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Damping off - a potentially fatal fungal disease, usually of seedlings or cuttings.
Dasyacanthus - with dense spination.
Deciduous - shed periodically or annually or at maturity. Often applied to the seasonal shedding of leaves.
Decomposer - an organism (fungus, bacterium etc.) that breaks down and digests the remains of other organisms.
Decumbens - lying on the substrate with an ascending apex.
Decurrent - of a terminal spine extended down the leaf margin.
Decussate - of leaves arranged on a stem in opposite pairs at right angles to adjacent pairs of leaves above and below to form four vertical rows of leaves. Crossed or intersected in the form of an X.
Deflexed - curved or bent back down on itself or towards the plant surface.
Dehiscent - splitting or bursting open along defined lines e.g of a fruit.
Dehydrate - to lose a high proportion of water content.
Deliquescent - a tendency to absorb moisture and become liquid.
Dentate - with sharp teeth perpendicular to the margin.
Depressed - flattened from above.
Desert - a very dry area that receives less than 10 inches (25 cm) of precipitation each year.
Dessicated - dried, wrinkled.
i. of a growing tip terminated by a bud or flower which may stop or limit growth.
ii. of sequential flowering from a central or apical bud to lateral or basal buds.
See also: indeterminate
Dichotomous - forking equally, divided into two parts e.g. division of a single headed cactus into a double-headed plant.
Dichotomous key - a systematic procedure for determining the identity of an organism (or other object) by making a series of binary choices, according to the characteristics of the object, each of which leads to other choices or to the correct name.
Dicotyledon - a large group of flowering plants with two cotyledons that initially emerge from the seed. The Cactaceae are dicotyledons.
Differentiation - the progressive physiological and morphological changes in cells, tissues, organs or the whole plant during development from meristem tissue to a mature or adult form. Differentiated tissues are usually more functionally specialised than undifferentiated ones.
Dimorphic - having two forms.
Dioecious - with male and female flowers on separate plants.
Diosgenin - a steroid sapogenin extracted from the tubers of Dioscorea (wild yam) species, especially D. mexicana and D. villosa. Diosgenin is used for the commercial synthesis of cortisone, pregnenolone, progesterone, and other steroid pharmaceuticals. Diosgenin also occurs in Fenugreek. Structure of Diosgenin
Diploid - having the basic two sets of chromosomes.
Disc flower -one of the multiple inner tubular florets on flower heads of Compositae (Sunflower family).
Dispersal - distribution, usually of seed.
Dissected - deeply divided or cut into parts.
Distal - towards the unattached or developing end of an organ.
Distichous - of opposite leaf pairs arranged along the stem in two ranks or planes, so each pair is parallel to those above and below. The result is a plant with a flat appearance.
Division - a major taxonomic grouping and primary division of a Kingdom, as in Kingdom Plantae, ranking above Class in size. See also: Phylum
DNA - deoxyribonucleic acid- the genetic material, contained in the chromosomes of all cellular organisms and some viruses e.g. potato spindle tuber virus. An extremely long double-helical linear polymer in which the gene sequences are encoded as the order in which four types of nucleotide bases occur, each paired with a complementary base. Groups of three base pairs in a structural gene specify the code for the 20 types of amino acids (and one or two rare ones such as selenocysteine) that make proteins.
Other sequences encode control functions such as start or stop (transcribing a sequence) signals. Much of the DNA in many organisms consists of apparently non-coding or repetitive sequences of unknown function.
Doctrine of Signatures - a theory dating back to Dioscorides (AD 40 - 90) that herbs resembling body parts are useful in treating ailments of that part. This philosophy was expanded by numerous writers including Coles, Galen, Paracelsus and Böhme. Böhme (1575 - 1624) suggested in "Signatura Rerum = The Signature of all Things" (1621) that God marked plants with a sign or signature so that by careful observation their purpose could be determined. This signature could also relate to the type of place in which such plants grow. The common names of many plants (hepatica, liverwort, lungwort, spleenwort etc.) originate in this belief.
Dormant - in a temporary state of suspended activity. Not in active growth, but protected (e.g. bud scales) from environmental stresses.
Dorsal - on or attached to the back of an organ.
Drip tip - a sharp elongated point on which atmospheric water accumulates from dews, mists and rain until the water drop is large enough to fall off the tip, to the ground around the roots of the plant. Drip tips are common on the leaves of tropical trees in areas of high rainfall and the spines of some Cactaceae also function in this way to scavenge moisture.
Drupe - a fleshy or pulpy indehiscent fruit with a thin outer skin, containing a single central hardened stone or pit containing a seed e.g. almond, cashew, cherry, date, elderberry, mango, olive, peach, plum. The hard, lignified stone originates as the ovary wall of the flower.
Druse - roughly spheroidal crystalline (e.g. calcium oxalate) or colloidal inclusions within tissues.