| raceme | A flower stalk on which the florets start blooming from the bottom of the stem and progress toward the top. |
| radial spacing | The horizontal spacing of branches around a trunk. |
| radicle | The first part of a seedling to emerge from the seed. Grows downward and develops into the primary root. |
| raphide | A needle-shaped crystal of calcium oxalate in certain species’ cells that deter herbivores. |
| ray flower | One of several small flowers often forming a ring around the disc flowers in a composite head. |
| receptacle | The enlarged end of a flower stalk to which the flower parts are attached. |
| recessive trait | A genetic characteristic that is masked by a dominant gene. |
| relative humidity | The ratio of water vapor in the air to the amount of water vapor the air could hold at the current temperature and pressure. |
| resin | A viscous, protective secretion of many conifers that is insoluble in water and hardens in contact with air. |
| resistant | A plant having qualities that make it retard the activities of a pathogen or insect pest. See immune, tolerant. |
| respiration | The process within plants where sugars and starches are converted into energy. |
| reticulate venation | A net-like vein pattern in some leaves. |
| reversion growth | A stem that originates from, and has the characteristics of, the plant’s rootstock. See sucker. |
| Rhizobia bacteria | Bacteria that live in association with the roots of legumes and convert atmospheric nitrogen to plant-available forms, a process known as nitrogen fixation. |
| rhizome | A thickened underground stem that grows horizontally with bud eyes on top and roots below. Bearded iris is an example of a plant that produces rhizomes. |
| rhizosphere | The thin layer of soil immediately surrounding plant roots. |
| ribosome | A celluar organelle which is the site of protein synthesis. |
| ripeness-to-flower | The minimum vegetative size a plant must achieve before it is capable of flowering. |
| root | Generally, the underground portion of a plant. It anchors the plant and absorbs water and nutrients. |
| root cap | A protective cover over a root tip. |
| root cutting | A section of root prepared for the purpose of vegetative propagation. |
| root hair | A delicate, elongated epidermal cell that occurs just behind a root’s growing tip. Root hairs increase the root’s surface area and absorptive capacity. |
| root nodule | A small swelling on a root resulting from invasion of nitrogen-fixing bacteria. |
| root pressure | The pressure developed by living cells in the root pushing water up the xylem. |
| root pruning | The cutting or removal of some of a plant’s roots. |
| root tuber | An enlarged, food-storage root bearing adventitious shoots. |
| root-bound | A condition in which a plant’s roots have completely filled its container. Typically, the roots begin to encircle the pot’s outer edge. Further growth is prevented until the plant is removed from the container. |
| rootstock | The root or stem onto which a scion or interstock is grafted. |
| rosette | A small cluster of leaves radially arranged in an overlapping pattern. |
| rot | Decomposition and destruction of tissue. |
| rotation | The practice of growing different plants in different locations each year to prevent the buildup of soil-borne diseases and insect pests, or the depletion of specific nutrients. |
| row cover | A sheet of synthetic material used to cover plants in order to retain heat and exclude insect pests. |
| rugose | Wrinkled. |
| runner | See stolon. |
| russet | Yellowish-brown or reddish-brown scar tissue on a fruit’s surface. |
