Protoplasmic Circulation - The streaming motion that may take place in the protoplasm of living cells.

Protoplasmic Respiration - Respiration taking place at the expense of protein materials in a starved plant.

Protoplast - The protoplasm, as distinct from the cell-wall.

Protoplast Fusion - Used for describing induced fusion of naked (wall-less) plant cells under culture conditions. Naked cells are obtained by enymataic digestion of cell walls, osmotic shock etc.

Protosome - A hypothetical central body present body in a gene.

Protospore - Refers to the Synchtriaceae when a uninucleate piece of protoplasm becomes the sporangium.

Protostele - Used for the most simple and primitive form of stele which consists of a central rod of xylem surrounded by a cyclinder of phloem and occurs in the stems of some ferns and lycopods, and is found more or less universally in roots.

Protothallus - Used for describing the first stages in the formation of a lichen thallus, often before the fungus and alga have become associated.

Proximal - (1) Situated towards the point of attachment.
(2) The part of a chromosome arm which is nearer to the centromere than another part.

Pruinose - (1) Covered with a waxy or powdery bloom.
(2) Covered with minute points which give a frosted appearance to the surface.

Prunasin - A glucoside in the bark of wild cherry. It is d-mandelo-nitrile glucoside.

Pruning - Used for describing the practice of cutting back some or all of the branches of a woody plant. It is done for removing dead or diseased wood, for changing plant into a desired shape, for promoting vigour of plant in case of fruit trees, for maintaining balance between vegetative growth and fruit formation.

Psammon - Refers to group of organisms that live between sand particles on a lake or sea shore. The food for bacteria protozoa, and other heterotrophic organisms of psammon is provided by diatoms and other algae.

Psammosere - Refers to the complete set of communities that develop in sandy habit during succession and ultimately gets stabilised such a habitat.

Pseudoalleles - Two or more mutations that are taking place in different parts of the same gene but causing similar effects on the phenotype.

Pseudocarp - (1) (false fruit) used for describing a fruit that incorporates tissues other than those derived from gynoecium. It may be developed from a single flower and include receptacle (pome) bracts etc., or may develop from whole inflorescence (multiple fruit).
(2) Used for describing a particular type of pseudocarp composed of a number of achenes that remain embedded in the outer surface of a fleshy receptale e.g., fruits of strawberry.

Pseudocopulation - Refers to the attempted copulation by a male insect with a flower or its par, which appears as female insect of its own species due to strikingly similar shape, colour or scent of the flower. Such attempt causes the pollination of the flower. This pollinating mechanism frequently occurs in Orchidaacae.

Pseudoendosperm - (1) Refers to the endosperm which is developed without involving fusion of polar or definitive nucleus with a male gamete but arising spontaneously from definitive nucleus or from somatic tissue.
(2) Refers to the tissue of female gametophyte in gymnosperms that nourishes the embryo. It is haploid and thus it is not analogous to endosperm of angiosperms.

Pseudogamy - Refers to a form of apomixis in which a diploid embryo is produced without fertiliation though stimulus from male gamete is needed to initiate it and so pollination becomes necessary.

Pseudoparenchyma - Refers to the plectenchyma having the appearance of parenchyma due to regular divisions in the fused hyphae. It is usually seen in fungi but also in certain red algae.

Pseudoperianth - Said of a relatively late-developing membranous sheath which is surrounding the young sporophyte in some liverworts e.g., Fossombronia, Sometimes also called perigynium (See perichaetium).

Pseudoplasmodium - A small slug-like aggregation of myxamoebae which is acting as a unit but within which each myxamoebaremains distinct. Its fruiting structure is a sorocarp.

Pseudopodium - (1) A leafless stalk used to raise the capsule in mosses that do not have the seta e.g., in Sphagnum.
(2) An extension in the body which is formed by flowing of cytoplasm into that area of the body that serves as a means of movement in amoepoid gametes.

Pseudothecium - A type of ascocarp which characteristically occurs in Loculoascomycetes and is superficially similar to perithecia but having double-walled acid that also differ in their development. They are often called perithecia.

Pubescent - Having fine short hairs.

Puff Balls - Refers to certain members of order Lycoperdales of Gasteromycetes in which basidiocarp do not open until maturity and its contents become dry and powdery. The ball, shaped basidiocarp relaeases spores in a series of puffs when it is disturbed e.g., Lycoperdon pyriforme.

Pulvinus - (1) Refers to the prominent swollen base of a petiole or pinna composed of motor cells that can rapidly lose water into or out of their vacuoles thereby resulting rapid changes in their turgour which is responsible for certain sleep movements and haptotropic movements.
(2) Used for thickened region in the stem or leaf sheath of grasses, often having intercalary meristem.

Punctuated Equilibrium - Refers to a theory of evolution postulating that species often arise rapidly in a short period of speciation lasting less than 100,000 years and then it is followed by a very long period of stasis that may last several million years before next such cycle of speciation and stasis comes. It means that evolution takes place in bursts puncturated by periods of stasis rather than by gradual change.

Punnet Square - Used for describing a chequerboard diagram which is attributed to geneticist R.C. punnet. It is used to illustrate how gametes involved in a particular cross result in the formation of various genotypes in various frequencies.

Pure Culture - Said of a culture of a pure stock of one species of plant. The term is used especially for beacteria, fungi and bacteria.

Pure Line - (1) The descendants which are obtained from self fertilisation of a single homogyous parent.
(2) An inbred homogyous strain.

Purine Group - A group of cyclic diureides which are derived from one molecule of a dibasic hydroxyacid and two molecules of urea. The simplest is purine.