Knight — 116 — Dictionary 



Pleiotropism. — The controlling of more than one character 

 by a single gene. Adj. Pleiotropic. See under Genuine 

 Pleiotropism. 



Pleiotropy. — Pleiotropism, q.v. 



Pleurotribe. — Of flowers : having the stamens and stigmas 

 so arranged as to encourage cross-pollination by automati- 

 cally rubbing against any insect visitor. 



Ploidy. — Duplication of the chromosome set. 



Plurivalent. — An association of more than two chromosomes 

 held together by chiasmata. 



P-M-C. — Pollen mother-cell, q.v. 



Point Mutation. — A heritable change taking place at a single 

 gene locus, presumably caused by chemical alteration of the 

 individual gene ; a mutation proper, transgenation or genovaria- 

 tion. 



Polar Bodies. — Small cells produced during the two divisions 

 of the oocyte nucleus in animals. See under Oocyte. 



Polar Cap. — The radiating fibrillae or striae which appear 

 at the nuclear poles at the commencement of nuclear division. 



Polar Fusion Nucleus. — The product of the fusion of the 

 two polar nuclei. This, after fusing with a male nucleus, gives 

 rise to the endosperm. 



Polarization, Polarized Chromosomes. — See under Pole- 

 field. 



Pole. — The point at either end of the spindle, from which the 

 spindle fibres radiate to the equator. 



Pole-field. — That side of a nucleus towards which the attach- 

 ment constrictions of the chromosomes are orientated at late 

 telophase and early prophase. This orientation of the chromo- 

 somes with centromeres towards one side of the nucleus is 

 termed 'polarization' and the chromosomes are said to be 

 'polarized.' 



Pole-plates. — Condensed plate-like bodies at the ends of the 

 spindle in certain forms of mitosis (Hertwig; Wilson). 



Pollen-grain. — A microspore which germinates to form the 

 male gametophyte (pollen grain plus pollen-tube) which con- 

 tains three nuclei, one of these fertilises the ovum, a second 

 fuses with the two polar nuclei to form the endosperm, whilst 

 the third (the tube-nucleus) degenerates. 



Pollen-lethals. — Genes which act directly on the pollen, pre- 

 venting those pollen grains which carry them from functioning. 



Pollen Mother-cells. — Sporogenous cells which lie within 

 the pollen-sac. They are derived from the hypodermal cells 



