Passerines
A group of small or medium-sized perching songbirds.  More than half of all birds belong to this group.

PEP carboxylase
An enzyme used by C4 and CAM plants during the initial step of carbon fixation.  Unlike rubisco, PEP carboxylase does not have the ability to fix oxygen, which contributes to lower photorespiratory carbon losses in C4 and CAM plants.

Perennial plants
Plants that are long-lived and may persist for years in a given location.

Permafrost
Subsoil that remains frozen year round.

Petri dish
A shallow, circular, glass or plastic dish with a loose-fitting cover over the top and sides, typically used for culturing bacteria and other microorganisms.

Phenology
The study of plant growth and development with respect to the timing of various growth stages, i.e. flowering, fruit development and senescence.

Phloem
The specialized vascular plant tissue used for the transportation of dissolved sugars and other organic solutes within a plant.

Phosphorus
An essential mineral element that is required for proper growth and development of plants.  It is a component of many compounds within cells and plays a major role in energy production.  Its most common deficiency symptoms in plants include stunted growth and malformed leaves that may contain necrotic spots.

Photoinhibition
A reversible process that reduces the efficiency at which absorbed light is used during the light-dependent reactions of photosynthesis.

Photorespiration
A process that begins when rubisco fixes molecular oxygen, as opposed to carbon dioxide, which ultimately leads to the evolution of CO2 from plants.  This process of carbon loss is stimulated by conditions of high light, temperature and oxygen concentration.

Photosynthate
Carbohydrates, including glucose and sucrose among others, that are produced from the end products of photosynthesis.

Photosynthesis
The process by which plants use sunlight, water, and carbon dioxide to produce their food.

Photosystem II
A protein complex embedded within the thylakoid membranes that plays an integral role in the light-dependent reactions of photosynthesis.

Pigments
Organic molecules located within the thylakoid membranes of leaves that have the capacity to absorb sunlight during the process of photosynthesis.

Planetary degassing
Any process that results in the escape of volatile substances from the crust of a planet.  An example is the release of CO2 to earth's atmosphere via vulcanism.

Planktonic foraminifera
Small, single cell aquatic organisms having a shell perforated by pores (foramina) through which slender filaments project.

Plate tectonics
A theory explaining the present and past locations of continents due to massive crustal movement.

Pleistocene
The period of time in earth's history, beginning about 2,000,000 years ago and continuing to the present day, during which the planet has experienced a succession of back-to-back 100,000-year glacial-interglacial climate cycles.

Poikilochlorophyllous desiccation-tolerant plants
Species that lose their chlorophyll and thylakoid membranes during desiccation but resynthesize them upon rehydration.

Polar vortex
The system of winds that circles each of earth's polar regions and effectively isolates them from the rest of the atmosphere during each hemisphere's winter season.  Under such conditions, temperatures within the polar vortex can drop to the very low values that are needed to sustain the polar stratospheric clouds that provide the catalytic surfaces for the chemical reactions thought to lead to stratospheric ozone depletion.

Poor man's biosphere
The experimental unit that is employed in our Global Change Laboratory, wherein a range of atmospheric CO2 concentrations may be readily established for studying the responses of plants to atmospheric CO2 enrichment and depletion.

Postulate
A claim that is considered to be self-evident or so plain as to require no illustration or proof.

Pothos
The common Devil's Ivy or Golden Pothos (Scindapsus aureus) plant that we have utilized in many of our "poor man's biosphere" experiments to demonstrate the effects of atmospheric CO2 enrichment and depletion on plant growth and development.  It is sold in most plant nurseries worldwide.

Precipitation Efficiency
The efficiency with which atmospheric moisture is converted to precipitation, often described as the ratio of precipitation to total available moisture.

Primary producers
The assemblage of plant species that fix the majority of the carbon that sustains an ecosystem.

Primary root
A root that extends from some portion of a plant other than another root.

Primates
The most highly developed order of mammals, including man, the apes, lemurs, and monkeys.

Protozoa
Small, usually single-celled microorganism that lives in the soil.  They are non-photosynthetic and feed upon dead or live bacteria and fragments of organic matter.

Proxy data
Data obtained from objects that are sensitive to climatic phenomena.  Some examples are tree ring widths, ice cores, pollen deposits, glacier lengths and deep sea sediments.  Analyses of such data can be used to provide estimates of past climate conditions, such as temperature, precipitation, or wind speed.