Pistillar - Club-shaped.

Pistillate - A flower which has carpels but no anothers.

Pit - (1) Refers to a small sharply-defined area of a plant cell-wall which remains un thickened when the rest of the wall gets thickened.
(2) . The two opposite thin areas in the walls of two cells or vessels in contact.
(3) A local thin spot in the wall of the oogonium of some Oomycetes.

Pit Cavity - Refers to the excavation in a cell-wall where the thinning is apparent.

Pitcher - An urn-shaped or vase-shaped modification of a leaf or part of a leaf. It serves to trap insects or other small animals which are killed or digested.

Pitted Thickening - Refers to the type of secondary wall patterning in tracheary elements in which continuity of secondary cell wall has been broken only by pits and performation plates (if present).

Placenta - (1) The part of an ovary to which the seeds get attached.
(2) Any mass of tissue to which sporangia or spores get attached.

Placental Cell - A food-supplying cell for the developing carpo-sporophyte or some red algae.

Placentation. Refers to the arrangement of the placentas in a syncarpous ovary.
(a) pariental-the carpels are fused only by their margins so that the placentas then appear as internal ridged on the ovary wall.
(b) axile-the margins of the carpel fold inwards fusing together in the centre 'of the ovary, forming a single central placenta.
(c) free central-the placenta is a central up growth from the base of the ovary.

Plagioclimax - An equilibrium is developing in an area in response to extensive grazing.

Plankton - The more or less free-floating animals and plants that are living near the surface of a sea or lake.

Plank Root - A root very markedly flattened so that it stands out from the base of a stem like a plank set on edge. They give additional support to the plant.

Plaque - A clear area in a plate of bacteria formed due to lysis of bacterial cells following infection of bacteriophage.

Plasmagene - Refers to the self-reproducing particle in the cytoplasm of a cell, affecting the characteristics of the cell bearing it.

Plasmalemma - A thin membrane which is surrounding the protoplasm, or a cell organelle. It is about 1/100µ thick and consists .of fat and protein. It is responsible for the restricted penetration ofmany substances into the cell. Severe damage to it kills the cell. In plants it is in contact with the cell-wall, and around vacuoles.

Plasmid - Refers to any small piece ofDNA which is found in cytoplasm of bacteria. It is being capable ofautonomous replication e.g. R-factors (residance factors that carry genes for antibiotic resistance in pathogenic bacteria of some types).

Plasmodesmata - The strands of cytoplam passing through cells walls and connecting the protoplasts of adjacent cells.

Plasmodium - A multinucleate, naked, amoeboid plant-body as found in the Myxothallophya and some hrysophyta.

Plasmgamy, Plastgamy - (1) The fusion of cytoplasm, in contrast to the nucleoplasm.
(2) The fusion of2 sexual cells. .

Plasmolysis - Refers to the collapsing ofthe cell-protoplasm from the cellulose cell-wall, due to the movement ofwater from the cell-sap into a solution of higher osmotic concentration surrounding the cell.

Plasmon - The cytoplasm ofan individual cell, which is considered as a single hereditary unit. The sum ofthe plasmagenes.

Plasmosome (Nucleolus). A Small cytoplasmic granule. A type ofnucleolus which stains with acidic dyes, and disappears during mitosis without mingling with the chromosomes.

Plasticity - The ability ofa material to change shape continuously under applied stress, and retains the impression after the removal ofthe stress. The plasticity ofa soil gives an indication ofits moisture content.

Plastic Material - Any substance used up in growth processes.

Plastid - A small, variously shaped, self-propogating body present in the cytoplasm of plant cells. They are possibly special centres of chemical activity. They may be coloured (chromoplast) of colourless (leucaplast). Chloroplasts are specialized plastids.

Plastid Inheritance - Inheritance determined by plastogenes.

Plastid Mutation - A change in a chloroplast which is affecting its ability to produce chlorophyll.

Plastocyanin - A blue copper-containing protein pigment occurring in chloroplast. It mediates transfer of electrons from photosystem II too photosystem I by oxidation and reduction Of its copper. Its redox potential is + 0.37 volt and it has been situated possibly very close to photosystem I, between cytochrome f and photosystem I.

Plastogene - A gene attached to a plastid, which determines the likeness of the daughter plastid to the parent.

Plastoglobuli - Refer to lipid droplets, 10-500 nm in diameter, occurring singly or in groups in the chloroplast stroma. These are not surrounded by membrane but retain their form even when they are isolated from stroma.

Plastoquinone - Said of an electron transferring substance present in chloroplast having a redox potential of zero. It mediates transfer of electrons from photosystem II to photosystem I. It is situated between cytochrome b3 and cytochrome f and ATP is synthesised during transfer of electron to cytochrome f.

Platyspermic - Used for seeds that are flat, bilaterally symmetrical as in Cordaitales. Compare radiospermic.

Plectostele - A type of protostete in which xylem is present in several plates, each surrounded by phloem on all sides as in some species of Lycopodium.

Plectenchyma - A form of 'tissue' which is occurring commonly among higher fungi. It is formed by a mass of interwoven anastomosing hyphae. It is called prosenchyma when formed by long fused hyphae and pseudoparenchyma when it appears cellular due to regular divisions in hyphae.

Pleiocene (Pliocene). Refers to the last epoch of Tertiary period of Cenozoic era between 13-1 million years ago. In this era climate was rather cold and dense forests were declined. Like woody plants, herbaceous angiosperms and monocotyledonous plants spread rapidly.