Appefidix 851 



Plasma membrane, living semipermeable membrane covering the cytosome of 

 certain cells {see Cell membrane). 



Plasmodesma (plaz mo -dez' ma) (Gr. plasma^ something formed; desma, bond)', 

 protoplasmic connection between cells (plural, plasmodesmata). 



Plasmolysis (plaz -mol' i sis) (Gr. plasma, liquid; lysis, loosening), shrinking of 

 the cytoplasm in a living cell due to loss of water. 



Plasmodium (plaz -mo' di um) (Gr. plasma, formed), naked, protoplasmic mass, as 

 in slime mold. 



Plasmosome (plaz'mosom) (Gr. plasma, liquid; soma, body), body known as the 

 nucleolus within the liquid of the nucleus. 



Plastid (plas'tid) (Grt plastes, to form), specialized protein body in a cell con- 

 cerned with producing a certain substance. 



Platyhelminthes (plat i hel -min' thez) (Gr. platus, flat; helmins, worm), flat- 

 worms. 



Plecoptera (pie -kop' ter a) (Gr. plekos, folded; ptera, wings), order of insects to 

 which the stone flies belong. 



Pleura (ploor' a) (Gr. pleura, rib or side), membranous lining of thoracic cavity 

 of mammals and covering the lungs in the cavity. 



Plexus (plek'sus) (L. plexus, interwoven), network of nerves or blood vessels. 



Plumule (ploo'mul) (L. pluma, feather), primary bud of an embryo seed plant. 



Poison (poy'sin) (L. poto, to drink), substance harmful to an organism. 



Polar body, see Polocyte. 



Polarity (po -lar' i ti) (Gr. polos, pivot), having two opposite poles with different 

 physiologic values. In an egg, there is usually a formative animal pole 

 and a nutritive vegetal pole. 



Polar transportation, movement of plant hormones in young tissues in a basipetal 

 direction. 



Polarized growth, development of younger plant tissues in length rather than 

 another direction due to specific plant hormones and certain environ- 

 mental conditions. 



Polian vesicle (after the Italian, Poli), bulblike organ of the water vascular sys- 

 tem of certain echinoderms. 



Pollen (pol' en) (L. pollen, fine flour), dustlike grains of material produced by 

 the male anthers of flowers. 



Pollen tube, formed by a pollen grain and transports sperm to the eggs in ovules. 

 A pollen grain and its mature tube^ are the male microgametophyte. 



Pollination (pol i -na' shun), application of male pollen to the female stigma, or 

 ovule, of a plant. 



Polocyte (po' lo site) (Gr. polos, pole; kytos, cell), small cell separated from the 

 egg during maturation; function unknown; also called polar body. 



Polygamy (po-lig'ami) (Gr. poly, many; gamos, marriage), more than one mate 

 at one time. 



Polymorphism (poly -mor' fizm) (Gr. poly, many; morphe, form), more than two 

 types or castes of individuals in a colony or community which belong to 

 the same species and are derived from the same parents. The various 

 castes of honeybees, ants, termites, etc., are typical. 



Polyp (pol' ip) (Gr. poly, many), sessile phase of the life history of certain 

 coelenterates. 



