D                      Biological TERMS  

deciduous Refers to plants that show a coordinated, seasonal loss of leaves. This strategy allows plants to conserve water during a time they could perform little photosynthesis anyway.

decomposer A type of detritivore that, in feeding on dead or cast-off organic material, breaks it down into its inorganic components, nutrients that can then be taken up by plants. Decomposers are all fungi or bacteria.

defecation The excretion of solid wastes through the rectal opening (anus).

deletion A chromosomal condition in which a piece of a chromosome has been lost. This occurs when a chromosomal fragment that breaks off and does not rejoin any chromosome.

denatured Pertaining to a protein that has lost its precise conformation, and therefore also its function.

dendrite An extension of the cell body of a nerve cell that generally receives information that it transmits to the neuron. Typically, a neuron has many dendrites.

density-dependent Having an effect that is proportional to the size of a population. These are biological factors, such as food supply and disease, that act to limit the population size.

density-independent Having an effect that is not related to the size of a population. These are physical forces, such as temperature and rain, that act to limit the population size.

deoxyribonucleic acid (DNA) A molecule, composed of a huge number of nucleotides, that provides the information for the construction of proteins and RNA. DNA can replicate itself, and it carries genetic information between cells and between generations.

derived character A character that is found in the descendents of a common ancestor, but was not found in that ancestor. Cladistics distinguishes ancestral from derived characters and uses these characters to determine evolutionary relationships.

dermal tissue The epidermis, or outer layer of cells of a plant. In addition to covering the plant, dermal tissue forms trichomes such as root hairs.

dermis The thick layer of the skin, composed mostly of connective tissue, that underlies, nourishes, and supports the epidermis.

desertification The transformation of an area into a desert, sometimes through human activity.

determined Referring to a cell during development whose fate is predictable.

detritivore An organism that feeds on the remains of dead organisms or the cast-off material from living organisms.

dicotyledon A type of plant that has two embryonic leaves within the seed. More than three-quarters of all flowering plants are dicotyledons.

diffusion The movement of molecules or ions from areas of their higher concentration to areas of their lower concentration. Over time, the random movement of molecules will result in the even distribution of the material.

digestive system The organ system that transports food into the body, secretes digestive enzymes that help break down food to allow it to be absorbed by the body, and excretes waste products. This system consists of esophagus, stomach, and large and small intestines, plus the accessory glands that produce the enzymes along the way.

digestive tract The muscular tube that runs through the body from mouth to anus, through which food passes.

dihybrid cross An experimental cross in which the plants used differ in two of their characters.

dikaryotic Having two haploid nuclei in one cell. A dikaryotic phase occurs in the life cycle of fungi because there is a delay between cytoplasm fusion and fusion of the nuclei, so all cells of a growing mycelium are dikaryotic.

diploblastic In animals, the state of developing from two layers of germ cells.

diploid Possessing two sets of chromosomes. All human cells are diploid with the exception of human gametes (eggs and sperm), which are haploid. Such haploid cells possess only a single set of chromosomes.