| ecology | The science of relationships between organisms and their environment. |
| economic threshold | The level at which pest damage justifies the cost of control. In home gardening, the threshold may be aesthetic rather than economic. |
| egg | A female sex cell. |
| emasculate | To remove a flower’s anthers. |
| embryo | The dormant, immature plant within a seed, the “germ” referred to in wheat germ. |
| embryo culture | See tissue culture. |
| enation | Epidermal outgrowths on leaves or stems. |
| endodermis | A layer of cells in roots between the cortex and the vascular tissues. |
| endosperm | The nutritive tissue within the seed of a flowering plant. Surrounds and is absorbed by the embryo. |
| enzyme | A biological catalyst that aids in a specific biochemical process, such as converting food from one form to another. |
| epidermal hair | A filament of cells arising from an epidermal cell. |
| epidermis | The outermost layer of cells covering a plant’s leaves, roots, and young parts. |
| epigeous germination | Seed germination in which the cotyledons are raised above the soil surface. |
| epinasty | An abnormal downward-curving growth or movement of a leaf, leaf part, or stem. |
| epiphyte | A plant growing on another plant for support. |
| espalier | The training of a tree or shrub to grow flat on a trellis or wall. Espalier patterns may be very precise and formal or more natural and informal. |
| etiolation | Development of yellow, long, spindly growth on a plant as a result of insufficient light. |
| ethylene | A gaseous plant hormone (C2H4) produced in abundance by ripening fruits and damaged tissues. |
| etiolation | The condition where a plant is grown in darkness, resulting in pale and elongated stems and underdeveloped leaves. |
| evergreen | A plant that never loses all of its foliage at the same time. |
| excise | To remove or extract, as an embryo from a seed or ovule. |
| excurrent | A tree form in which the main trunk remains dominant with small, more or less horizontal branches. Fir and sweetgum are examples. |
| exfoliating | Peeling off in shards or thin layers, as in bark from a tree. |
| exoskeleton | The outer support structure of an insect. |
| exotic | Non-native. |
