• plankton - drifting organisms
• planozygote - a motile zygote
• plurilocular - having many small chambers or locules
• pneumatocyst - gas bladder or float as in the brown alga Macrocystis
• polyglucan granules - polymers of glucose somewhat like glycogen in animals
• polysiphonous - having many filaments or pericentral cells as in the red alga Polysiphonia
• polysporangium - a sporangium producing many spores, homologous with a tetrasporangium
• polystichous - type of thallus constructed of true parenchyma as in some brown algal orders