| tannin | A substance occurring in the bark and leaves of some plants that helps protect against predators. |
| taproot | A thick central root attached directly to a plant’s crown. Taproots branch little, if at all. |
| taxonomy | Classification or naming of plants or animals. |
| temporary branch | (1) A small shoot or branch left on a young tree’s trunk for protection and nourishment. (2) A low lateral allowed to remain until a tree is tall enough to have scaffolds at the desired height. |
| tender | Not tolerant of frost or cold temperatures. In horticulture, tender does not mean weak or susceptible to insect pests or disease. |
| tendril | A slender projection used for clinging, usually a modified leaf. Easily seen on vines such as grapes and clematis. |
| terminal | The tip (apex), usually of a branch or shoot. |
| terminal bud | The bud at the tip of a stem, trunk, or branch. Its development extends the plants growth. |
| thatch | A tightly intermingled layer of stems, leaves, and roots, living and dead, that forms between the soil surface and green vegetation of grass. |
| thermoperiod | The change in temperature from day to night. |
| thermophillic | High temperature, as in microorganisms that break down organic matter in a hot compost pile. |
| thigmotrophism | A growth response to touch. |
| thin | (1) To remove an entire shoot or limb where it originates. (2) To selectively remove plants or fruits to allow remaining plants or fruits to develop. |
| thorn | A hard, sharp-pointed, leafless branch. Hawthorne is an example of a plant that produces thorns. See prickle. |
| tiller | A shoot that arises from a plant’s crown. Generally associated with grass species. |
| tissue | A group of cells of the same type having a common purpose. |
| tissue culture | The process of generating new plants by placing small pieces of plant material onto a sterile medium. Also called embryo culture. |
| tolerant | A plant that will produce a normal yield even if infested by a disease or insect pest. See immune, resistant. |
| topiary | A tree or shrub shaped and sheared into an ornamental, unnatural form, usually a geometric shape or the shape of an animal. |
| totipotency | The ability of any cell to develop into a complete plant. |
| trace element | See micronutrient. |
| transpiration | The process of losing water in the form of vapor through stomata. |
| transpirational pull | The force exerted by transporation from the leaves which draws water up through the plant. |
| tree | A woody plant that typically grows more than twelve feet tall and has only one main stem or trunk. |
| tropism | The tendency of a plant to turn in response to an external stimulus, either by attraction or repulsion, as a leaf turns toward the light. See geotropism, phototropism. |
| trunk | The main stem of a tree. Also called a bole. |
| truss | A flower cluster, usually growing at the terminal of a stem or branch. |
| tuber | An underground storage organ made up of stem tissue. Contains buds on the surface, from which shoots may arise. Potatoes are an example. |
| tuberous root | An underground storage organ made up of root tissue. Sprouts only from the point where it was attached to the parent plant. Dahlias are an example. |
| turgor | Cellular water pressure; responsible for keeping cells firm. |
| twig | A young stem (one year old or less) that is in the dormant winter stage (has no leaves). |
| twiner | A stem growing in a spiral fashion around a support. |
