GLOSSARY 457 



Prothallus. The small heart-shaped sexual generation or gametophyte 



in the life cycle of ferns. 

 Prothrombin. An enzyme contained in the blood platelets concerned in 



the formation of a blood clot. 

 Proton. A component of an atom consisting of a positive electrical 



charge. 

 Protophyta. Plants consisting of single cells or of simple associations 



of cells that show no tissue specializations. 

 Protoplasm. A definitely organized colloidal substance consisting of at 



least 70 per cent water, about 15 per cent proteins, also fats, carbo- 

 hydrates, and inorganic substances, which is the physical basis of 



life. 

 Protozoa. Whole animals composed of single cells or of colonies of 



unspecialized cells. 

 PsEUDoccELE. A type of body cavity that appears secondarily in such 



forms as the Platyhelminthes. 

 PsEUDOPODiA. Temporary extrusions of the body of such forms as 



Amceba, which serve as organs of locomotion. 

 Pulp cavity. The cavity in the centre of the tooth and of the placoid 



scale. 

 Radial symmetry. The structure of the organism is such that all parts 



are symmetrically arranged with reference to an axis. 

 Radical. A group of atoms that forms a part of a molecule and may be 



replaced by a single atom, or which acts as a unit in reactions. For 



example, in H2SO4 the SO4 is a radical that is replaceable by some 



single atoms and in reactions forms the negative ion of metallic 



salts. 

 Ramus. A connecting nerve fibre. 

 Recapitulation. See Biogenetic law. 

 Receptacle. The structure in the base of a flower which contains the 



Receptor. An organ for the reception of stimuli. 



Recessive. A member of a pair of alternative characters is said to be 



recessive when the progeny of a cross between a parent having the 



character and one having its alternative all resemble the alternative 



contributed by the second-mentioned parent. 

 Reductional division. That division during maturation of the germ 



cells in which the number of chromosomes is reduced from the 



diploid to the haploid in the daughter cells. 



