thorn forest hawthorn (Crataegus monogyna). Compare prickle; spine.
rounds the large central vacuole of plant cells.
thorn forest See forest.
torus (pl. tori) A disk-shaped structure formed from lignified primary cell-wall material on the middle lamella of a bordered pit. The structure is found mainly in the conifers. It is thought to act like a valve, sealing the pit when the pressure on the two sides is unequal, e.g. when an adjacent tracheid is damaged.
threonine An AMINO ACID derived from aspartic acid. It is broken down to form glycine and acetyl CoA. Isoleucine can be synthesized from threonine. thylakoid An elongated flattened fluidfilled sac that forms the basic unit of the photosynthetic membrane system in chloroplasts and photosynthetic bacteria. See chloroplast. thymidine The nucleoside formed when thymine is linked to D-ribose by a b-glycosidic bond. See nucleoside.
thymine A nitrogenous base found in DNA, but not in RNA. It has a PYRIMIDINE ring structure. It base pairs with adenine in the complementary strand of DNA. See base pairing; thymidine.
tissue In a multicellular organism, a group of cells that is specialized for a particular function, e.g. palisade mesophyll. Several different tissues may be incorporated into an organ, e.g. a leaf. Compare organ. tissue culture The growth of cells, tissues, or organs in suitable media in vitro. Such media must normally be sterile, correctly pH balanced, and contain all the necessary micro and macronutrients and hormones for growth. Studies of such cultures have shed light on physiological processes that would be difficult to follow in the living organism. For example, the cytokinins were discovered through work on tobacco pith tissue culture.
tocopherol (vitamin E) A terpenoid-like substance found especially in certain seeds, such as cereals, where it probably prevents oxidation of lipids, thus prolonging seed viability.
tonoplast The membrane that sur-
totipotency The ability shown by many living cells to form all the types of tissues that constitute the mature organism. This may be achieved, even if the cells have completely differentiated, provided that the appropriate balance of nutrients and hormones is given. The best example of this phenomenon is the formation of adventitious embryos in carrot tissue cultures.
toxin A
chemical produced by a pathogen (e.g. bacteria, fungi) that causes damage to a host cell in very low concentrations. The toxin may be formed within the pathogen and released when it dies (endotoxin), or secreted through its cell wall (exotoxin). Toxins are often similar to the enzymes of the host and interfere with the appropriate enzyme systems.
trabecula (pl. trabeculae) An elongated cell wall, cell, or line of cells across a cavity. In plants, trabeculae may be found in Selaginella stems, where they suspend the steles in large air spaces. trace element See micronutrient. tracheid An elongated xylem conducting element with oblique end walls. Tracheids have heavily lignified walls and the only connection between adjacent tracheids is through paired pits, which are mainly concentrated in the end walls. Many tracheids lack protoplasm at maturity and have lignified secondary cell walls. Tracheids form the only xylem conducting tissue of vascular plants other than an-
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