Rheotropism 



The response of an organism to a current. 



Rhesus Factor 



See RH factor. 



RH Factor 



An Antigen (q. v.) found in the blood of a large number 

 of human beings who consequently possess Antibodies against 

 the antigen. Syn. Rhesus factor. 



Rhizine 



An organ that serves for the attachment of certain organ- 

 isms such as the many intergrown filaments in many lichens. 



Rhizobia 



Bacteria that occur in the nodules of certain plants, 

 especially in the pea family, Leguminosae, and fix free litro- 

 gen into forms useful to the Host. 



Rhizoid 



A filamentous organ, one cell thick, found in mosses, 

 fern Gametophytes, and other plants, used for attachment 

 and probably also for absorption of water and nutrient salts. 



Rhizome 



An underground stem that produces shoots and roots at 

 the nodes, cf. Rootstock, Runner. 



Rhizosphere 



The portion of the soil close to and under the influence 

 of the root of a plant. 



Rhythm 



The more or less regular recurrence of phenomena such 

 as day and night, differences in animal behaviour, cf. 

 Periodicity. 



Ridge Terrace 



A long, low ridge with gently sloping sides and a shallow 

 channel along the upper side for the purpose of collecting 



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