GLOSSARY 423 



Nec'tar (Gr. nectar, drink of the gods) : a sweet fluid secreted by certain 

 groups of cells known as nectar glands in a flower. From this substance 

 bees make honey. 



Neu'ron : a nerve cell. 



Nic'titating membrane (Lat. nidare, to wink) : the third eyelid, a delicate 

 membrane covering the eye in birds and frogs. 



Ni'trate : a soluble salt of nitric acid. 



Ni'trogen (Lat. nitrum, natron ; -gen, producing) : a gaseous element, found 

 in many organic compounds and forming almost four fifths of the atmos- 

 phere. 



Nu'cleus (Lat. nucleus, a kernel) : the center of activity in the living cell. 



Nu'trient (Lat. nutrire, to nourish) : nourishing substance contained in foods. 



Nu'tritive ratio : the proportion of protein in the diet. 



Oils : a class of nutrients composed of much carbon and hydrogen, with a 



little oxygen. 

 Ommatid'itmi (Gr. omma, eye) : one of the elements of a compound eye. 

 Oper'culum (Lat. operculum, a lid) : a lid or flap covering the gills. 

 Op'sonin (Gr. opsonein, to cater for) : a substance in the blood which helps 



colorless corpuscles destroy bacteria. 

 Organ : each part in an animal or plant which performs some special work. 

 Or'ganism (Gr. organon, an instrument) : a body which is made up of organs 



or parts, each of which has a special function ; any animal or plant. 

 Osmo'sis : diffusion of water through a semi-permeable membrane, the greater 



flow being toward the lesser concentration of water. 

 O'vary (Lat. ovum, egg) : in a plant, the base of a pistil, containing the ovules ; 



in an animal, the egg-forming gland. 

 Ovipos'itor (Lat. ovum, egg ; ponere, to place) : a specialized structure for 



depositing eggs, found in insects. 

 O'vule : a rounded structure in the ovary, which may become a seed. 

 Oxida'tion : the chemical union of oxygen with some other substance. 

 Ox'ygen (Gr. oxus, acid ; -gen, producing) : a gaseous element found in the 



air and in many compounds. 

 Oxyhaemoglo'bin : a combination of oxygen with haemoglobin. 



Pal'ate (Lat. palatum) : the roof of the mouth. The hard palate is sup- 

 ported by bone ; the soft palate is a fold of mucous membrane lying pos- 

 terior to the hard palate. 



Palisade layer : a layer of green cells under the upper epidermis of a leaf. 



Pal'pus or palp (Lat. palpare, to touch) : in arthropods, an appendage at- 

 tached to a mouth part ; usually an organ of touch or taste. 



