nators. Sterile leaf-like (white, colorless, but usually colored) structures in flowers that serve to attract pollinators. PICTURE

petiole The generally non-leafy part of the leaf that attaches the leaf blade to the stem; celery and rhubarb are examples of a leaf petiole that we use as food. The stalk connecting the leaf blade to the stem. PICTURE

petrifaction Mode of fossilization where f organic matter is replaced with silica.

PGA (phosphoglycerate)  A three-carbon molecule formed when carbon dioxide is added to ribulose biphosphate (RuBP) during the dark reaction of photosynthesis (Calvin, or Calvin-Benson Cycle). PGA is converted to PGAL, using ATP and NADPH.

PGAL (phosphoglyceraldehyde)   A substance formed from PGA during the dark reaction of photosynthesis. Some PGAL leaves the cycle and can be converted to glucose, while other PGAL molecules are used to reform ribulose biphosphate (RuBP) to continue the dark reaction.

pH The negative logarithm of the H⁺ ion concentration. The pH is a measure of the acidity or basic character of a solution. Since it measures a fraction, the larger the pH number, the less H ions are present in a solution. PICTURE

phagocytes  White blood cells that can engulf (by phagocytosis) and destroy microorganisms including viruses and bacteria; cells in this category include neutrophils and monocytes.

phagocytosis  A form of endocytosis in which white blood cells surround and engulf invading bacteria or viruses. PICTURE

pharynx  The passageway between the mouth and the esophagus and trachea. Food passes from the pharynx to the esophagus, and air passes from the pharynx to the trachea.

phenotype  The observed properties or outward appearance of a trait. The physical expression of the alleles posessed by an organism.

pheromones  Chemical signals that travel between organisms rather than between cells within an organism; serve as a form of communication between animals.

phloem  Tissue in the vascular system of plants that moves dissolved sugars and other products of photosynthesis from the leaves to other regions of the plant. Phloem tissue consists of cells called sieve tubes and companion cells. Cells of the vascular system in plants that transport food from leaves to other areas of the plant. PICTURE 1 | PICTURE 2 | PICTURE 3

phosphate group A chemical group composed of a central phosphorous bonded to three or four oxygens. The net cha