- Major. Greater in size
- Malacophily. Pollinated by snails or slugs
- Male or Staminate. Plant with staminate flowers only
- Marbled. A surface traversed by irregular veins of color; as block of marble often is
- Marcescent. Usually ephemeral with persistent remains; withering persistent
- Marginal. Pertaining to the border or edge
- Massula. A clump of microspores, as in Azolla
- Matutinal. With flowers opening in the morning
- Medial. Upon or along the longitudinal axis
- Median. With stipules adnate to petiole with free part of stipules near middle of petiole
- Median Leaf. Leaf on top of stem, as in heterophyllous species of Selaginella
- Megasporangium. The sporangium in which megaspores are produced
- Megaspore. A spore that gives rise to a female gametophyte
- Megasporophyll. Modified leaf bearing ovules; e.g., Zamia
- Melittophily. Pollinated by bees
- Membranous. Thin and semi-translucent; membrane-like
- Meniscoidal. Thin and concaveconvex
- Mericarp. A portion of fruit that seemingly matured as a separate fruit
- Mermaphrodigynous. Inflorescence with hermaphroditic flowers inside or above and pistillate outside or below (hermaphrodigynecephalous)
- Mesocarp. Middle layer of pericarp
- Metaphyll. Adult leaf
- Micro. Axial embryo in minute seeds, less than 0.2 mm long; minute and undifferentiated to almost total size of seed
- Micromelittophily. Pollinated by small bees
- Micropyle. Hole through integument(s)
- Microsporangium. The sporangium in which microspores are produced
- Microspore. A spore that gives rise to a male gametophyte
- Microsporophyll. Modified leaf bearing microsporangia or pollen sacs
- Midrib. The central conducting and supporting structure of the blade of a simple leaf
- Midvein. The central conducting and supporting structure of the blade of a leaflet
- Minor. Smaller in size
- Minute. Very small
- Mixed. Contains flower, leaf, and stem primordia; will give rise to branch with leaves and flower(s)
- Monad. Pollen grains occurring singly
- Monadelphous. With one group of stamens connate by their filaments
- Moniliform. Elongate roots with regularly arranged swollen areas
- Monocarpellate. One-carpelled
- Monocarpic, Hapaxanthic. Perennial or annual, flowering and fruiting once, then dying; fruiting once
- Monocephalous. One-headed, as in composites
- Monochasium. A cymose inflorescence with one main axis
- Monocotyledonous. With one cotyledon
- Monocyclic. One-whorled
- Monoecious. With staminate and carpellate flowers on same plant
- Monoecious. Plant with all flowers imperfect, but staminate and pistillate flowers on same plant
- Monolete Spore. Bean-shaped, with a single scar line
- Monomerous. Whorl with one member
- Monomorphic. All of the same shape and size
- Monophyllous. One-leaved
- Monopodial. Branching with a main axis and reduced or missing laterals; excurrent
- Mucilaginous. Gummy or gelatinous
- Mucronate. Less than 3:1 l/w, straight and stiff
- Mucronulate. 1:1 l/w or broader than long; straight
- Multicellular. Many-celled
- Multicipital. With many axes or stems from one rootstock or caudex
- Multicostal. Many-ribbed
- Multilocular. Many-locular
- Multinodal Shoot. Spring shoot developing from the terminal winter bud and producing 2 or more whorls of branches; the cones are partly lateral in the middle of the shoot; e.g., Pinus echinata
- Multiseriate. Many-rowed; in many series
- Multistriate. Many-lined
- Muricate. Covered with short, hard protuberances
- Muriculate. Minutely muricate
- Muticous. Without a vein extension, awn or hair
- Myiophily. Pollinated by diptera
- Myrmecophily. Pollinated by ants
- Nanophyllous. With leaves to 225 sq. mm in size.
- Napiform. Turnip-shaped
- Navicular. Boat-shaped
- Neck. Narrowed portion of hypanthium, between the base and a flared limb
- Necrocoleopterophily. Pollinated by carrion beetles
- Nectar Secreting Glands. Sugary compound producing glands found mostly on floral parts that produce attractants for pollinators
- Nectary. A specialized nectar-secreting structure or area
- Needle. Acicular; slender, elongated leaf, usually over 2 in long; e.g., Pinus
- Net. Veins uniting to form a network
- Neuter or Agamous. Without stamens and carpels in flower; or sex organs abortive
- Nocturnal. Opening during night
- Nodal. At the nodes
- Node. Point on the stem where leaves are attached; or the point of branching of the stem
- Nodiform or Nodulose. Knotty or knobby, as the roots of most of the Fabaceae
- Nucellus. Female sporangium within ovule; megasporangium in seed plants
- Nut. A one-seeded, dry, indehiscent fruit with a hard pericarp, usually derived from a one-loculed ovary
- Nutlet. A small nut
- Obconic. Inversely conical
- Obcordiform. Inversely cordiform
- Obdiplostemonous. With stamens in two whorls, outer opposite petals, inner opposite the sepals
- Oblique. Having an asymmetrical base
- Oblong. With widest axis at midpoint of structure and with margins essentially parallel
- Obovate. Inversely ovate
- Obscure. Not clearly visible macroscopically, usually owing to incomplete differentiation
- Obtriangular. Inversely triangular
- Obtrullate. Inversely trullate
- Obtuse. Margins straight to convex, forming a terminal angle more than 90
- Ocellated. A broad spot of some color has another spot of a different color within it
- Ocreate. Having a stipular tube surrounding stem above insertion of petiole or blade
- Ocreolate. Diminutive of ocreate; usually applied to bract bases
- Oligomerous. With reduction in number of members within whorl
- Oligotaxy. Reduction in number of whorls
- Opaque. With a dull surface
- Operculate Capsule. One that dehisces through pores, each of which is covered by a flap, cap, or lid, as in Papaver
- Opposite. Two leaves or other structures per node, on opposite sides of stem or central axis
- Ornithophily. Pollinated by birds
- Orthotropous or Atropous. With straight body so that funicular attachment is at one end and micropyle at other
- Osseous. Bony
- Ovary. Ovule-bearing part of pistil
- Ovate. With widest axis below middle and with margins symmetrically curved; egg-shaped
- Ovulate. Female; bearing ovules only in the flowers
- Ovule. Embryonic seed consisting of integument(s) and nucellus
- Ovuliferous Scale. Highly modified lateral branch in the axil of a leaf (bract), and bearing ovules; may be flat or peltate, woody or fleshy; e.g., Pinaceae