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 PALEOBOTANY Paleobotany is the branch of botany that studies the plants that existed in former geologic periods, chiefly by studying fossils. |  PALEONTOLOGY Paleontology is the branch of biology that studies the forms of life that existed in former geologic periods, chiefly by studying fossils. "Paleo" means old or ancient. "Ontology" is the study of existence ("onto-" means existence, "-logy" is the study of something). |  PALEONTOLOGIST A paleontologist is a scientist who studies paleontology, the forms of life that existed in former geologic periods, chiefly by studying fossils. "Paleo" means old or ancient. "Ontology" is the study of existence ("onto-" means existence, "-logy" is the study of something). This "-ist" at the end means a person who is involved in the field. |
| PALEOZOIC ERA The Paleozoic Era (540 to 248 million years ago) saw an explosion of new life forms. The Paleozoic (meaning "ancient life") ended with the largest mass extinction in geologic history and was followed by the Mesozoic Era. It is divided into the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods. It was followed by the Mesozoic Era, the time of the dinosaurs. | PALISADE PARENCHYMA The palisade parenchyma (also called the palisade layer) is a layer of tightly-packed, tube-shaped, chlorophyll-containing cells (chlorenchyma) that is the major photosynthetic layer of a leaf. The palisade layer is located in the upper mesophyll of a leaf (just beneath the upper dermal layer of the leaf). | PALM A palm is an evergreen tree and a monocot. Classification: Division Magnoliophyta (Angioperms), Class Liliopsida (Monocots) , Subclass Arecidae, Order Arecales, Family Arecaceae (or Palmae) (Palms). |
 PALMATE A palmate leaf has a hand-like structure. It has more than three lobes that branch from a single point at the base of the leaf. |  PALMATE VENATION A leaf with palmate venation has the main veins arising from a point at the base of the leaf. |  PANGAEA (pronounced pan-GEE-ah) Pangaea was a supercontinent consisting of all of Earth's land masses. It existed during the Permian through the Jurassic period. It began breaking up during the Jurassic, forming the continents Gondwanaland and Laurasia. |
PANTHALASSA (pronounced pan-tha-LASS-ah) Panthalassa (meaning "All seas") was the super-ocean that existed on Earth during the time of the super-continent Pangaea. Panthalassa existed during the Permian through the Jurassic period, when Pangaea began to break up; the Tethys sea formed between the northern and southern parts of pangaea as they drifted apart. |  PARAPHYLETIC A paraphyletic group (also called a grade) consists of a common ancestor and some, but not all, of its descendants. These are incomplete groups based primarily on physical characteristics rather than directly on evolutionary relationships. An example of a paraphyletic group is the dinosaurs (without including the birds). |  PARALLEL VENATION A leaf with parallel venation (also called longitudinal-striate venation) has its veins arranged almost parallel to one another. |
PARASITE A parasite is an organism which uses its host for food and/or shelter. A parasite gives its host nothing in return, and often makes it sick or even kills the host. Termites are a parasite of many trees. | PARASITISM Parasitism is arelationship between two organisms in which one organism benefits at the other organism's expense. Lice are an example of a parasite that affects many animals; termites are a parasite that are destructive to many trees. Parasitism is a type of symbiosis. | PARENCHYMA Parenchyma are generalized (undifferentiated) cell or tissue in a plant. Parenchyma cells make or store food; they can often divide or differentiate into different types of cells and have thin cell walls. Parenchyma is the most common type of plant cell. The pith is parenchyma cells at the center of the primary stem of a dicot. |
PARSIMONY Parsimony is the scientific idea that the simplest explanation of a phenomenon is the best one. |  PARTED A parted (also called cleft) leaf is one in which the margins between the irregular teeth go more than halfway to the midrib. |  PEANUT PLANT The peanut plant (Arachis hypogea) is a legume that is native to South America (it now grows around the world). The peanut plant grow to about 60 cm tall and has yellow flowers (1-2 cm long). The flower blooms for about half a day. About 4 days later, a stem (also called a peg) will grow from the flower and head into the soil. At the end of each stem, the seed pods (peanuts in the shell) will grow. The peanut is an annual plant (it completes its life cycle in one year). |
PEAT Peat is a type of soil that is composed of incompletely decomposed plant material that water-logged and low in oxygen. | PEDICEL A pedicel is plant stalk that attaches a single flower or fruit to the main branch of the inflorescence. | PEDOGENESIS Pedogenesis is the natural process in which soil forms. |
