phylogenetic Pertaining to a phylogeny.

phylum  The broadest taxonomic category within kingdoms (pl.: phyla). PICTURE

phytochrome  A pigment in plant leaves that detects day length and generates a response; partly responsible for photoperiodism.

phytoplankton  A þoating layer of photosynthetic organisms, including algae, that are an important source of atmospheric oxygen and form the base of the aquatic food chain.

pilus Projection from surface of a bacterial cell (F+) that can donate genetic Material to another (F-).

pineal gland  A small gland located between the cerebral hemispheres of the brain that secretes melatonin.

pioneer community  The initial community of colonizing species.

pistil Female reproductive structures in flowers, consisting of the stigma, style, and ovary. Also known as a carpel in some books. PICTURE 1 | PICTURE 2

pith Central area in plant stems, largely composed of parenchyma tissue modified for storage. PICTURE 1 | PICTURE 2

pituitary gland  A small gland located at the base of the brain; consists of an anterior and a posterior lobe and produces numerous hormones. The master gland of the endocrine system, the pituitary releases hormones that have specific targets as well as those that stimulate other glands to secrete hormones. Part of the pituitary is nerve tissue, the rest is glandular epithelium. PICTURE 1 | PICTURE 2

placenta  An organ produced from interlocking maternal and embryonic tissue in placental mammals; supplies nutrients to the embryo and fetus and removes wastes.

placental mammals One of three groups of mammals that carry their young in the mother's body for long periods during which the fetus is nourished by the placenta. Humans are placental mammals.

planaria  Small free-living þatworms (Phylum Platyhelminthes) with bilateral symmetry and cephalization. The freshwater type is often used as an experimental organism.

planktonic organisms  "Floaters"; one of the two main types of organisms in the pelagic zone of the marine biome.

Plantae The plant kingdom; nonmobile, autotrophic, multicellular eukaryotes. Kingdom of the plants, autotrophic eukaryotes with cellulose in their cell walls and starch as a carbohydrate storage product, with chlorophylls a and b as photosynthesis pigments. PICTURE

plasma  The liquid portion of the blood.