photosynthesis (foe-toe-sin'lhe-sis) : manufacture 

 of carbohydrates out of carbon dioxide and 

 water by chlorophyll, using light energy, usu- 

 ally from sunlight. 

 phototropism (fo-tot'row-pism) : tropism in which 



the stimulus is light. 

 phylum (fy'lum) : used in classification; largest 

 division within the animal or plant kingdom. 

 pistil (pis'till) : organ in the center of a flower 

 within which the egg cell or cells are lo- 

 cated. 

 pistillate (pis'till-ate) : applied to flowers having 



pistils but no stamens. 

 pith: tissue of thin-walled cells sometimes found 



in the center of dicot stems. 

 pituitary (pit-two'i-ter-ree) body: ductless gland 

 at the base of the brain, secreting many hor- 

 mones, one of which regulates the growth of 

 the skeleton; often called the "master gland." 

 placenta (pla-sen'ta) : in mammals that part of 

 the wall of the uterus to which the embryo is 

 attached and through which it is nourished. 

 The placenta consists of membranes formed 

 by the uterus and by tlie embryo. In the 

 ovary of plants the placenta is the place of 

 attachment of the ovules. 

 plasma (plaz'ma): liquid part of the blood, usu- 

 ally straw colored, 

 plasma membrane: see cell membrane, 

 platelet (plate'let) : tiny blood cells that start the 

 process of blood clotting; under certain con- 

 ditions they break down and release a sub- 

 stance that aids the clotting. 

 pleura (ploo'ra) : thin, moist membrane that cov- 

 ers each lung and lines the chest cavity in 

 mammals. 

 plexus (plex'us) : in the autonomic nervous sys- 

 tem of the higher vertebrates it is a network 

 of ganglia and nerves lying near or within 

 the walls of some of the internal organs; in 

 the central nervous system, it is a network of 

 nerve fibers. 

 plumule (plume'you-1 ) : in the embryo of a seed 

 plant, the part that will grow into the shoot. 

 pollen (pol'en) : tiny grains produced by the 

 anther of the stamen of a flower (in conifers 

 by the male cone) ; within each grain there 

 is formed, besides other nuclei, the sperm nu- 

 cleus which will unite with the egg nucleus. 

 Pollen tube: tube that grows from the pollen 

 grain lying on the stigma; normally, it ex- 

 tends to an ovule, carrying the sperm nucleus 

 and the other nuclei toward the egg cell, 

 pollination (pol-in-ay'shun ) : transfer of pollen 

 from anther (of stamen) to stigma (of pis- 

 til). Self-pollination: transfer within the same 

 flower or between different flowers of the 

 same plant. Cross-pollination: transfer be- 

 tween flowers of different plants. 

 Polydactyly (polly-dak'tih-lee) : condition of hav- 

 ing more than the normal number of fingers 

 or toes. 



Porifera (pore-if'er-a) : phylum of invertebrates 

 whose bodies are pierced with many holes, 

 such as sponges. 

 posterior (poss-tee'ree-or) : hind end; referring to 

 the trunk of an anim.al, the end opposite the 

 head. 

 primates ( prim-ay'tees) : order of mammals in- 

 cluding those with the best developed brains, 

 such as monkeys, great apes, and man. 



protective resemblance: see adaptation, protec- 

 tive, 

 proteins (pro'tee-ins) : group of nitrogenous com- 

 pounds needed by all living things for mak- 

 ing protoplasm; plants make proteins from 

 simpler substances. 



prothallus (pro-thall'us) : in the ferns and their 

 relatives a liny, flat, green plant that grows 

 out of a spore, producing in time eggs and 

 sperm; sexual phase in the life history of 

 the fern. 



protonema (pro-toh-nee'ma ) : in mosses and their 

 relatives a tiny green, branching filament that 

 grows out of a spore; buds forming on the 

 filament grow into leafy plants. 



protoplasm ( pro'toh-plasm ) : living matter that 

 makes up all plants and animals. 



Protozoa ( proe-toe-zoe'ah ) : phylum of single- 

 celled animals. 



pseudopod (siu'doe-pod) : temporary projection 

 from the body of certain kinds of cells; used 

 in locomotion and food getting. 



psychiatrist (sy-kye'a-trist) : physician who is an 

 expert on mental illness. 



psychologist (sy-kol'o-jist) : person trained in the 

 science of hiunan and animal behavior. 



pteridophytes (ter'id-oh-fites) : in plant classifica- 

 tion the large group or phylum that includes 

 the ferns, club mosses, and horsetails. 



ptyalin (ty'a-lin) : digestive enzyme in saliva that 

 changes starch into one kind of sugar (malt- 

 ose) . 



pulse: regular expansion (and relaxation) of the 

 arteries caused by the successive contractions 

 of tbe heart; can be easily felt in the wrist. 



pupa (pew'pa): in the complete metamorphosis 

 of insects, the nonfeeding stage between 

 larva and adult. 



pure: in genetics a term applied to an organism 

 in which the two genes of a pair are alike; 

 the organism is then pure with respect to 

 that one pair of genes and to the character 

 tbey determine; the organism as a whole 

 would be pure only if this were true of every 

 pair of genes. 



pus: yellow-white substance that may form in an 

 infected part of the body, consisting of dead 

 white cells, dead and living germs, and a 

 little plasma. 



pyloric sphincter (pie-lor'ik sfink'ter) : a circular 

 band of nHis(l(> at the opening between the 

 stomach and small intestine; when relaxed, 

 food can go iiUo tbe iiUestine. 



.'591 



