| adelgids | a family of aphid-like insects that feed exclusively on conifers and typically are covered with white waxy material. |
| aesthetic | has to do with appearance; beauty. |
| alkaline | having a basic reaction, that is containing the OH- radical and turning red litmus blue when in solution. |
| alternate | an arrangement of leaves or other parts not opposite or whorled; parts situated one at a node, as leaves on a stem; like parts succeeding each other singly with a common structure. |
| alternate host | one of two kinds of plants on which a parasitic insect or fungus (e.g., rust) must develop to complete its life cycle. |
| annual | maturing and living one season only. |
| anthers | pollen-bearing part of a stamen, borne at the top of a filament, or sessile. |
| anthracnose | a leaf-, stem-, or fruit-spot type of disease caused by fungi that produce their asexual spores in an acervulus. |
| appendages | subordinate or derivative body parts. |
| armyworms | caterpillar species closely related to cutworms. They often move in mass through crop stands while stripping plants of their foliage. |
| auricles | an ear-shaped lobe, generally used to describe the base of leaves or petals. |
| axils | the main stem or central support of a plant. |