Plant Science Terms - P

P - see Phosphorus

Palisade mesophyll - cells just beneath a leaf’s upper epidermis that contain most of the leaf’s photosynthesis

Palmate or palmately compound - leaf whose veins radiate outward from a single point somewhat like the fingers of a hand; form of espalier training

Palmate venation - major veins radiate from one point

Panicle - panicle is a much-branched inflorescence. In an indeterminate inflorescence a panicle is a branched raceme in which the branches are themselves racemes (e.g., Yucca recurvifolia). A panicle can also be a compound indeterminate inflorescence, a branched raceme in which each branch has more than one flower, as in the astilbe (Astilbe x. arendesii)

Parallel venation - major veins are parallel to each other, or nearly so

Parasite - organism that lives in or on another organism (host) and derives its food from the latter

Parasitic seed plant - plant that lives parasitically on other seed plants. An example is mistletoe

Parent material  - underlying geological material (generally bedrock or a superficial or drift deposit) in which soil horizons form

Parterre - formal garden in which shrubs, flowers, and paths form a geometric pattern of matched pairs

Parthenocarpic - development of fruit without fertilization

Partial Sun or Partial Shade - 4 to 6 hours of direct sun a day

Pathogen - organism that can cause a disease

Pathology - study of plant diseases

Ped - cluster of individual soil particles

Pedicel - stem of an individual flower

Peduncle - main stem supporting a cluster of flowers (as opposed to a pedicel, which is the stem of an individual flower)

Pendulous - hanging loosely; suspended so as to swing or sway

Perennial - plant that lives more than two years and produces new foliage, flowers, and seeds each growing season

Perfect flower - type of flower with both stamens and pistils

Perianth - collectively, sepals and petals form the perianth

Permanent wilting point - point at which a wilted plant can no longer recover

Permeability - rate at which water moves through a soil

Persistent - adhering to a position instead of falling, whether dead or alive, e.g., flowers or leaves; pesticide that retains its chemical properties in the soil for a long time

Pet Friendly - plants that are unlikely to be harmful to pets

Petals - highly colored portions of the flower, inside the sepals, that protect the inner reproductive structures. Often attract insects with their color or may contain osmophores which are scent structures both of which facilitate pollination

Petiole - stalk that joins a leaf to a stem; leafstalk

pH - acidity or alkalinity of a solution on a scale of 0-14, with a value of 7 signifying neutral, values below 7 signifying acidic, and values above 7 signifying alkaline. Relates to the concentrations of hydrogen (H+) ions in the soil. pH values are logarithmic

Phenological stage - crop development stage

Phenoxy - herbicides work to mimic IAA or auxin in broadleaf plants causing uncontrolled growth and eventual death

Pheromone - vapor or liquid emitted by an insect that causes a specific response from a receiving insect. Some pheromones are used to find a mate. Synthetic pheromones are used as attractants in insect traps

Phloem - principle nutrient-conducting structure of vascular plants

Phosphate - form of phosphorus listed in most fertilizer analyses

Phosphorus (P) - primary plant nutrient, especially important for flower production. In fertilizer, usually expressed as phosphate

Photoperiod - amount of time a plant is exposed to light

Photoperiodism - plant responses to light and dark periods that induce certain physiological reactions

Photosynthate - food product (sugar or starch) created through photosynthesis

Photosynthesis - process in which green plants convert light energy from the sun into chemical energy in order to produce carbohydrates; Formation of carbohydrates from carbon dioxide and a source of hydrogen (as water) in the chlorophyll-containing tissues of plants exposed to light

Phototropism - phenomenon of plants growing toward the direction of a light source

Physiographic - geological terrain that defines the range in which some plants occur, such as alpine plants that occur only in decomposed granite high on the slopes of mountains and survive extreme cold and winds, or desert plants which grow in sand and live on less than 10 inches of rain a year and survive temperatures over 100 degrees

Physiology - study dealing with the functioning of plants, their mechanisms of response, and their physical and biochemical processes

Phytoplasm - microscopic, single-celled organisms that lack distinct cell walls and that cause destructive diseases in plants

Phytotoxic - toxic to a plant

Picotee - pattern of flower petal coloration in which the edges of the petal are in a color that contrasts with the flower body

Pinch - remove a portion of the plant usually only the tip of the shoots, to encourage branching.  Often this is done by using your finger nails to pinch off the newest growth but scissors, pruning shears, or a knife can also be used. This causes axillary shoots or buds to develop. (See also Deadhead, Shear)

Pine fines - finely ground pine mulch also sold as a soil conditioner. A byproduct of the bark mulch industry, pine fines are too small to be sold as bark mulch, but make an excellent mulch for flower beds and container plantings (direct-seeded annual flowers can still push up through) and an excellent soil amendment to introduce organic matter into heavy clay soil

