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.