Photosynthetic Bacteria - A group of phototropic bacteria having a green pigment, bacteriochlorophyll, which is similar to chlorophyll and thus it can photosynthesize organic food involving light energy but derives hydrogen from H2S or other reduced sulphur compounds instead of water. CO2 flation occurs by involving Calvin cycle reactions. Green sulphur bacteria, purple sulphur bacteria and purple non-sulphur bacteria are the examples of photosynthetic bacteria.

Photosynthetic Capacity - Said of the efficiency of a plant cell or a chloroplast in carrying out photosynthesis.

Photosynthetic Efficiency - Refers to the ratio of light-energy absorbed by the chloroplasts of a tissue in unit time, to the amount of energy fixed per unit time.

Photosynthetic Number - Defined as the ratio between the number of grams of carbon dioxide absorbed per hour by a unit of a leaf to the number of grams of chlorophyll which that unit has.

Photosynthetic Quotient, Photosynthetic Ratio - Defined as the ratio between the volume of carbon dioxide absorbed to the volume of oxygen set free, during a given time, by plant material occupied in photosynthesis.

Photosynthetic Unit The minimum unit of chlorophyll which is necessary to reduce 1 molecule of carbon dioxide, e.g., it is estimated that 2,500 chlorophyll molecules are necessary.

Photosystem I and II - (Pigment systems I and II, PS I and PS 11). Refers to two photochemical systems which consist of photosynthetic and accessory pigments and electron carriers that work in sequence to carry out non-cyclic photophosphorylation and NADPH formation sequences of light reactions during photosynthesis. These systems occur in chloroplast grana and are linked to each other. PS II has from P680 of chlorophyll-a that is having light absorption peak at 684 nm. PSI has from P700 of chlorophyll-a, that is having light absorption peak at 700 nm.

Phototaxis - Taking place of the movement of a whole organism in response to light.

Phototonus - The condition of a leaf which is able to respond to a stimulus, because it has received an adequate amount of light.

Phototroph - Refers to an organism that obtains energy required for the synthesis of organic compounds directly from solar radiation e.g., photoautotroph, photoheterotroph etc.

Phototropic Conduction - The differential conduction of auxin down either side of a shoot which is unilaterally illuminated so that growth occurs towards the light.

Phototropic Induction - The effect of light on the apex of the plant affects the growth of the stem below it, i.e., the tip, under the influence of light, induces growth lower down.

Phototropism - The growth curvature of part of a plant in response to light.

Phragmobasidium - A basidium which becomes septate, and is then divided into. four cells.

Phycobilin (biloprotein). Refers to any of the blue or red accessory, photosynthetic pigments which occur in blue-green and red algae. These are different from chlorophylls and carotenoids in being water soluble but are structurally very similar to porphyrin part of chlorophyll molecule except that magnesium is not present in them.

Phycobilisome - Said ofa small particle which is about 30 nm in diameter; many such particles are present on the membranes oflamellar system of blue-green algae.

Phycobiont - Refers to an algal partner in a lichen that is often a member of genus Tribouxia. If a blue-green algae (e.g., Nostoc,Tivularia etc.) is present, the lichen is generally gelatinous. 24 genera of algae are so far found in lichens.

Phycocyanin - The blue pigment present in the chloroplasts pf the Rhodophyta, and distributed through the cells of the Cyanophyta.

PhyleticClassification - A scheme ofplant-classification which is based on the presumed evolutionary descent oforganisms.

Phyllid - Used for describing leaf-like structure in mosses and leafy liver-worts.

Phylloclade - Describing modification ofstem where a portion ofstem having few internodes and nodes becomes flattend and leaf-like and takes up the function of leaf. The leaves in such plants are reduced into spines, protuberances etc., and are present on the phylloclade also.

Phyllody - Refers to the transformation of parts of flower into leaf like structure. It may take place in response to some infections as in maize infected with Sphacelothecareiliana which develops phyllody in both male and female flowers.

Phyllotaxis (phyllotaxy). Refers to the arrangement of leaves on the stem. Various types are shown in Fig. 25

Phylogenetic (phyletic). Used for the study of stages in the evolutionary history of groups of organisms. This study is carried out on the basis of fossil studies, comparative anatomy etc., and such study forms a basis for classification. In such classification, more weight is given to characters that are assumed to be the result of evolutionary relationships.

Phylogeny - Said of the evolutionary history of an organism or a group.

Phylum - A major division of the plant or animal kingdom. The term 'division' is used more frequently in the plant, kingdom.

Physiographic Factor - Refers to any of the factors besides the climatic, edaphic and biotic factors that influences the prevailing conditions within a habitat and distribution of living organisms in it; These include such factors that are due to topography, altitude, drainage conditions, degree of erosion, slope of land etc.

Physiological Balance - The balance between the ions in a culture solution, so that none of them can have an adverse effect on the growth of a plant.

Physical Barrier - Any physical object, e.g., a sea or mountain range which imposes a barrier on the migration of plants or animals.

Physiognomy - The characteristic appearance of a plant community by which it becomes possible to be recognized at a distance.

Physiographic Climax - A plant community which is maintained in a certain stage of development by some natural features of the habitat.

Physiological - Relating to the functions of a plant (or animal) as a living organism.

Physiological Anatomy - The study of the relation between structure and function.

Physiological Drought Refers to the condition when a plant failes to take in water, because of the low ground-temperature, or because it holds substances in solution which hinder the absorption of water by the plant.

Physiological Race - Said of a population that is morphologically identical but physiologically different from other populations of the same species. Such races exhibit genetic diversity of the species.

Physiology - The study of the physical and chemical processes which go on in living organisms.

Phytoalexin - A chemical produced by a host that is inhibiting the growth of a pathogenic fungus e.g., phenolic compound pisatin accumulating in pea tissues in presence of various fungi.