photoperiodism Response by an animal or plant to changes in daily, seasonal, or yearly cycles of light and darkness. Among animals, sleep, migration, reproduction, and the changing of coats or plumage are regu¬ lated to some extent by day length. In the poultry industry, photoperio¬ dism is commonly induced by artificial lighing to maximize egg laying and body weight. Plant growth, seed setting, germination, flowering, and fruiting are also affected by day length. Other environmental factors that modify an organism’s responses include temperature and nutrition.
photoreception Biological responses to stimulation by light, most often referring to the mechanism of vision. In one-celled organisms such as the amoeba, the whole cell may be sensitive to light. Earthworms have photoreceptive cells scattered over their bodies to help orient themselves by comparison of light intensities in different directions. Most animals have localized photoreceptors of varying complexity. In humans, photo¬ reception relies on the chemical response of a light-sensitive pigment, rhodopsin, in photoreceptor cells in the retina of the eye. Stimulation of those cells results in a stimulus being conducted toward the nervous sys¬ tem. Humans, like other vertebrates, have two types of photosensitive cells, rod cells and cone cells. Rod cells are responsible for vision when there is little light; cone cells mediate daylight vision and colour. Photo¬ reception also refers to photosynthesis in plants. See also sense.
range from about 18,000 °F (10,000 °C) at the bottom to 8,000 °F (4,000 °C) at the top; its density is about 1/1,000 that of air at the surface of Earth. Sunspots are photospheric phenomena. The photosphere has a granular structure. Each grain (cell), a mass of hot gas several hundred miles in diameter, rises from inside the Sun, radiates energy, and sinks back within minutes to be replaced by others in a constantly changing pattern.
photosynthesis Process by which green plants and certain other organisms transform light into chemical energy. In green plants, light energy is captured by chloro¬ phyll in the chloroplasts of the leaves and used to convert water, carbon dioxide, and minerals into oxygen and energy-rich organic compounds (simple and complex sugars) that are the basis of both plant and animal life. Photosynthesis consists of a number of photochemical and enzy¬ matic reactions. It occurs in two stages. During the light-dependent stage (light reaction), chlorophyll absorbs light energy, which excites some electrons in the pigment mol¬ ecules to higher energy levels; these leave the chlorophyll and pass along a series of molecules, generating for¬ mation of NADPH (an enzyme) and high-energy ATP molecules. Oxy¬ gen, released as a by-product, passes into the atmosphere through pores in the leaves. NADPH and ATP drive the second stage, the dark reaction (or Calvin cycle, discovered by Melvin Calvin), which does not require light. During this stage glucose is generated using atmospheric carbon dioxide. Photosynthesis is crucial for maintaining life on Earth; if it ceased, there would soon be little food or other organic matter on the planet, and most types of organisms would disappear.
phototube See photocell
photovoltaic effect Process in which two dissimilar materials in close contact act as an electric cell when struck by light or other radiant energy. In crystals of certain elements, such as silicon and germanium, the electrons are usually not free to move from atom to atom. Light striking the crys¬ tal provides the energy needed to free electrons from their bound condi¬ tion. These electrons can cross the junction between two dissimilar crystals more easily in one direction than another, so one side of the junc¬ tion acquires a negative voltage with respect to the other. As long as light falls on the two materials, the photovoltaic battery can continue to pro¬ vide voltage and current. The current can be used to measure the bright¬ ness of the light or as a source of power, as in a solar cell.
Phrachomklao See Mongkut Phrachunlachomklao See Chulalongkorn Phramongkutklao See Vajiravudh
phratry \'fra-tre\ In ancient Greece, tribal subdivisions of households or families claiming a common kin relation. At the apex of each phratry were aristocratic clans that had certain hereditary rights, such as the right to hold priestly offices. Both phratries and clans were patrilineal. The Athenian phratry is well known. Every native Athenian male belonged to a phratry; they were concerned with such matters as legitimacy of descent and inheritance.
photosphere Visible surface of the Sun, about 250 mi (400 km) thick. Phraya Taksin See Taksin It emits most of the Sun’s light that reaches Earth directly. Temperatures
© 2006 Encyclopaedia Britannica, Inc.
1494 I phrenology ► physics
phrenology \fri-'na-l3-je\ Study of the shape of the skull as an indication of mental abilities and character traits. Franz Joseph Gall stated the principle that each of the innate mental faculties is based in a specific brain region (“organ”), whose size reflects the faculty’s prominence in a person and is reflected by the skull’s surface. He examined the skulls of persons with par¬ ticular traits (including “criminal” traits) for a feature he could identify with it. His followers Johann Kaspar Spurzheim (1776-1832) and George Combe (1788-1858) divided the scalp into areas they labeled with traits such as combativeness, cautiousness, and form perception. Though popu¬ lar well into the 20th century, phrenology has been wholly discredited.
Phrygia Vfri-je-oN Ancient district, west-central Anatolia. It was named for a people whom the Greeks called Phryges and who dominated Ana¬ tolia between the Hittite collapse (12th century bc) and ascent of Lydia (7th century bc). The Phrygians were possibly of Thracian origin (see Thrace) and had their capital at Gordium. The kingdom of their legendary ruler, Midas, ended c. 700 bc with the invasion of the Cimmerians, who burned the capital. The Phrygians excelled in metalwork, wood carving, carpet making, and embroidery. Their religious cult of the Great Mother of the Gods was passed on to the Greeks. Excavations conducted since 1945 have uncovered carved stone tombs and shrines there.
Phumiphon Adunlayadet See Bhumibol Adulyadej
Phyfe \'fif\, Duncan orig. Duncan Fife (b. 1768, near Loch Fan- nich, Ross and Cromarty, Scot.—d.
Aug. 16, 1854, New York, N.Y.,
U.S.) Scottish-born U.S. furniture designer. His family settled in Albany, N.Y., c. 1784; there he became an apprentice cabinetmaker and eventually opened his own shop.
In 1792 he moved to New York City, where he changed the spelling of his name and became so successful that he employed 100 carvers and cabi¬ netmakers. He was one of the first Americans to use the factory method of manufacturing furniture success¬ fully. Though he did not originate a new furniture style, he interpreted fashionable European styles—
Sheraton, Regency, Directoire,
Empire —with such grace that he became a major exponent of Neo- classicism. His furniture, decorated with typical period ornaments such as harps and acanthus leaves, was generally of high-quality mahogany.
phyle Vf!-le\ Any of several tribes wm that formed the largest political subgroups in all Dorian and most Ionian
Greek city-states. Phylae were simultaneously kinship groups, corpora¬ tions with their own officials and priests, and local units for administra¬ tive and military purposes. Athens’s original four phylae were replaced 3 by 10 under a reorganization (508/507 bc) initiated by Cleisthenes; Spar¬