Pinnately compound - arrangement of leaflets attached laterally along the rachis of a compound leaf

Pinnatifid - leaf shape cleft nearly to the midrib in broad divisions not separated into distinct leaflets

Pistil - female component of the flower. It is in the center of the flower and has three parts, the stigma, the style, and the ovary

Pistillate - female flowers; flowers with no stamens (pistils only), also called imperfect because they lack the stamen

Plant classification - scientific grouping and naming of plants by characteristics

Plant disease - any lasting change in a plant’s normal structure or function that deviates from its healthy state

Plant growth regulator - see Growth regulator

Plant nutrition - plant’s need for and use of basic chemical elements. (See also Macronutrient, Micronutrient)

Plant pathology - study of diseases in plants: what causes them, what factors influence their development and spread, and how to prevent or control them

Plant tissue culture - plant material grown in vitro under sterile conditions in an artificial medium. A primary means of rapidly increasing the number of plants from a single mother plant

Pleach - intertwine branches of trees, vines, or shrubs to form an arbor or hedge

Pleniflora - term used in botanical names to indicate a double-flowered cultivar. (See also Double)

Plug - 2- to-4-inch chunks of sod, either round or square, with soil around their roots

Plumule - shoot portion of an embryo

Point source - single, identifiable source of pollutants such as a factory or municipal sewage system. (See also Nonpoint source)

Pollard - method of tree pruning that involves heading back severely to main branches each year so as to produce a thick, close growth of young branches

Pollen - plant’s male sex cells, which are held on the anther for transfer to a stigma by insects, wind, or some other mechanism

Pollen tube - slender tube growing from the pollen grain that carries the male gametes and delivers them to the ovary

Pollination - first step in fertilization; the transfer of pollen from anther to a stigma

Pollinator - agent such as an insect that transfers pollen from a male anther to a female stigma

Pollinizer - plant whose pollen sets fruit on another plant. (See also Cross-pollination)

Polychromatic - use of all the colors and their tints, shades, and tones

Pome fruit - fruit having a core, such as an apple, pear, or quince

Pomology - science of fruits and the art of fruit culture, especially tree fruits

Porespace - spaces within a rock body or soil that are unoccupied by solid material

Postemergent - product applied after crops or weeds emerge from the soil

Potash - form of potassium listed in most fertilizer analyses

Potassium (K) - primary plant nutrient, especially important for developing strong roots and stems. In fertilizers, usually expressed as potash

Powdery mildew - fine, white to gray, powdery fungal coating on leaves, stems, and flowers

Power raking - using fixed knife-type blades that slice thatch as opposed to ripping it out

Predator - animal that eats another animal

Preemergence - product applied before crops or weeds emerge from the soil

Preharvest interval - amount of time that must elapse (legally) after application of a pesticide before harvest takes place

Preplant - product applied before a crop is planted

Prickle - rigid, straight, or hooked outgrowth of bark or stems. Often called a thorn, but technically different. Roses are examples of plants with prickles. (See also Thorn)

Primary growth - growth that occurs via cell division at the tips of stems and roots

Primary nutrient - nutrient required by plants in a relatively large amount (nitrogen, phosphorus, and potassium)

Primocane - first-year growth, usually vegetative, on cane berries. Only fall-bearing raspberries produce fruit on primocanes in late summer

Processed fertilizer - fertilizer that is manufactured or refined from natural ingredients to be more concentrated and more available to plants

Production - nursery or greenhouse growing area used before plants are put up for retail sales

Prolegs - plump, fleshy, and often hooked to allow the caterpillar to hold onto a plant

Propagate - start new plants by seeding, budding, grafting, dividing, etc

Propagule - structure capable of being propagated or acting as an agent of reproduction

Protozoa - from of a diverse group of eukaryotes, of the kingdom Protista, that are primarily unicellular, existing singly or aggregating into colonies

Provenance - area within a plant’s native range that seed or propagation materials such as cuttings were collected

Prune - using pruning shears, scissors, a knife, or loppers to remove plant parts to improve a plant’s health, appearance, or productivity. Removal can shape or rejuvenate a plant, not to increase branching.  Generally pruning is much more drastic than pinching.  Pruning is most commonly used on shrubs, trees, and perennials

Pseudobulb - thickened, aboveground, modified stem that serves as a storage organ. Found in some orchids

Psocoptera - major order of insects that have two pairs of wings, or are wingless, and chewing mouthparts (barklice, booklice)

Pubescent - leaf textures that are hairy

Pupa - stage between larva and adult in insects that go through complete metamorphosis

Pupae - insect in the non-feeding stage between the larva and adult, during which it typically undergoes complete transformation within a protective cocoon or hardened case. Only insects that undergo complete metamorphosis have pupal stage


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