 PEDUNCLEA peduncle is plant stalk that bears an inflorescence or single flower. | PENNSYLVANIAN PERIOD The Pennsylvanian Period lasted from 325 to 280 million years ago. During this time, the first reptiles (like Hylonomus) appeared and ferns dominated the warm, swampy landscape. | PERENNATE A plant that perennates lives from year to year (it is a perennial). |
PERENNIAL A perennial plant has a life cycle that lives for more than two years. Perennials usually flower each year. | PERFECT FLOWER A perfect flower has both male (stamen) and female (ovary) reproductive organs on the same flower. (Compare with imperfect flowers.) | PERIANTH The perianth (which means "around the anthers") is the sepals and petals of a flower. |
PERICARP The pericarp is the fruit wall that develops from the ovary wall. The pericarp is divided into the endocarp, mesocarp and exocarp. | PERICARPAL The pericarpal is the upper part of the flower stem, the receptable, and the lower part of the pistil. | PERIOD The period is the basic unit of geological time in which a single type of rock system is formed, lasting tens of millions of years. |
PERMIAN EXTINCTION The Permian extinction was the largest mass extinction that ever occurred on Earth, but its causes are unknown. It occurred at end of the Permian period, about 248 million years ago. It in, trilobites went extinct, as did 50% of all animal families, 95% of all marine species, and many trees. Groups that went extinct included: the fusulinid foraminifera, trilobites, rugose and tabulate corals, blastoids, acanthodians, placoderms, and pelycosaurs (like Dimetrodon). Groups that were substantially affected included: bryozoans, brachiopods, ammonoids, sharks, bony fish, crinoids, eurypterids, ostracodes, and echinoderms. This extinction was followed by the Triassic period of the Mesozoic Era. |  PERMIAN PERIOD The Permian period (named after the Perm Province in northeast Russia where rocks from this period were first described) is known as "The Age of Amphibians" (280 to 248 million years ago), this is when Pangaea formed and Earth's atmosphere was oxygenated to modern levels. In the early Pemian, labyrinthodonts dominated the land. The Permian ended with the largest mass extinction and was followed by the Mesozoic Era. | PERMINERALIZATION Permineralization is the process in which minerals are deposited into a bony fossil. |
| PETAL A petal is one of the leafy structures that comprise a flower. Petals are often brightly-colored and have many different shapes. They are located between the sepals and the flower's reproductive organs. | PETIOLATE Petiolate means having a petiole. |  PETIOLE A petiole is a leaf stalk. On a compound leaf, the petiole extends from the stem to the first set of leaflets. A leaf without a petiole is sessile. |
| PETRIFICATION Petrification is the process in which an organic tissue turns to stone. The original materials are repaced by minerals. |  PETRIFIED WOOD Petrified wood is fossilized wood. Minerals seeped into buried logs, replacing the original tissues with rock. | PHANEROZOIC EON The Phanerozoic (meaning "visible life") is the time in which life forms with skeletons or hard shells existed. It is the period from about 540 million years ago until the present. |
PHENETICS Phenetics is a method of attempting to classify biological organisms that does not use genetic or evolutionary information; it was invented by Sokal and Sneath in 1963. In a phenogram, organisms are grouped by superficial overall similarity. Phenetics was abandoned by most scientists in the 1980's because its classifications were arbitrary, mostly useless, and unstable. Paul Ehrlich was a proponent of this system. | PHLOEM Phloem is plant tissue that conducts nutrients (food) through the plant. In woody-stemmed plants, the phloem is the inner layer of the bark. (Compare with xylem.) | PHOMA (pronounced Pho-ma) Phoma is a genus of fungus that lives on a wide variety of plants (including sunflower, cabbage, banana, etc.). Phoma is characterized by large, black lesions on the stem and/or leaves of the infected plant. About 80 species of phoma have been described. |
PHOTOSYNTHESIS Photosynthesis is the process in which plants convert sunlight, water, and carbon dioxide into food (sugars and starches), oxygen and water. Chlorophyll or closely-related pigments (substances that color the plant) are essential to the photosynthetic process. | PHOTOTROPISM Phototropism is the bending of a plant in response to sunlight. This reaction is caused by the growth hormone auxin that is contained in the stem. | PHYLLOCLADE A phylloclade is a flattened stem that looks like a leaf. |
