| P | The chemical symbol for phosphorus. (see below) |
| palisade mesophyll | The cells just beneath a leaf’s upper epidermis that contain most of the leaf’s chlorophyll and are responsible for most photosynthesis. |
| palmate | A form of espalier training. |
| palmate venation | A leaf whose veins radiate outward from a single point somewhat like the fingers of a hand. |
| palmately compound leaf | A leaf in which the leaflets radiate from one point. |
| panicle | A highly branched inflorescence. |
| parallel venation | A vein pattern in which the veins are parallel to each other. |
| parasite | Any animal or plant that lives in, or on, another animal or plant and withdraws nutrients from its host. |
| parasitic seed plant | A plant that lives parasitically on other seed plants. An example is mistletoe. |
| parenchyma | A thin-walled, undifferentiated cell. |
| parterre | A formal garden in which shrubs, flowers, and paths form a geometric pattern of matched pairs. |
| parthenocarpic | Development of fruit without fertilization. |
| pathogen | Any organism that causes disease. Generally applied to bacteria, viruses, fungi, nematodes, and parasitic plants. |
| pathology | The study of diseases. |
| pectin | A substance in cell walls binding cells together. |
| ped | A cluster of individual soil particles. |
| pedicel | The stem of an individual flower. |
| peduncle | The main stem supporting a cluster of flowers (as opposed to a pedicel, which is the stem of an individual flower). |
| pendulous | More or less hanging or declined. |
| perennial | A plant that lives two or more years and produces new foliage, flowers, and seeds each growing season. |
| perianth | Collectively, all external flower parts. |
| pericarp | The fruit wall, derived from the ovary wall. |
| pericycle | A root tissue giving rise to branch roots. |
| permeability | The rate at which water moves through the soil. |
| persistent | (1) Adhering to a position instead of falling, whether dead or alive, e.g., flowers or leaves. (2) A pesticide that retains its chemical properties in the environment for a long time. |
| petals | The usually showy structures around a flower’s reproductive organs. |
| petiolate leaf | A leaf in which the blade is attached to a stem by a petiole. |
| petiole | The stalk of a leaf. |
| pH | A scale measuring the acidity or alkalinity of a sample. What the pH scale actually measures is the hydrogen ion (H+) concentration present. pH values run from 0 (the most acidic value possible) to 14 (the most alkaline value possible). pH values from 0 to 7 indicate acidity, a pH of 7 is considered to be neutral, while pH values from 7 to 14 indicate alkalinity. The scale is logarithmic, thus a difference of 1 pH unit is equal to a 10-fold change in acidity or alkalinity (depending on the direction), a difference of 2 pH units indicates a 100-fold change, and a difference of 3 pH units indicates a 1,000-fold change. |
| phenological stage | Crop development stage. |
| phenotype | The physical appearance of an organism. |
| pheromone | A vapor or liquid emitted by an insect that causes a specific response from a receiving insect. Some pheromones are used to attract a mate. Synthetic pheromones are used as attractants in insect traps. |
| phloem | Photosynthate-conducting tissue. See xylem. |
| phosphate | The form of phosphorous listed in most fertilizer analysis (P2O5). |
| phosphorous (P) | A primary plant nutrient, especially important for flower production. In fertilizer, usually expressed as phosphate (P2O5). |
| photoinduce | To initiate a physiological process as a result of being exposed to a specific photoperiod. |
| photoperiod | The amount of time a plant is exposed to light. |
| photosynthate | A food product (sugar or starch) created through photosynthesis. |
| photosynthesis | The process in green plants of using sunlight to convert water and carbon dioxide into sugar. |
| phototropism | A growth response to light. Growth of a plant toward a light source is the most common example. |
| phytoplasma | A microscopic, bacteria-like organism that lacks a cell wall. Previously called mycoplasma. |
| phytotoxic | Toxic to a plant. |
| picotee | A pattern of flower petal coloration in which the edges of the petal are a contrasting color to the body. |
| pilose | Having long, soft hairs. |
| pinch | To remove a growing tip from a stem, thus causing axillary shoots or buds to develop. See deadhead, shear. |
| pinnately compound leaf | A leaf in which the leaflets are arranged on both sides of a common axis. |
| pinnate venation | A leaf vein pattern in which the major veins are arranged in rows on each side of the midrib. |
| pistil | The female sexual organ of a flowering plant, made up of the stigma, style, and ovary. |
| pit | A small opening in a cell wall. |
| pith | A region of parenchyma cells at the center of a stem. |
| plagiotrophic | Growth of a branch at an angle. |
| plant growth regulator | See growth regulator. |
| plant nutrition | A plant’s need for and use of basic chemical elements. See macronutrient, micronutrient. |
| plasmolysis | Shrinkage of cytoplasm away from cell walls due to water loss. |
| pleach | To intertwine branches of a tree, vine, or shrub to form an arbor or hedge. |
| plena | A term used in botanical names to indicate a double-flowered variety. See double. |
| pleniflora | See plena (see above). |
| pleno | See plena (see above). |
| point source | A single, identifiable source of pollutants such as a factory or municipal sewage system. See nonpoint source. |
| pollard | A 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 | A plant’s male sex cells, which are held on the anther for transfer to a stigma by insects, wind, or some other mechanism. |
| pollinizer | A plant whose pollen sets fruit on another plant. See cross-pollination. |
| pollination | The transfer of pollen from a male anther to a female stigma, enabling fruits to set and develop. |
| pollinator | An agent, such as an insect, which transfers pollen from a male anther to a female stigma. |
| polyploid | Having three or more sets of chromosomes per cell. |
| pome fruit | A fruit having a core, such as an apple, pear, or quince. |
| pomology | The science of fruits and the art of fruit cultivation, especially fruit trees. |
| post emergent | A product applied after crops or weeds emerge from the soil. See preemergent, preplant (see below). |
| potash | The form of potassium listed in most fertilizer analysis (K2O). |
| Potassium (K) | A primary plant nutrient, especially important for developing strong roots and stems. In fertilizers, usually expressed as potash. See potash. |
| predator | An animal that eats another animal. |
| preemergent | A product applied before crops or weeds emerge from the soil. See post emergent (above), preplant (below). |
| preharvest interval | The period of time that must pass from the time a pesticide is applied to a crop until the crop is safe to pick and use. |
| preplant | A product applied before a crop is planted. See post emergent, preemergent (above). |
| prickle | A rigid, straight, or hooked outgrowth of bark or stems. Often called a thorn, but technically different. Roses are examples of plants with prickles. See thorn. |
| primary growth | Growth arising from cellular activities in apical meristems. |
| primary nutrient | A nutrient required by plants in a relatively large amount (nitrogen, phosphorus, and potassium). See macronutrient. |
| primocane | First-year growth, usually vegetative, on caneberries. Only fall-bearing raspberries produce fruit on primocanes late in summer. |
| processed fertilizer | A fertilizer that is manufactured or is refined from natural ingredients to be more concentrated and more available to plants. |
| prop root | A supportive root growing from an aboveground stem. |
| propagate | To start new plants by seeding, budding, grafting, dividing, etc. |
| protoplasm | The living substance of cells, including cytoplasm and nucleus. |
| prune | To remove plant parts to improve a plant’s health, appearance, or productivity. |
| pseudobulb | A thickened, aboveground, modified stem that serves as a storage organ. Found in some orchids. |
| pubescent | Having short hairs. |
| pupa | The stage between larva and adult in insects that go through complete metamorphosis. |
