One Gene: One byme Hypothesis - According to this theory every enyme is coded by a particular gene. An evidence in favour of this theory comes from the fact that presence or absence of an enyme in a biochemical pathway is a genetically determined characteristic inherited as a single gene.

Ontology - Used for the changes which are taking place during the life history of an organism.

Oogamy - Said of the fertilization of a large non-motile female gamete by a small, motile male gamete.

Oogenesis - The development of an egg from an oocyte.

Oogonium - A cell which is giving-rise to oocytes, directly or by mitosis.

Oolysis - The conversion into leafy structures of carpels and ovules.

Oophyte - A gametophyte in the Bryophyta and Pteridophyta.

Ooplasm - The central plasma in the Oogonium of some Oomycetes, representing a more or less undifferentiated egg.

Oosphere. (1) The large non-motile fertile gamete of some algae and fungi. (2) An ovum.

Oospore. The thick-walled resting zygote which is formed from a fertilized Oosphere Operational Taxonomic Units (OTUs) - Saidoftheentities whose affinities have been studied by numerical taxonomy. Depending on the level and type of investigation, an OTU may be of any taxonomic rank.

Operator Gene - Said of a site on DNAP adjacent to a structural gene which is acting as a 'switch' starting or 'stopping' transcription from structural gene.

Operculum - (1) A cap covering the ostiole in certain ascomycete fungi.
(2) A lid which is covering the aperture in many pollen grains which gets pushed aside by the emerging pollen tube.
(3) The membranous cap which is covering the peristome in the ca psule of many mosss sporophytes before dehiscence of the capsule.

Operon - Used for describing a group of genes which are lying adjacent to each other and function as a single unit (either all transcribing,together or none doing so) and code for a set of eneymes in a particular pathway.

Opposite - (1) Inserted at the same level.
(2) Said of leaves that are inserted in pairs at each node, with one on each side of the stem.
(3) Said of a stamen which lies next to the middle of a petal.

Orbicular - Said of the structure where shape is almost circular . and flattened e.g., leaves of Nymphoides pletata.

Order - Describing a major category in taxonomic hierarchy which is usually comprising of families.

Ordination - Said of a technique which is used by ecologists following the continum concept of community in community ecology for relating the composition of different stands of vegetation to each other. Certain properties of each stand are made to plot against one or more axes, where each axis is representing some enviromental gradient. Thus, it gives an arrangement in which different stands are ordered in the manner that best reflects their similarities and dissimilarities.

Ordonician - Used for describing the second period of Palaeozoic era, about 500-440 million years ago when temperature was quite high, even at poles and most of the earth was covered by seas. There was no terrestrial life and the only dominant plants were marine algae.

Organ Culture - Said of culture of excised organs e.g., roots, leaves, embryos, meristerms etc., a suitable aseptic medium.

Organelle - Used for describing any of the membrane-bound structures which are present in the cytoplasm of a cell. It is for carrying out a specific metabolic process essential to the life of cell e.g., mitochondrion, chloroplast etc.

Organogenesis - Said of developmental changes that take place during formation of a particular organ.

Origin of Life. Describing the way in which living organisms have originated on earth. It is accepted that life has originated about 3000-3500 million years ago from inorganic matter. Probably long-chained protein and carbohydrate molecules were formed from simple molecules by catalytic reaction.

Orinthophily - Used for pollinationin which pollens are being carried by birds. It is important in many tropical species.

Orthotropism. Used for the type of tropism in which growth response is directly towards or away from the source of stimulus e.g., vertical growth of stem is described as negative orthotropism.

Orthotropous - (1) Used for an ovule which is straight, i.e., with the micropyle in a straight line with the funicle.
(2) Used for an organ which shows a sharp positive or negative tropism in respect to a given stimulus.

Osmosis. The phenomenon of the passage of water (or solvent) rom a dilute solution to a more concentrated one through a semipermeable membrane.

Osmotic Potential (solute potential). Symbol Ψ p said of the decrease in water potential of a solution in water which occurs due to dissolved Ionic or non-ionic solute particle in it or the decrease in chemical potential of solvent in a solution which a occurs due to presence of ionic or non-ionic solute particles. It depends on the number of particles rather than their nature or charge. It in generally expressed in bars with negative sign.

Osmotic Pressure.' The pressure which is developed due to the passage of water by osmosis. The maximum osmotic pressure of a given solution is developed when it is separated from pure water by a semi-permeable membrane.

Ostiole - (1) A general term for an opening.
(2) The opening by means of which spores etc., escape from conceptacle or a perithecium.

Outbreeding - The mating of relatively unrelated gametes.

Outer Fissue - The upper arm of the cleft in the raphe (wall) of some Bacillariophyceae.

Ovary - (1) The hollow basal region of a carpel, having one or more ovules. In a flower with 2 or more united carpels, they form a single compound ovary.
(2) Loosely used as meaning the pistil.

Ovate - Hat and thin which is sphaped like the longitudinal section of an egg, widest below the middle.

Ovule - (1) The nucleus having the embryo-sac and is enclosed by 1 or 2 integuments, which after fertilization, and subsequent