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Home >>Botany Dictionary>> Desmolases - Dinucleotide
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Desert - A major type of regional community in areas of low rainfall and supporting little or no vegetation. True deserts are regions which are devoid completely of higher plant life. Cold deserts include tundra and regions which are permanently covered with ice and snow. Hot deserts are having annual mean rainfall U1\der 250 mm. Plants generally are few, usually small and sparsely distributed.
Desmids - Used for unicellular members of class Zygnemaphceae of Chlorophyta that are characterised by their distinctively symmetricall cells, anatomically complex plastids and absence of free swimming swanners and reproduction by conjugation.
These are of two types; saccoderm desmids commonly occurring in acidic pools and peat bogs and placoderm desmids (or true desmids). The cells of placoderm desmids are characteristically splitted up into two virtually identical halves, joined by a narrow bridge at the centre.
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Desmolases - Enzymes breaking, or forming a C-Clink, without hydrolysis e.g., Carboxylkase catalyses the change of pyruvic acid to acetaldehyde and carbon dioxide.
Desoxyribose - A pentose sugar, haying the following structure;
Desoxyribose Nucleic Acid - See Deoxybose Nucleic Acid (DNA).
Desynapsis - The abnoromally early breaking of synapsis in meiosis.
Deutobroque - A stage which is proceeding leptotene in oogenisis. The chromosomes radiate from the nucleolus and wind about just under the nuclear membrane.
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Devolopment - Used for the succession of stages in the life of the
plant, as distinct from simple growth.
Devonian - A geological period 310-270 million years ago.
Dextrinase - A plant enzyme which hydroloyses dextrin .
Dextrins - A series of polysaccharides intermediate in structure between glucose and starch, with 5-10 glucose units in straight clains. Formed during starch hydrolysis.
Dextrose - A hexose sugar (D-glucose), a solution of which rotates a beam of polarized light to the right. The former name of glucose.
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Diadelphous - Describing stamens which have the filaments, either in two bundles or in a group with one fee stamen.
Diadromous - Said of venation which is resembling the ribs of a fan.
Diageotropism - The growth of a plant member so that it becomes at right angles to the gravitational field.
Diakinesis - The last stage in the prophase of meiosis in which the nuclear membrane and nucleolus disappear and the bivalents become more contracted, the chiasmata tend to move to the ends of the chromosomes, and the tetrads pass on to the spindle.
Diallele Cross - A breeding experiment in which each of a number of males is made to cross with each of a number of females.
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Dialysis - A process of separating small particles (ions or small molecules) from big ones (colloids or large molecules) in solution. This is carried out by placing the mixed solution in a membrane which has pores large enough for the small molecules to pass through, but which obstruct the passage of the larger ones ( e.g., cellophane, collodian, parchment). It is then placed in water, and the smaller particles diffuse out into it.
Diaphototrophic - Used for a plant member which grows into a fixed position across the direction of incident light.
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Diaphragm - (1) Transverse plalte of cells across a stem, generally at the nodes.
(2) A cross-partition in the air chambers of hydrophytes, generally perforated.
Diarch - Having two strands of xylem.
Diaschistic: Used for tetrads which divide once transversely and once longitudinally, in meiosis.
Diastase - An enzyme complex which breaks down starch to glucose.
Diaster - In cell division, when the daughter chromosomes are situated in two groups near the poles of the spindle and ready to form the dauther nuclei.
Diastole - The growth and epansion of a nucleus from the end of one mitosis to the beginning of the next.
Diatoms - Used for unicellular or colonial non-flagellate alagae of Bacillariophyata which have delicately scupltured si11ica cell walls divided into two overlapping halves (see frustule). These may be radially symmetrical (centric diatoms) or bilaterally symmetrical (pennate diatoms). Cell walls are extremely resistant to decay and form diatomaceous earth (kieselguhr) in lake or ocean beds.
Diatropism - The growth of an organ or organism so that its axis becomes at right-angles to the line of action of the stimulus.
Dichasium (dichasial cyme) - A cymose inflorescence in which apecies are developed into flowers and two lateral flowers are developed below each apex from axillary buds of common node. This gives symmetrical appearance to the inflorescence as in Stellaria holostea. Compare monochasim.
Dichogamy - Used for describing the maturing of the anthers and ovules in the same flower at different times.