PHYLLODE A phyllode is a leaf that has an enlarged midrib and no blades. | PHYLOGENY Phylogeny is the evolutionary relationship between organisms. The phylogeny of an organism reflects the evolutionary branch that led up to the organism. | PHYLUM (plural plyla) In classification, a phylum is a group of related or similar organisms. A phylum contains one or more classes. A group of similar phyla forms a Kingdom. The plant phyla include: Ginkgophyta, Lycophyta (lower ferns like clubmosses), Pterophyta (ferns), Psilophyta (whisk ferns), Anthophyta (flowering plants), Gnetophyta, Sphenophyta, Coniferophyta (conifers), Cycadophyta (cycads), Sphenophyta, and Bryophyta (mosses, liverworts, hornworts). |
 PINNATE COMPOUND LEAF A pinnate compound leaf is made up of many small leaflets arranged in pairs on either side of a long central midrib (the rachis). There is often a single terminal leaflet at the end of the midrib. |  PINNATELY LOBED LEAF Pinnately lobed leaves have many lobes arranged along the midrib. |  PINNATE VENATION A leaf with pinnate venation has its veins arranged in pairs coming from a main central midrib vein (the rachis). |
PIONEER A pioneer is a plant that is the first (or among the first) to live in a new area. | PISTIL The pistil is the central set of female reproductive organs in a flower. The pistil is composed of one or more carpels and produces the ovule. | PISTILLATE Pistillate flowers have a pistil or pistils. |
PITH The pith is parenchyma cells at the center of the primary stem of a dicot. | PLANT A plant is a member of the kingdom Plantae, a living organism that undergoes photosynthesis. | PLANTAE Plantae is a kingdom in the classificaton of life on Earth. Plantae (plants) is divided into non-vascular plants, vascular plants, and seed plants. |
PLANT SUCCESSION Plant succession is the natural pattern of ecosystem growth and change over time for a particular environment. Plant life follows established patterns of growth and change after major distruptions, like fires, floods, agricultural damage, logging, etc. Generally, smaller, fast-growing herbaceous species and grasses grow first in an open field, followed in a few years by softwood tree seedlings and larger herbaceous species. As a young forest develops into a mature forest (30 to 70 years), an understory of smaller hardwood trees develops. The final stage is a climax hardwood forest (100 plus years). | PLASMA MEMBRANE A plasma membrane (also called the cell membrane or plasmalemma) surrounds each cell and separates it from the environment. In pants, the cell membrane contains cellulose (a carbohydrate). | PLASMALEMMA A plasmalemma (also called the cell membrane or plasma membrane) surrounds each cell and separates it from the environment. In pants, the cell membrane contains cellulose (a carbohydrate). |
| PLASMID A plasmid is a circular loop of DNA found in prokaryotic cells (like those of bacteria). Eukaryotic cells prokaryotic cells (like plant and animal cells) do not have plasmids, they have chromosomes. | 
PLATE TECTONICS Plate tectonics is the now-established theory that chunks of the Earth's crust (plates) float on the surface and change both position and size over time. | PLATYSPERMS Platysperms (meaning "flat seeds") are plants with flattened ovules. Platysperms include gingkos, conifers, glossopterids, and cordaites. |
PLEISTOCENE The Pleistocene was an epoch of geologic time that lasted from 1.8 million to 11,000 years ago; it was a period of widespread glaciation and large Ice Age animals (caled Pleistocene Megafauna). The first humans (Homo sapiens) evolved during the Pleistocene. Mammoths, mastodons, saber-toothed cats, giant ground sloths, and other Ice Age mammals evolved. A mass extinction of large mammals and many birds occurred about 10,000 years ago, probably caused by Ice Ages. | PLESIOMORPHY Plesiomorphy (meaning "old form") is a primitive character of a group. | PLUMULE The plumule is the shoot of a plant embryo (in the seed before germination). |
PNEUMATOPHORE A pneumatophore is an air-transporting vessel in plants. | POD A seed pod is an elongated, two-sided vessel that contain several fertilized seeds. It is a dehiscent fruit or pedicarp - the pod splits open when the seeds are mature. Beans and peas are some plants that have pods. | PODZOL Podzol is a kind of soil that is very low in humus. |
POLLEN Pollen is the male reproductive cell of flowering plants and cone-bearing plants. Pollen grains are produced in the anther of a flower. | POLLENATION Pollination is the transfer of pollen from the anther to the stigma. | POLLEN TUBE After the male's pollen grains have landed on the stigma during fertilization, pollen tubes develop within the style. The pollen tubes transport the sperm from the grain to the ovum (where fertilization of the egg occurs and the seeds will develop). |
