| gall | A growth on plant stems or leaves caused by abnormal cell growth stimulated by the sucking of some insects (e.g., aphids) or by viral, fungal, or bacterial infection. |
| gamete | A sex cell, male or female. |
| gene | A unit of genetic inheritance. |
| genus | A group of related species, each of which is distinct and unlikely to cross with any other. A group of genera forms a family, and a group of families forms an order. See species. |
| geotropism | The turning or curving of a plant’s parts in response to gravity. A root growing downward is an example. Geotropism is controlled largely by the hormone auxin. |
| germination | The initial sprouting stage of a seed. |
| germination inhibitor | A chemical substance preventing seed germination. |
| girdling | The cutting, removing, or clamping of bark all the way around a trunk or branch. Sometimes, girdling is done deliberately to kill an unwanted tree, but often it results from feeding by insects or rodents. Wires and ties used to support a tree can cause girdling, as can string trimmers. |
| glabrous | Hairless, but not necessarily smooth. |
| glaucous | Covered with a grayish, bluish, or whitish waxy coating that is easily rubbed off. Blue spruce needles are an example of glaucous leaves. |
| gradual metamorphosis | See simple metamorphosis. |
| graft union | See bud union. |
| grafting | The act of inserting a shoot or bud of one plant into the trunk, branch, or root of another, where it grows and becomes a permanent part of the plant. |
| gravitational water | Water in excess of a soil’s capacity. Drains downward to groundwater. |
| green cone | An enclosed composting unit often used for composting food waste. |
| green manure | See cover crop. |
| groundcover | Plants used in lieu of grass for holding soil and providing leaf texture. |
| growing season | The period between the beginning of growth in the spring and the cessation of growth in the fall. |
| growth regulator | A compound applied to a plant to alter its growth in a specific way. May be a natural or synthetic substance. See hormone. |
| guard cell | Leaf epidermal cells that open and close to let water, oxygen, and carbon dioxide pass through the stomata. |
| gum | A sticky, water-soluble plant secretion that hardens on contact with air. |
| gymnosperm | A member of a class of plants that forms seeds in an exposed condition, often in cones. |
