- Imbricate. Having margins overlapping
- Imparipinnate. Pinnate with a conform terminal leaflet. Imparipinnately Compound - Odd-pinnately compound, with a terminal leaflet
- Imperfect or Unisexual. With stamens or carpels absent in the flower
- Implicate. With both lamina margins folded sharply inward
- Incised. Margins sharply and deeply cut, usually jaggedly
- Inclinate. With lamina folded or curved transversely near the apex
- Inclined. Ascending at 46-75° angle of divergence
- Included Veinlets. Veins ending inside areoles
- Incomplete. Leaf without one or more parts: blade, petiole, stipules; one or more types of floral parts absent
- Incrassate. Thickened
- Incumbent or Notorhizal. Reclinate with sides of cotyledons against hypocotyl
- Incurved. Curved inward or upward
- Indehiscent Capsule. One that does not dehisce at maturity, as in Peplis
- Indeterminate or Evergrowing. Continual growth of plant parts, not limited by a cessation of meristematic activity
- Induplicate. Having margins bent inward and touching margin of each adjacent structure
- Indurate. Hardened
- Indusium. A flap of tissue covering a sorus
- Inequilateral. With unequal sides
- Inermous. Unarmed, without prickles or spines
- Inferior. Other floral organs attached above ovary with hypanthium adnate to ovary
- Inflated. Swollen or thickened, as in Eichhornia
- Inflexed. Bent abruptly inward or upward
- Infrafoliar. On the stem below the leaves, as in the Arecaceae
- Infrapetiolar or Subpetiolar. Axillary bud surrounded by base of petiole
- Infundibular. Funnel-shaped
- Integuments. Outer covering of ovule; embryonic seed coat
- Intercalary. Growth region near the base of an internode or base of blade
- Interfoliar. On the stem between the leaves, as in the Arecaceae
- Intermittent. A renewal and cessation of meristematic activity which produces clusters of stems and/or leaves along an axis
- Internode. A section or region of stem between nodes
- Interpetiolar. With connate stipules from two opposite leaves
- Interrupted or Discontinuous. Symmetry of arrangement broken, with uneven lengths of internodes
- Interstitial. Growth all-over in an organ, no localized meristems, as in some fruits
- Investing. Axial embryo usually erect with thick cotyledons overlapping and encasing the somewhat dwarfed hypocotyl; endosperm wanting or limited
- Involucel. Small involucre; secondary involucre
- Involucre. A group or cluster of bracts subtending an inflorescence
- Involute. Margins or outer portion of sides rolled inward over upper or ventral surface
- Irregular. With floral parts within a whorl dissimilar in shape and/ or size
- Isocotylous. With cotyledons of same size and shape
- Isodynamous. With equally developed structures
- Isomerous. With same number of members in different whorls
- Isopetalous. With petals of same size and shape
- Isophyllous. With leaves of same size and shape
- Isosepalous. With sepals of same size and shape
- Isostichous. With equal rows
- Jointed. With stems that can be pulled apart easily at the nodes, as in Equisetum
- Keel. The two united petals of a papilionaceous flower; any structure ridged like the bottom of a boat
- Lacerate. Margins irregularly cut, appearing torn
- Laciniate. Cut into closely parallel ribbonlike or straplike projections
- Lacuna. Chamber or internal air space
- Lamina. The leaf tissue other than the veins or axes
- Laminar. Leaf-like stamen without a distinct anther and filament but with embedded or superficial microsporangia, as in Degeneria
- Laminate. With the placenta over the inner surface of the ovary wall
- Lammas Shoots. Abnormal late season shoots that develop from the terminal bud, not a recurring phenomenon as in flushing shoots
- Lanate. Covered with long, intertwined trichomes, cottony
- Lanceolate. Lance shaped, much longer than wide; widened at or above the base and tapering to the apex
- Lanuginose. Cottony, similar to lanate but trichomes shorter
- Lateral or Axillary. On the side of a structure or at the nodes of the axis. Lateral Embryo - Basal or baso-lateral embryo, discoid or lenticular, usually surrounded by copious endosperm. Lateral Leaf - Leaf on the side of the stem, as in heterophyllous species of Selaginella
- Laterospermous. On the side of the seed
- Latiflorous. With broad-flowers
- Leaf. A photosynthetic and transpiring organ, usually developed from leaf primordium in the bud; an expanded, usually green, organ borne on the stem of a plant. Leaf Primordium - Meristematic tissue that gives rise to a leaf. Leaf Scar - A mark indicating former place of attachment of petiole or leaf base. Linear Leaf - Narrow, flattened, triangular, or quadrangular leaf usually 1/2-2 in. long; e.g., Taxus, Picea
- Leaflet. A distinct and separate segment of a leaf
- Leaf-opposed. On stem opposite the base of the leaf, as in Alchemilla
- Legume. A usually dry, dehiscent fruit derived from one carpel that splits along two sutures
- Lemma. Outer scale subtending grass floret
- Lenticel. A pore in the bark
- Lenticular. Biconvex, usually elongate and flattish
- Lepidote or Squamulose. Covered with minute scales
- Leptophyllous. With leaves to 25 sq. mm in size.
- Leptosporangiate. Having the entire sporangium develop from a periclinal division of a superficial cell or small group of cells
- Ligneous. Woody
- Ligulate or Ray. Strap-shaped
- Ligule. An outgrowth or projection from the top of the sheath, as in the Poaceae; the strap-shaped portion of a ray or ligulate corolla; a small membranous outgrowth or projection at the base of the leaf, appearing above the sporangium in fertile leaves, as in Selaginella and Isoetes
- Limb. Expanded portion of corolla or calyx above the tube, throat or claw
- Lineate. In lines, stigmatic surface linear
- Lingulate. Tongue-shaped, plano-convex in cross section
- Lip or Labellum. Either of two variously shaped parts into which a corolla or calyx is divided, usually into an upper and lower lip, as in the Lamiaceae and Orchidaceae. Lip Cells - The line of cells between which the sporangium dehisces
- Lobate. Lobe rounded; sinus depth variable; outer and inner margins concave
- Lobe. Any, usually rounded, segment or part of the perianth
- Lobed. Divided into lobes
- Locule. Compartment of an anther; ovary cavity
- Capsule. Loculicidal Capsule One that dehisces longitudinally into the cavity of the locule, as in Epilobium
- Lodicule. Abortive perianth part in the Poaceae; hyaline scales at base of ovary in the Poaceae
- Loment. A legume that separates transversely between seed sections
- Long Shoot. Elongated internodes, rapid annual growth. Long Bud Shoots - Abnormal buds or shoots which elongate, then have arrested growth without the development of leaves and lateral branches
- Longitudinal. Dehiscing along long axis of theca
- Loose, Distant, or Scattered. Parts widely separated from one another, usually irregularly
- Lorate. With elongate vertical waves in the margins or sides at right angles to the longitudinal axis
- Lunate. Crescent-shaped, with acute ends
- Lyrate. Lyre-shaped; pinnatifid with large terminal lobe and smaller lower lobes