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Nature of the Checks to Increase - Chapter III - Origin of Species

Page 124 Contents - 'The Origin of Species' by Charles Darwin prev page     next page
 

PARTHENOGENESIS.--The production of living organisms from unimpregnated eggs or seeds.

PEDUNCULATED.--Supported upon a stem or stalk. The pedunculated oak has its acorns borne upon a footstool.

PELORIA or PELORISM.--The appearance of regularity of structure in the flowers of plants which normally bear irregular flowers.

PELVIS.--The bony arch to which the hind limbs of vertebrate animals are articulated.

PETALS.--The leaves of the corolla, or second circle of organs in a flower. They are usually of delicate texture and brightly coloured.

PHYLLODINEOUS.--Having flattened, leaf-like twigs or leafstalks instead of true leaves.

PIGMENT.--The colouring material produced generally in the superficial parts of animals. The cells secreting it are called PIGMENT-CELLS.

PINNATE.--Bearing leaflets on each side of a central stalk.

PISTILS.--The female organs of a flower, which occupy a position in the centre of the other floral organs. The pistil is generally divisible into the ovary or germen, the style and the stigma.

PLACENTALIA, PLACENTATA.--or PLACENTAL MAMMALS, See MAMMALIA.

PLANTIGRADES.--Quadrupeds which walk upon the whole sole of the foot, like the bears.

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 unisexual and others hermaphrodite. The unisexual (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 passed in perfect repose. The CHRYSALIS is the pupal state of butterflies.

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 class 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 destitute 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.

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