Photometer 



An instrument for measuring the intensity of light. 



Photon 



One of the particles in a beam of radiant energy, cf. 

 Quantum. 



Photonasty 



The response of an organism or organ to a diffuse light 

 stimulus, e. g., closing of oxalis flowers in the evening. 



Photoperiod 



The duration of light during a 24-hour period. 



Photoperiodism (Photoperiodicity) 



The response of plants and animals to the relative dura- 

 tion of light and darkness, e. g. a chrysanthemum blooming 

 under short days and long nights. 



Photosynthesis 



The synthesis of carbohydrates from carbon dioxide and 

 water by Chlorophyll using light as energy with oxygen as a 

 by-product. 



Phototaxis 



The directed movement of a motile organism in response 

 to a light stimulus, cf. Photokinesis. 



Phototrophic 



Refers to organisms that can obtain energy from sunlight, 

 cf. Autotrophic, Heterotrophic, Chemotrophic. 



Phototropic 



The growing or turning of an organ of a plant when un- 

 equally illuminated, toward the light of greater intensity. 



Phragmosis 



The process of opening or closing holes by animals, e. g., 

 an ant closing its entrance to the nest in a plant stem. 



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