448 APPENDIX 



POLLINATION. The transference of pollen to the stigma of the pis- 

 til in higher Flowering Plants. 



POLYHYBRID. The progeny of parents which differ in regard to 

 more than three given characters. 



POLYMORPHISM. Occurrence of several types of individuals during 

 the life history, or composing a colony; e.g., in some Hydroids. 



POTENTIAL ENERGY. Energy possessed by virtue of stresses, i.e., 

 two forces in equilibrium. Criterion is work done against any 

 restoring force; e.g., kinetic energy of sunlight through agency 

 of chlorophyll separates C0 2 into C and 2 and thereupon is 

 represented by an equal amount of chemical potential energy. 

 Restoring force is here chemical affinity. Similarly a raised 

 weight possesses gravitational potential energy in amount 

 equal to kinetic energy expended in raising it. See Kinetic 

 Energy and Conservation of Energy. 



PREFORMATION. The abandoned doctrine that development is es- 

 sentially an unfolding of an individual ready-formed in the germ. 

 See Epigenesis. 



PRONEPHROS. Primitive kidney of Vertebrates. 



PRONUCLEI. The nuclei of the male and female gametes ready to 

 unite at fertilization. 



PROPHASE. Preparatory changes during mitosis leading to the dis- 

 position of the chromosomes in the center of the cell (equatorial 

 plate) ready for division. See Metaphase. 



PROSTOMIUM. A lobe which projects from the first segment of the 

 body of the Earthworm and forms an upper lip. 



PROTEIN. A class of complex chemical molecules, containing nitro- 

 gen, which form the chief characteristic constituent of proto- 

 plasm. 



PROTHALLUS. The gametophyte of Ferns. 



PROTISTA. Protophyta and Protozoa; all unicellular organisms. 



PROTONEMA. A filamentous growth from a Moss spore which gives 

 rise to the leafy Moss plant. 



PROTOPHYTA. Unicellular plants. See Protista. 



PROTOPLASM. The physical basis of life. Living matter. 



PROTOPLAST. The cell exclusive of the cell wall, especially in plants. 



