OPERCULUM, lid; applied to the separable part of the theca of mosses; also applied to the lid of certain seed-vessels. OPPOSITE, applied to leaves placed on opposite sides of the same stem at the same level. ORBICULAR, rounded leaf with petiole attached to the centre of it. ORGANOGRAPHY, the description of the organs of plants. ORTHOTROPAL, ovule with foramen opposite to the hilum; embryo with radicle next the hilum. OSMOSE, the force with which fluids pass through membranes in experiments on exosmose and endosmose. OVAL, elliptical, blunt at each end. OVARY, the part of the pistil which contains the ovules. OVATE, shaped like an egg; applied to the broader end of the egg next the petiole or axis. OVOID, egg-shaped. OVULE, the young seed contained in the ovary. P PALE, the part of the flower of grasses within the glume; also applied to the small scaly laminæ which occur in the receptacle of some _Compositæ_. PALÆPHYTOLOGY, the study of fossil plants. PALEACEOUS, chaffy, covered with small, erect, membraneous scales. PALMATE and PALMATIFID, applied to a leaf with radiating venation, divided into lobes to about the middle. PALMATIPARTITE, applied to a leaf with radiating venation, cut nearly to the base in a palmate manner. PANDURIFORM, shaped like a fiddle. PANICLE, inflorescence of grasses, consisting of spikelets on long peduncles coming off in a racemose manner. PANICULATE, forming a panicle. PAPILIONACEOUS, corolla composed of vexillum, two alæ, and carina, as in the pea. PAPILLOSE, covered with small nipple-like prominences. PAPPUS, the hairs at the summit of the ovary in _Compositæ_. They consist of the altered calycine limb. _Pappose_, provided with pappus. PARAPHYSES, filaments, sometimes articulated, occurring in the fructification of mosses and other cryptogams. PARASITE, attached to another plant, and deriving nourishment from it. PARENCHYMA, cellular tissue. PARIETAL, applied to placentas on the wall of the ovary. PARIPINNATE, a compound of pinnate leaf ending in two leaflets. PARTHENOGENESIS, production of perfect seed with embryo, without the application of pollen. PATENT, spreading widely. PATULUS, spreading less than when patent. PECTINATE, divided laterally into narrow segments like the teeth of a comb. PEDATE and PEDATIFID, a palmate leaf of three lobes, the lateral lobes bearing other equally large lobes on the edges next the middle lobe. PEDICEL, the stalk supporting a single flower. PEDUNCLE, the general flower-stalk or floral axis; sometimes it bears one flower, at other times it bears several sessile or pedicellate flowers. PELAGIC, growing in the ocean. PELLUCID, transparent. PELORIA, a name given to a teratological phenomenon, which consists in a flower that is usually irregular becoming regular; for instance, when _Linaria_, in place of one spur, produces five.