primary growth Cells produced by an apical meristem. The growth a plant by the actions of apical meristems on the shoot and root apices in producing plant primary tisues.

primary macronutrients  Elements that plants require in relatively large quantities: nitrogen, phosphorus, and potassium.

primary meristems The apical meristems on the shoot and root apices in plants that produce plant primary tissues.

primary root  The Þrst root formed by a plant.

primary structure  The sequence of amino acids in a protein. PICTURE

primates The taxonomic order of mammals that includes prosimians (lemurs and tarsiers), monkeys, apes, and humans; characteristics include large brain, stereoscopic vision, and grasping hand.

principle of independent assortment  Mendel's second law; holds that during gamete formation, alleles in one gene pair segregate into gametes independently of the alleles of other gene pairs. As a result, if enough gametes are produced, the collective group of gametes will contain all combinations of alleles possible for that organism.

principle of segregation  Mendel's Þrst law; holds that each pair of factors of heredity separate during gamete formation so that each gamete receives one member of a pair.

prions  Infectious agents composed only of one or more protein molecules without any accompanying genetic information.

producers  The Þrst level in a food pyramid; consist of organisms that generate the food used by all other organisms in the ecosystem; usually consist of plants making food by photosynthesis.

progesterone One of the two female reproductive hormones secreted by the ovaries.

prokaryote  Type of cell that lacks a membrane-bound nucleus and has no membrane organelles; a bacterium. Prokaryotes are more primitive than eukaryotes. Cells lacking membrane-bound organelles and having a single circular chromosome, and ribosomes surrounded by a cell membrane. Prokaryotes were the first forms of life on earth, evolving over 3.5 billion years ago.

prolactin  A hormone produced by the anterior pituitary; secreted at the end of pregnancy when it activates milk production by the mammary glands.

promoter  The speciÞc nucleotide sequence in DNA that marks the beginning of a gene. PICTURE

prophase  1) The Þrst stage of mitosis during which chromosomes condense, the nuclear e