Pleuron. The lateral plate on either side of the arthropod somite.

Plexus. a network of interlaced.

Pneumonia. A human lung disease caused by a number of bacterial and viral pathogens.

Polar. (Of bacterial flagella) arising at one or both ends of cell.

Polar Body (Polocyte). Minute cell produced during development of oocyte containing one of the nuclei derived from first or second division of meiosis but practically no cytoplasm.

Polarity. The morphological and / or physiological difference between the two ends of an axis such as root and stem.

Pollen Tube. Tube formed on germination of pollen grain that carries male gametes to egg. In flowering plants each pollen tube penetrates tissues of stigma and style into ovary.

After entering ovary pollen tube grows towards an ovule, passing down micropyle to nucellus and finally penetrating nucellus tissues into embryo sac where it ruptures at the tip, setting free two male gametes one of which fuses with egg nucleus.

Polocyte. Polar body.

Polyandry. A system of polygamy in which the female has several mating partners at one time.

Polygymy. A system of polygamy in which the male has several female mating partners at one time.

Polymer. A compound of high molecular weight formed of long chain of repeating units.

Polymorphism. Having several different forms. Particularly used of distinct kinds of individual belonging to one species, and occurring in fairly constant proportions within a freely interbreeding population.

Polyp (Hydranth). Sedentary form of coelenterate, of subphylum Cnidaria, e.g. Hydra, sea-anemone. Cylindrical trunk, fixed at one end; mouth at other end, surrounded by ring of tentacles.

Many polyps reproduce themselves by budding, and may form colonies.