Tertiary. Geological period ; lasted approximately from 65 till 1 ½ million years ago.

Testa. Seed coat. Protective covering of embryo of seed plants formed from integument ; almost hard and dry.

Testis. Organs of animal which produces sperms. In vertebrates it also produces sex hormones.

Testosterone. An androgen probably the principal one produced within male vertebrates.

Tetrad.
(1) (Bot.). A group of four spores formed by meiosis within a spore mother-cell, e.g. formation of micro spores within a microscope mother-cell. The process is often described as tetrad division.
(2) Paired chromosome of meiosis, after each chromosome has become visibly two-s.

Tetraploid. A form of polyploidy. Having four times the number of chromosomes in a nucleus.

Tetrapoda. Four-footed animals. A grouping of vertebrate classes sometimes used in classification. Includes amphibia birds reptiles, and mammals, i.e. all the essentially alnd living vertebrate classes.

Tetrapterous. Having four wings.

Tetraspore. First cell or gametophyte generation in many brown and red algae; formed in fours in tetrasporangium following meiosis.

Thermophilic. (Of a micro-organism), with optimum temperature for growth above 45°C.

Thermoregulation. The control of body heat in Homoiotherms.

Thoracic Cavity. The space within the thorax.

Thoracic duct. Main lymph vessel of mammals receiving lymph from trunk (including lacteals) and hind-limbs, running up the thorax close to the vertebral column. Discharging into vena cava superior or an associated vein.

Thorax.
(1) In terrestrial vertebrates region of the body containing heart and lungs (chest). Only in mammals it is from the abdomen by the diaphragm.
(2) In insects, the group of three segments behind the head which bears the three pairs of legs and the wings.

Thorn.  A sharply pointed woody plant structure formed from a modified branch.