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Home >>Botany Dictionary >> 2,4-D - Dermatosome
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2,4-D - A synthetic auxin called 2, 4-dichlorophenoxyacetic acid. It is used as weed killer.
Damping Off - A disease attacking young seedling at ground level causing them to rot and fall over. It is usually caused by species of the fungus Phythium, and is favoured by overcrowding in damp conditions.
Dark Seed - A seed which will germinate, only when kept in the dark, when other conditions would normally favour germination.
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Dark Reactions - The sequence of light independent reactions utlising the energy in the form of ATP and reducing power in the form of NADPH which are formed during the light reactions to reduce carbon dioxide.
Deamination - Used for the removal of an amino group from compound. Amino acid breakdown takes place by oxidative deamination. For example, the deamination of glutamate by enzyme glutamate dehydrogenase yields a-ketoglutarate, NH4+ and reduced NAD or NADP.
Decarboxylase - Any enzyme catalysing the removal of carbon dioxide from a substrate e.g., pyruvate decarboxylase catalysing the formation of acetaldehyde from pyruvate.
Dauermodification - A lasting inheritable change which is possibly cytoplasmic, and is produced by some treatment.
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Dating - Said of the determination of the age of rocks, minerals and organic matter. Organic materials may be dated by radio carbon dating and dendrochnorlogy. However, these methods can be used only for specimens of later part of quaternary period. Dating of older rocks rely on radiometric dating.
Dauer Humus - Humus which is resistant to the attack of microorganisms.
Decumbent - Lying flat, or being prostrate, but having the tip growing upwards.
Decurrent - (1) Of a leaf whose base is extending down the stem as two wings.
(2) Of the gills of an agaric when they run some distance down the stipe.
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Day Neutral Plant - Describing a plant without definite photoperiod i.e., it can flower regardless of the length of exposure to daylight or darkness each day.
Deciduous - Used for describing woody perennial plants that shed their leaves before the winter or ary season.
Decomposer - Describing an organism that feeds on dead organic matter deriving energy by its breakdown to simpler substances e.g., many bacteria and fungi.
Decompound - Describing a compound leaf, when the leaflets are made up of distinct parts.
Deconjugation - Used for the separation of chromosomes before the end of prophase of meiosis.
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Day Position - The situation adopted by leaves of plants during the day, if they alter this position when the light intensity decreases.
Day Sleep - Used for the folding of leaflets of a compound leaf in bright light. This brings the stomatal surfaces together, thereby reducing the wate -loss.
Decussate - Describing leaves, when they are opposite, but each pair is at right angle to the ones above and below it.
Dedifferentiation - Used for the reduction of the cells of a differentiated tissue to a common undifferentiated form.
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Dediploidization - Used for the production of haploid cells (or hyphae) from a dikaryotic diploid cell (or mycelium).
Daylength - See critical daylength.
Deficiency - Used for the loss of a terminal acentric segment of a chromosome. Sometimes used to include 'deletion'
Deficiency Disease - An abnormaity which is caused by the absence of an essential chemical element.
Definite Growth - Describing the form of growth in which growth of the part is stopped after a certain maximum size is reached.
Definite Variation - A change which is taking place in a definite direction in the history of a race.
Definitive Nucleus - The diploid nucleus present in the centre of embryo sac formed after the fusion of two haploid polar nuclei. One male gamete from pollen tube fuses with it to form triploid primary endosperm nucleus.
Degeneration - (1) The change from a complex to a simpler form during evolution.
(2) The loss of morphological or physiological features by a fungus or bacterium kept in culture.
Degree of Freedom. A comparison of items within a body of data, independent of any other comparison used in the analysis. The number of degrees of freedom represents the number of independent comparisons which can be made within the body of data.
Dehiscence - Used for splitting of certain plant organs along predetermined lines for releasing their contents as in anthers, spore capsules and fruits. It is usually caused by gradual drying out of the enclosing walls.
Dehydrase - An enzyme catalysing the removal of water, i.e., condensation, e.g.,
2C6H12P6 →C12H22 Oll + H2O.
Dehydrogenase - An enzyme catalysing the removal of hydrogen, and by so doing oxidizes the substrate.
Dehydrogenation - A biological oxidation, involving dehydrogenases, when hydrogen is removed from a donor, which gets oxidized, to a receptor which gets reduced, e.g., AH2 + B → A + BH2
Delayed Inheritance - When each successive genera tion is having the genotype of the female parent for a particular character.
Deletion - Describing the loss of an intercalary acentric segment of a chromosome.
Delignifitation - Describing the destruction of lignin in wood by a fungus.
Deliquescence - (1) Used for the softening and liquefying of a sporangium membrane etc., at maturity.
(2) Used for a stem that breaks into separate branches.
Denaturation - Reducing the solubility of a protein, by altering its structure, chemically or by heating.
Dendritic - (1) Much branched.
(2) Having markings which are tree like or moss like.
Dendrochronology - Used for a method of dating historical events by making use of annual growth rings of old trees. Dating of archeological sites by cross-dating is achieved by doing comparison between timber at the site and small samples of cores of very old living trees there. Year of timber felling can be known by finding the point at which ring patterns of both correspond. In absence of sufficiently old trees in the area, chronologies are built up successively further back by matching the type ofring patterns ofnumber of wood samples with overlapping ages.
Dendrogram - A branching generally rooted diagram which reflects the relationships ofa group oftaxa. It is possible to represent the taxonomic hierarchy in a kingdom with kingdom at the base and subform as the terminal branches.
Dendrograph - An instrument used to measure the periodic shrinking and swelling of tree trunks.
Denitrification - Used for the decomposition of nitrates in the so be Bacterium denitrificans, resulting in the production of nitrogen.
DenitrifyingBacteria - A group of soil bacteria which breakdown nitrites and nitrates anaerobically to evolve nitrogen.
Density Gradient Centrifugatin - A technique which is used for separating and isolating pure samples of cell organelles. In this technique cells are first broken down in a homogenizer to release the ccontents and then the homogenate is filtered to remove cell fragments. It is now allowed to pass through prepared salt or sucrose solutions of different concentrations which are layered according to their density in a glass tube. Application of appropriate centrifugal forces, causes banding together of cell inclusions at regions in density gradient that corresponds to their own density and then these get separated off by various methods.
Denizen - A species of plant which could maintain itself in the wild, but was probably introduced by man.
Deoxynucleoside - A 2-desoxyribose sugar having adenine, guanine, cytosine, or thymine attached to it. A base unit of nucleic acid.
Deoxyrinucleorides - The products of hydrolysis of DNA.
Deoxyribose Nucleic Acid (DNA) - The hereditary material of the cell. The molecule has deoxyrobose (a sugar), phosphoric acid, two pyrimidines (thiamine and cytosine) and two purines (adenine and guanine). The pyrimidines and purines form bases. The molecule is a double helix of sugar-phosphate linkages (forming the sides of a twisted ladder) having the bases joined across to form the rungs. Adenine and thiamine are always paired, as are guanine and cytosine. E.g.
Dermatogen - The external layer of a stem or root apex, one cellithick, and giving rise to the epidermis.
Dermatophyte - A parasitic fungus causing disease of skin, hair, or nails.
Dermatosome - One of the small pieces into which a cell wall can be resolved by prolonged treatment with dilute hydrochloric acid, followed by heating at 50-60◦ C.
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