- Quadrate. Nearly square to form
- Quadrifid. Cut or divided into four lobes or parts
- Quincuncial. Having five structures, two of which are exterior, two interior, and a fifth with one margin covering interior structure and other margin covered by that of one of the exterior structures
- Raceme. Unbranched, indeterminate inflorescence with pedicelled flowers
- Rachilla. Secondary axis of compound leaf; central axis of a grass or sedge spikelet
- Rachis. The main axis of a pinnately compound leaf; major axis within an inflorescence; axis of a compound fern blade
- Radical. Leaves basal, near ground, usually from caudex or rootstock
- Radicle. Basal end of embryo axis that gives rise to root system
- Rameal Sheath. Leaf sheath on the stem joints, as in Equisetum
- Ramentaceous. Having many thin scales, as on the epidermis of some ferns
- Ramose. Branched
- Raphe. Ridge on seed coat formed from adnate funiculus
- Ray. Secondary axis in a compound inflorescence
- Receptacle or Torus. The region at end of pedicel or on axis to which flower is attached; point on a leaf where sporangia are attached
- Receptaculum. A fleshy structure below the seed formed from the bases of bracts and the swollen receptacle or cone axis; e. g., Acmopyle, and some Podocarpus spp
- Reclinate. Bent down upon the axis, no angle of divergence
- Reclined. Descending at 106-135° angle of divergence
- Rectangular. Box-shaped, longer than wide
- Recurved. Curved outward or downward
- Reduced. Decreased in size
- Reflexed. Bent or turned downward
- Regular. With floral parts within a whorl similar in shape and size
- Reniform. Kidney-shaped, with shallow sinus and widely rounded margins
- Repand. Sinuate with indentions less than 1/16 distance to midrib or midvein
- Repent. Creeping or lying flat and rooting at the nodes
- Replicate. With lamina folded once abaxially along midrib or midvein
- Replum. Persistent septum after dehiscence of fruits, as in the Brassicaceae
- Resinous. Having a yellowish, sticky, exudate
- Resupinate. Inverted or twisted 180°, as in pedicels in the Orchidaceae
- Reticulate. Netted; with veins forming a network
- Reticulodromous. With a single primary vein, the secondary veins not terminating at the margin and losing their identities near the margin by repeated branching, yielding a dense reticulum
- Retinaculum, Jaculator or Echma. A persistent indurated, hook-like funiculus in the fruits of Acanthaceae
- Retrorse. Bent or directed downward. Retrorsely Crenate - Rounded teeth directed toward base. Retrorsely Serrate - Sharp or pointed teeth directed toward base
- Retuse. Lobe rounded; sinus depth to 1/16 distance to midpoint of blade; margins convex
- Revolute. Margins or outer portion of sides rolled outward or downward over lower or dorsal surface
- Rhizocarpic. Plants having the roots living for many years with the stems dying annually
- Rhizoid. A hair-like absorptive organ on gametophytes and rarely on sporophytes
- Rhizome. A horizontal underground stem
- Rhizotaxis. Arrangement of roots
- Rhombic. With widest axis at midpoint of structure, and with straight margins; elliptic but margins straight and middle angled
- Ribbed. With longitudinal nerves
- Ringed. With old bud scale scar rings
- Robust. Large
- Rolled. Sides enrolled, usually loosely, over upper or lower surfaces
- Root. An absorbing and anchoring organ, usually initially developed from the radicle and growing downward. Root Cap - Parenchymatous, protective apex of root. Root Hair - Lateral, absorbing outgrowth of the epidermal cell
- Secondary Root. Lateral root with root cap and hairs, derived from the pericycle
- Rootstock. A term applied to miscellaneous types of underground stems or parts
- Roridulate or Dewy. Covered with waxy platelets, appearing dewy
- Rostellum or Beak. Persistent stylar base on fruit
- Rosulate. Leaves in a rosette
- Rotate. Wheel-shaped, with short tube and wide limb at right angles to tube
- Rounded. Margins and apex forming a smooth arc
- Rudimentary. Basal, small nonperipheral embryo in small to large seed; relatively undifferentiated; endosperm copious
- Rugose. Covered with coarse reticulate lines
- Ruminate. Coarsely wrinkled, appearing as chewed
- Runcinate. Oblanceolate with lacerate to parted margins
- Runner or Stolon. An indeterminate, elongate, above ground propagative stem, with long internodes, rooting at the tip forming new plants