GLOSSARY 439 



Peltate. Shield-shaped, with the support attached to the lower surface instead of at 



the base or margin. 

 Pentacyclic. Having five cycles or whorls of floral parts. 

 Pentamerous. Having the floral parts in sets of five or multiples of five. 

 Perfect flower. Having both stamens and pistil in the same flower. 

 Perianth. The floral envelope; the calyx and corolla collectively. In the leafy liver- 

 worts, a group of specialized united leaves surrounding the archegonia. 

 Periblem. A group of embryonic cells, in a root tip or stem tip, that gives rise to the 



cortex. 

 Pericarp. The ripened wall of an ovary. 

 Perichaetium. A sheath or rosette of modified leaves surrounding the sex organs of 



many mosses. 

 Periclinal. Parallel to the outer surface, as a cell wall. 

 Pericycle. One or more layers of cells forming the outermost part of the stele in most 



vascular plants. 

 Peridium. The outer covering of the basidiocarp in the Gasteromycetales. 

 Perigynous. With the perianth and stamens borne on a disk or cup formed by the 



receptacle. 

 Perisperm. Nutritive tissue in a seed formed outside the embryo sac, mainly in the 



nucellus. 

 Peristome. A ring of teeth surrounding the open rim of a moss capsule. 

 Perithecium. A spherical or flask-shaped ascocarp opening by a small terminal pore. 

 Petal. One of the leaf-like parts of the corolla. 

 Petiole. A leafstalk. 



Phellogen. The cambium that produces cork tissue. 

 Phloem. In vascular plants, a complex tissue consisting of sieve tubes and often also 



of companion cells, parenchyma, and fibers, and serving for the conduction of 



food. 

 Phycocyanin. A blue pigment associated with chlorophyll in the blue-green algae. 

 Phycoerythrin. A red pigment associated with chlorophyll in the red algae. 

 Phylogeny. The evolutionary development of the race or group to which an organism 



belongs. 

 Pileus. The expanded or cap-like part of a mushroom. 

 Pinna. A leaflet or primary division of a pinnate leaf. 

 Pinnate. Feather-like ; with the leaflets or primary divisions arranged on each side of 



a common axis or rachis. 

 Pinnule. One of the ultimate divisions of a bipinnate leaf. 

 Pistil. In angiosperms, a carpel or an organization of two or more carpels. 

 Pith. The central portion of a siphonostelic stem, generally consisting of parenchyma. 

 Placenta. The place within an ovary to which the ovules are attached. 

 Plasma membrane. A thin membrane that encloses the cytoplasm of all living cells. 

 Plasmodium. A mass of naked, multinucleate protoplasm usuallj^ showing amoeboid 



movement. 

 Plastid. An organized protoplasmic body, other than the nucleus, occurring in some 



plant cells and concerned with some special metabolic activity. 

 Plerome. A group of embryonic cells, in a root tip or stem tip, that gives rise to the 



stele. 

 Plumule. The primary bud of the embryo of a seed plant. 

 Polar nucleus. One of two free nuclei in an embryo sac, one coming from each pole, 



that eventually unite to form the fusion nucleus. 

 Pollen chamber. A cavity developed at the apex of the nucellus, as in cycads. 

 Pollen sac. One of the pollen-containing cavities in an anther. 



