PLASTIC.--Readily capable of change.

PLEISTOCENE PERIOD.--The latest portion of the Tertiary epoch.

PLUMULE (in plants).--The minute bud between the seed-leaves of newly-germinated plants.

PLUTONIC ROCKS.--Rocks supposed to have been produced by igneous action in the depths of the earth.

POLLEN.--The male element in flowering plants; usually a fine dust produced by the anthers, which, by contact with the stigma effects the fecundation of the seeds. This impregnation is brought about by means of tubes (POLLEN-TUBES) which issue from the pollen-grains adhering to the stigma, and penetrate through the tissues until they reach the ovary.

POLYANDROUS (flowers).--Flowers having many stamens.

POLYGAMOUS PLANTS.--Plants in which some flowers are unis.e.xual and others hermaphrodite. The unis.e.xual (male and female) flowers, may be on the same or on different plants.

POLYMORPHIC.--Presenting many forms.

POLYZOARY.--The common structure formed by the cells of the Polyzoa, such as the well-known seamats.

PREHENSILE.--Capable of grasping.

PREPOTENT.--Having a superiority of power.

PRIMARIES.--The feathers forming the tip of the wing of a bird, and inserted upon that part which represents the hand of man.

PROCESSES.--Projecting portions of bones, usually for the attachment of muscles, ligaments, etc.

PROPOLIS.--A resinous material collected by the hivebees from the opening buds of various trees.

PROTEAN.--Exceedingly variable.

PROTOZOA.--The lowest great division of the animal kingdom. These animals are composed of a gelatinous material, and show scarcely any trace of distinct organs. The Infusoria, Foraminifera, and sponges, with some other forms, belong to this division.

PUPA (pl. PUPAE).--The second stage in the development of an insect, from which it emerges in the perfect (winged) reproductive form. In most insects the PUPAL STAGE is pa.s.sed in perfect repose. The CHRYSALIS is the pupal state of b.u.t.terflies.

RADICLE.--The minute root of an embryo plant.

RAMUS.--One half of the lower jaw in the Mammalia. The portion which rises to articulate with the skull is called the ASCENDING RAMUS.

RANGE.--The extent of country over which a plant or animal is naturally spread. RANGE IN TIME expresses the distribution of a species or group through the fossiliferous beds of the earth's crust.

RETINA.--The delicate inner coat of the eye, formed by nervous filaments spreading from the optic nerve, and serving for the perception of the impressions produced by light.

RETROGRESSION.--Backward development. When an animal, as it approaches maturity, becomes less perfectly organised than might be expected from its early stages and known relationships, it is said to undergo a RETROGRADE DEVELOPMENT or METAMORPHOSIS.

RHIZOPODS.--A cla.s.s of lowly organised animals (Protozoa), having a gelatinous body, the surface of which can be protruded in the form of root-like processes or filaments, which serve for locomotion and the prehension of food. The most important order is that of the Foraminifera.

RODENTS.--The gnawing Mammalia, such as the rats, rabbits, and squirrels. They are especially characterised by the possession of a single pair of chisel-like cutting teeth in each jaw, between which and the grinding teeth there is a great gap.

RUBUS.--The bramble genus.

RUDIMENTARY.--Very imperfectly developed.

RUMINANTS.--The group of quadrupeds which ruminate or chew the cud, such as oxen, sheep, and deer. They have divided hoofs, and are dest.i.tute of front teeth in the upper jaw.

SACRAL.--Belonging to the sacrum, or the bone composed usually of two or more united vertebrae to which the sides of the pelvis in vertebrate animals are attached.

SARCODE.--The gelatinous material of which the bodies of the lowest animals (Protozoa) are composed.

SCUTELLAE.--The h.o.r.n.y plates with which the feet of birds are generally more or less covered, especially in front.

SEDIMENTARY FORMATIONS.--Rocks deposited as sediments from water.

SEGMENTS.--The transverse rings of which the body of an articulate animal or annelid is composed.

SEPALS.--The leaves or segments of the calyx, or outermost envelope of an ordinary flower. They are usually green, but sometimes brightly coloured.

SERRATURES.--Teeth like those of a saw.

SESSILE.--Not supported on a stem or footstalk.

SILURIAN SYSTEM.--A very ancient system of fossiliferous rocks belonging to the earlier part of the Palaeozoic series.

SPECIALISATION.--The setting apart of a particular organ for the performance of a particular function.

SPINAL CORD.--The central portion of the nervous system in the Vertebrata, which descends from the brain through the arches of the vertebrae, and gives off nearly all the nerves to the various organs of the body.

STAMENS.--The male organs of flowering plants, standing in a circle within the petals. They usually consist of a filament and an anther, the anther being the essential part in which the pollen, or fecundating dust, is formed.

STERNUM.--The breast-bone.

STIGMA.--The apical portion of the pistil in flowering plants.

STIPULES.--Small leafy organs placed at the base of the footstalks of the leaves in many plants.

STYLE.--The middle portion of the perfect pistil, which rises like a column from the ovary and supports the stigma at its summit.

SUBCUTANEOUS.--Situated beneath the skin.

SUCTORIAL.--Adapted for sucking.

SUTURES (in the skull).--The lines of junction of the bones of which the skull is composed.

TARSUS (pl. TARSI).--The jointed feet of articulate animals, such as insects.

TELEOSTEAN FISHES.--Fishes of the kind familiar to us in the present day, having the skeleton usually completely ossified and the scales h.o.r.n.y.

TENTACULA or TENTACLES.--Delicate fleshy organs of prehension or touch possessed by many of the lower animals.

TERTIARY.--The latest geological epoch, immediately preceding the establishment of the present order of things.