Prosthetic Group - A non-protein group which is attached to a protein. If the protein is an enzyme, the prosthetic group is essential for carrying out its functioning.

Protandrous - (1) The male gametes which are developing before the female gametes. (2) The anthers which are ripening before the stigma(s) of the same flower is receptive.

Protamine - A simple protein of relatively low molecular weight, and a high proportion of bases.

Proteases - Enzymes that are attacking the peptide bond in protein molecules.

Protective Layer - A layer of suberized cells lying across the place where a leaf comes away at leaf-fall. It checks water-loss, and the entry of parasites.

Protein - A compound having carbon, hydrogen, oxygen, and nitrogen, and frequently sulphur and phosphorus. They are synthesized fromamino-acids and form one of the main constituents of protoplasm. They are mainly α-amino-acid residues which are joined by peptide linkages.

Protein Synthesis - Involves the multistage process by which there occurs the joining of amino acids by peptide bonds in the cell cytoplasm in accordance to the information derived from genetic material of cell nucleus thereby forming a specific protein molecule.

Proteolysis - Refers to the breadown of proteins into their constituent amino acids by proteolytic enzymes.

Proteolytic Enzyme - Any enzyme hydrolysing protein.

Prothallial Cell - (1) A single cell which is cut off early in the division of a microspore of some heterothallic pteridophytes. It represents the vegetative tissue of the male gametophte. (2) A small cell in the pollen grain of the gymnosperms, representing the male prothallus.

Prothallus, Prothallium - The independent gametophyte plant of the Pteriodophyta. It is a small, green parenchymatous thallus bearing antheridia and archegonia. It shows little differentiation, and is usually prostrate on the soil-surface, to which it is attached by rhizoids. (3) The similar stages in the life-cycle of the gymnosperms. (4) The earliest stages in the development of a lichen thallus.

Prothecium - A primitive or rudimentary perithecium.

Protista - A term sometimes used to include all unicellular organisms, both plant and animal.

Protobiont (coacervate). Refers to an aggregate of large complex organic compounds which are bounded by an organic membrane. Such aggregate are regarded as precursors of living cells in certain theories of origin of life.

Protocooperation (Facultative mutualism). Used for describing the mutually beneficial biotic interaction in which both organisms can also can independently e.g., symbiotic relationship between Rhizobium bacterium and leguminous plant.

Protoderm - Said of the outermost layer of apical meristem. Its derivatives give rise to epidermis and sometimes also to associated subepidermal tissue.

Protogyny - Refers to the maturation of female reproductive organs before there occurs the maturation of male organs as in Rosaceae and Cruciferae.

Pressure Potential (turgour potential). Refers to a component of water potential which is denoted by symbol ΨD. It may be defined as the pressure exerted by cell contents on the cell wall (turgour pressure) and is usually positive but may be zero in cells at incipient plasmolytic stage or that are plasmolyzed. Negative pressure potential may also be developing in xylem elements due to rapid transpiration and the resistance of tissue to water flow.

Prickle - Said of an outgrowth of epidermis which is short, hard, woody and pointed. It may be simply protective or may be helping the plant in hooking itself to some support as the curved prickles of roses. Compare spine, thorn.

Primary Consumer - Refers to an organisms whose position is just after producer in the food chain in the ecosystem. It uses producers as its food material. Herbivores are primary consumers in autotrophic ecosystem.

Primary Endosperm Nucleus - A nucleus formed after fusion of polar nuclei or definitive nucleus of embryo sac with one of the male gametes released from the pollen tube.

Primary Growth - Refers to an increase in size occurring due to cell division at apical meristems and subsequent cell enlargement. Tissues produced in this way are termed as primary plant tissues.

Primary Phloem - Said of phloem tissue which is derived from the procambium in the primary plant body.

Primary Pit Field - Used for an area in the primary plan t cell wall which is having greatly reduced thickness and usually penetrated by plasmodesmata. It is responsible for relatively easy transfer of materials between cells (a function similar to pits).

Primary Plant Body - Refers to the body of a plant that is made up of primary plant tissues only. It is derived from primary growth.

Primary Thickening Meristem - The meristem occurring in certain monocotyledons (e.g., palms) below the leaf primordia at the apex. It tends to increase width of the stem behind the apex by cutting rows of cells by periclinal divisions.

Primary Tissue - Tissue formed from cells which are derived from the primary meristems.

Primary Trisomic - A plant having the ordinary diploid chromosome complement together with one extra chromosome.

Primary Xylem, Primary Wood - The xylem that is formed from a procambial strand and present in a primary vascular strand. It consists of protoxylem and metaxylem.

Prime Type - In Datura, one of the homologous types of chromosome structure distinguished from other types by interchange, and used as a basis of reference.

Primitive - Original; first-formed; of early origin.

Primodium - The earliest recognizable rudiment of an organ or structure in development.

Probasidium - A teliospore, or the basidium from nuclear fusion to the start of the epibasidium, sterigma, or spore development in any basidiomycete.

Procambium - A tissue of elongated, narrow cells grouped into strands. It is differentiated in the plerome just behind the growing point in stem and root of vascular plants. It gives rise to the vascular system by further development.