Dichopodium - A sympodial branch system which is made up of successive parts of a dichotomizing branch system, of which only one of part is assisting in forming the axis.
Dichotomous - Forked branching which is produced by the subdivision of an apical meristem, to form two branches of the same size. It is a primitive form of branching.
Diclinous - (Unisexual). Male and female organs are borne on different plants.
Dictyospore - A multicellular spore divided by transverse and longitudinal walls.
Dictyostele - A siphonostele having overlapping leafgaps. This gives distinct vascular strands called meristeles. The whole is enclosed by an endodermis.
Didymous - Used for describing arrangement of anthers in two equal pairs in the androecium.
Didynamous - Used for the condition of stamens in androecium in which there are two long and two short stamens e.g., in Labiatae.
Differential Centrifungation - Used for describing a method of isolating cell organelles. This method involves the breaking up of cells in homogenizer, followed by filtering the homogente to separate cell fragments and then certifuging in and decanting several times in sequence with increasing the time and speed of certifugation at each successive stage. Nuclei, membranes and ribosomes have been collected in that order.
Defferentiation - The process in which a cell divides to give rise to dissimilar tissues within an individual. The process is repeated by heredity, and is therefore genetically determined.
Deffuse Nucleus - The chromatidia sometimes present in non nucleated cells.
Diffuse Porous - When the vessels are scattered evenly through out the xylem, or when there is little difference in the size of the vessels formed in different seasons.
Diffuse Porous Wood - Used for describing secondary xylem in which diameter of tracheary elements show no or very little seasonal variation as teh wood of Betuala lutea. Such woods lack discernible growth rings.
Diffusion Pressure Deficit (DPD) - The osmotic pressure of a solution particuarly when comparing it with that of another solution, e.g., a comparison of the osmotic pressure of the cell-sap, compared with that of the surrounding medium. It is calculate by DPD (diffusion pressure deficit)=OP [osmotic pressure] TP (turgour pressure.) See also osmotic pressure, turgour pressure, water potential.
Digitaliform, Digitate - Diverging from a central point, like the fingers on a hand.
Dihybrid - An organism that is heterozygous for two particular [genetical} factors (genes). It is produced by a cross taking place between parents that are homozygous for the two factors in consideration. For example, a cross taking place between homozygous parents (AABB x aabb or Aabb x aaBB) result in dihybrid AaBb. Dihybrid has expression of dominant allele for factors in consideration. See also monohybrid, trihybrid.
Dihybrid Ratio - A 9:3:3:1 ratio of phenotypes among the offspring of a single cross. This ratio can result only if a. Mendelian cross has involvement of two factors, both involved factors are assorted randomly and independently and both parents are heterozygous for both the factors in consideration. If dihybrid ( e.g., AaBb) is salfed i.e., crossed with other similar dihybrid ( e.g., AaBb) is salfed i.e., crossed with other similar dihybrid (AaBb x AaBb) and there occurs complete dominance of one allele of both factor, fertilization is random and all gemetes, zygotes and offspring are having equal chances of survival, such a phenotype ratio occurs among the offspring. It is an expression of monohybrid Mendelian ratio viz. (3:1)2. If in an experiment which should give such a ratio, some deviant ratio is observed, it reveals that some of the basic assumptions, singly or in combination, are not being applicable in the particular case.
Dikaryotic - A plant thallus (the fungus mycelium of Basidiomycetes) in which each segment is having two genetically distinct nuclei. Such mycelium is formed from fusion of two monokaryotic mycelia of different strains.
Dimerous - Used for a flower in which number of units in each whorl is two or its multiple e.g., in Ciracaea.
Dimorphism - Said of the phenomenon in which an individual has an organ in two morphologically distinct forms (e.g., chloroplasts in mesophyll and buyndle sheath cells of C4 plants) sun and shade leaves or juvenile and adult leaves, gametophte and sporophyte plants of a species or male and female individuals (sexual dimorphism) etc.
Dinoflagellates - Used for describing flagellate, unicellular, marine algal members of class Dinophyceae of division Dinophyta (Pyrrophyta). They are abundant in phytoplankton and often dark-brown in colour, they have characteristically one longitudinal and one transverse groove in the cell wall and two flagella in each groo
ve.
Dinucleotide - A unit which is derived from deoxyribose and is linkied through a phosphate radical. Some are important as prosthetic groups of eI1zymic globular proteins.
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