GLOSSARY 499 



Nitrogen-fixing bacteria. — Bacteria capable of changing nitrogen into 

 nitrates, found on root nodules of plants in the Pea Family. 



Nitrostarch. — A compound obtained by treating starch with nitric acid. 

 Used in high explosives. 



Nucleus. — ^A dense mass of protoplasm, usually found near the center 

 of the cell, governing most of the vital activities of the cell and impor- 

 tant in heredity as the location of chromatin. 



Nut. — A modified achene with a hard shell-like covering produced by 

 the ovary wall. 



Organ. — A part of a body, made up of tissues, that carries on a special 



function or group of functions; example, heart, leaf. 

 Organic. — Pertaining to a compound containing carbon, which has been 



derived directly or indirectly from living activities. 

 Organism.— Any living individual. 

 Osmosis. — Passage of water or other solvent through a semi-permeable 



membrane from a region of low concentration of a solute to one of 



greater concentration. 

 Ovary. — The enlarged region at the base of the pistil of a flower, where 



the ovules are found and where fertilization takes place. 

 Ovule. — The sporangium of a seed plant, in which the megaspores and 



female gametophytes are produced; found in the ovary of a flower. 

 Oxidation. — ^The union of oxygen with another substance. 



Pathogen. — A plant or animal which causes disease. 



Pathogenic. — Disease causing or producing. 



Palmate. — ^A type of leaf venation or compounding in which the veins 



or leaflets all radiate from a common point; example woodbine or 



horse-chestnut. 

 Parasite. — A heterotrophic organism which derives its nourishment 



from another living organism, living on or within it. 

 Parasitic. — Living at the expense of another, its host. See parasite. 

 Parenchyma. — Relatively unspecialized cells making up the fundamental 



or ground tissue of plants; common in leaves, flowers and fruits. 

 Pepo. — ^A type of fruit with an outer rind; example, squash or melon. 

 Perennial. — Any plant living for more than two years. 

 Petal. — One of the colored parts of the flower (see corolla) whose func- 

 tion is attraction of pollinating agents. 

 Phloem. — ^The special food-conducting channels in higher plants. 

 Photosynthesis. — ^The synthesis of carbohydrates from carbon dioxide and 



water, by plant cells containing chlorophyll, in the presence of 



sunlight. 



