500 PLANTS AND MAN 



Phylum. — The largest classification unit; a group of classes. Examples, 



Bryophyta or Spermatophyta. 

 Piling. — Long, straight poles driven deep into the ground to act as 



supports for bridges, docks and buildings. 

 Pinnate. — A feather-like type of venation or leaf composition in which 



all leaflets or veins extend outward from a common midvein or 



midrib, which serves as a line of origin; example locust or ash. 

 Pistil. — The part of the flower producing ovules. 



Pistillate. — The female flower cluster or flower part, containing pistils. 

 Pith. — The central portion of the stem in Dicots. 



Placenta. — The portion of the ovary to which the ovules are attached. 

 Plastid. — Specialized parts of the cytoplasm found in plant cells, usually 



associated with pigments; example, green plastids such as chloro- 



plasts. 

 Pod. — A dry fruit containing several seeds which splits open at maturity, 



characteristic of Pea and Bean Family. 

 Pollen. — The male gametophyte resulting from germination of the 



microspore. 

 Pollen tube. — The portion of the male gametophyte which grows out 



of the microspore, into the tissues of the pistil and ovule, and through 



which the male cell reaches the ^gg. 

 Pollination. — The transfer of microspores, or male gametophytes, to the 



pistil. 

 Pome. — An accessory fruit in which the ovary forms a core, and the 



receptacle and calyx form the fleshy portion; example, apple. 

 Protein. — ^Compounds made up of amino acids, chiefly composed of 



carbon, oxygen, hydrogen, nitrogen and sulphur. 

 Prothallus. — The small, often microscopic, gametophyte of a fern or a 



seed plant. 

 Protoplasm. — The living substance which is the physical basis of life; 



chiefly a mixture of proteins in the colloidal state. 

 Protozoa. — The group of unicellular animals; examples. Amoeba, 



Paramecium. 

 Pure stand. — ^A forest, or community of trees, made up of a single species, 



such as a pure white pine stand. 

 Pustule. — A small blister-like elevation on the surface of a plant part. 



It ultimately bursts to release the spores lying beneath it's covering. 

 Pyroxylin. — A cellulose nitrate compound produced by partial nitration 



of cellulose. 



Radial section. — A section of a tree trunk or of wood, which is cut along 

 the radius of a circle formed by the end of the trunk, or cut on a 

 line from the center of the trunk to the outside. 