 POLYPHYLETIC A polyphyletic group consists of organisms but not their common ancestors. This is an artificial group which is based primarily on physical characteristics rather than on evolutionary relationships. An example is "flying vertebrates" which includes birds, pteranodons and bats. | POLLINATION Pollination is the process in which the male's pollen fertilizes the female's ovule and creates a seed; the pollen is transferred from the male's anther to the female's stigma. Pollination is effected by the wind, insects, hummingbirds, etc. | POLLINIUM A pollinium (or pollinia) is a mass of fused pollen grains that is produces by a single anther. This mass sticks together and during pollination is transported as a single unit. Orchids have polliniums. |
PRECAMBRIAN The Precambrian is the time 540 before the Cambrian period (before million years ago). It is the time from when the Earth formed until simple life-forms evolved. | PREHISTORIC Prehistoric refers to the time before people began recording history in writing. This time varies from culture to culture. | PREY An animal becomes prey when another animal hunts and kills it for food. Insects are the prey of the Venus fly trap. |
PRICKLE A prickle is a a sharp outgrowth of a plant's epidermis (outer skin). Prickles are easily broken off a plant. Roses have prickles. |  PRICKLY PEAR CACTUS Prickly pear cacti (genus Opuntia, many species) are North American desert succulents that have flat, fleshy, leaf-shaped pads and large spines (modified leaves) growing from tubercles (small bumps on the pads). They have red, yellow, or purple flowers. Prickly pear cacti and their fruit are edible. | PRIMARY ROOT The primary root is the first root of a plant to develop in the germinating seed. The primary root develops from the radicle of the embryo. It is also called the taproot |
PRIMARY VEGETATION Primary vegetation is the plants life that has been in place since the area reached its climax state. |  PRIMARY CONSUMER A primary consumer is an herbivore (a plant-eater). Primary consumers occupy the second trophic level in the grazing food chain. | PRIMARY FOREST A primary forest is a forest that has never been logged or disturbed. |
PRIMARY GROWTH Primary growth is plant growth that occurs at the tips (terminal bud) of the stem or the tip of the roots, at the apical meristems. In seed plants, primary growth produces . |   PRODUCER A producer (or aurotroph) is an organism that makes its own food from light energy (using photosynthesis), or chemical energy (using chemosynthesis). Most green plants, many protists (one-celled organisms like slime molds) and most bacteria are producers. Producers are the base of the food chain. | PROTOPLAST A protoplast is a plant or bacterial cell whose cell wall has been removed. |
| PROTOZOA Protozoans (meaning "first animals") are a phylum of primitive animals that include the following classes: Mastigophora (flagellates), Sarcodina (amoebas), Sporozoa (Parasites), and Ciliata (Ciliates). | PSAMMOPHILE A psammophile (meaning "sand loving") is a plant that lives in sand or sandy soils. Some psammophytic plants include Salix psammophila (a willow), Dypsis psammophila (a palm) and Cocolobo diversifolia (the pigeon plum). | PSILOPHYTA Psilophyta are a division of primitive plants that are also called wisk ferns (they are not true ferns). Psilophytes are seedless vascular plants with underground rhizomes. They live in tropical to sub-tropical areas and are terrestrial or epiphytic. There are only 2 genera (Psilotum and Tmesipteri), and only a few living species. |
 PTERIDOPHYTESPteridophytes are a group of primitive vascular plants that include Lycopods (club mosses), Sphenopsids (horsetails, shown left), and ferns (shown, right). These plants reproduce with spores that germinate only in moist areas; they also reproduce using rhizomes (underground stems). Pteridophytes evolved during the Devonian and were mostly low-growing during the Mesozoic Era. These fast-growing, resilient plants were a source of food for plant-eating dinosaurs that lived in moist areas. |  PTERIDOSPERMS Pteridosperms (Seed ferns) were primitive seed plants (not ferns at all) that lived in swampy areas from the Mississipian Epoch through the Mesozoic Era. They had woody stems studded with dried out leaf bases. The tops had fern-like fronds which bore seeds. Some seed ferns include Glossopteris (pictured above), Dicroidium, Caytonia, Denkania, and Lidgettonia. | PTEROPHYTA Pterophyta (also called Pteridopsida) is a phylum of plants that includes some ferns, ancient vascular plants. There are about 11,000 living species of Pterophytes. |
PULSE A pulse refers to a leguminous plant that produces edible seeds or to the seeds themselves. Some pulses include peas, beans, and lentils. |  PUMPKIN The pumpkin (Cucurbita pepo) is an annual vine from the New World. |
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