ovule has more than one embryo. This condition is common to
certain grasses.
Pome . A fruit in which the loral cup forms a thick outer leshy layer and that has a papery inner pericarp
layer (endocarp) forming a multi-seeded core (e.g., apple, pear).
Poricidal capsule . A capsule that at maturity splits open at pores near the top, releasing mature seed (e.g.,
poppy).
Prechill . The practice of exposing imbibed seeds to cool (5-10°C) temperature conditions for a few days
prior to germination at warmer conditions. See deinition of stratiication.
Prehydration . Soaking seeds in water or gels prior to planting to enhance germination, stand establish-
ment, and seedling growth.
Priming . Soaking seeds in aerated, low water potential osmotica such as polyethylene glycol or salts fol-
lowed by subsequent drying to enhance germination, stand establishment, and seedling growth (also
known as osmoconditioning).
Primorida . Organs in their earliest stage of development as a leaf primordia or meristem.
Proembryo . The young embryo in its early stages of development.
Protein . An essential constituent of all living cells. Proteins occur naturally and are complex combinations
of amino acids linked by peptide bonds.
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