look exactly like the parent. See also apomixis.
partial parasite (hemiparasite) A parasitic plant that has chlorophyll and can photosynthesize, but depends on another plant for its water and mineral salts. An example is mistletoe.
particle size distribution The proportion of particles of different sizes in a sample, usually expressed as weight percentages. There are standard terms for particles of different sizes. However, the same terms may be assigned to different size ranges by soil scientists and geologists. One of the commoner scales in use is the Udden-Wentworth particle size scale.
particulates 1. Substances made up of separate particles.
2. Particles of pollutants. Two types are recognized: primary particulates, which occur in smoke and industrial emissions; and secondary particulates, which are formed in the air by chemical reactions such as those involved in producing photochemical smog. See also PM10.
pascal (Pa) The SI unit of pressure. 1 pascal is a pressure of 1 newton per square meter. (A newton is the force needed to give an acceleration of one meter per second to one kilogram of mass.)
passerine A songbird with a perching habit. More than half of all living species of birds are passerines.
pasture Land covered in grass or other small plants, used as grazing by livestock.
patch dynamics The population dynamics associated with migration between discrete patches of habitat. It provides the basis for some models of competition.
patchiness 1. The degree to which a habitat shows spatial variation in its suitability for a particular species, or the degree to which the distribution of a population shows spatial variation within the habitat.
2. The distribution of different types of habitat within a landscape. Habitat quality may be affected by the presence of different habitat types nearby, providing such resources as nesting sites or pollinators. Landscape patchiness affects population dynamics and the success and survival of particular species.
patch-use model An optimal foraging model concerned with how foraging animals choose between patches containing prey.
pathogen Any organism that is capable of causing disease or a toxic response in another organism.
peat Partially decomposed plant material that accumulates in waterlogged anaerobic conditions in temperate humid climates, often forming a layer several meters deep. Peat varies from a light spongy material (sphagnum moss) to a dense brown humidified material in the lower layers. If mineral salts are present in the waterlogged vegetation, neutral or alkaline fen peat is formed (the salts neutralize the acid produced by decomposition). If there are no mineral salts in the water (as in rain), acid bog peat is formed. Peat is used as a fuel and is the first step in coal formation. It is also used to improve soil and as a component of potting compost in horticulture. These uses have led to concern about the destruction of natural peat bogs. See also bog; fen.
peck order See dominance.
ped See crumb structure.
pedogenesis The process of soil formation from unconsolidated parent material.
pedology The study of the formation, structure, distribution, and classification of soils.
pelagic Inhabiting the open upper waters rather than the bed of a sea or ocean.
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