354 INTRODUCTION TO BOTANY 
Phycocyanin (seaweed blue). A bluish coloring matter found within 
certain alge. 
Phyllotaxy. Leaf arrangement. 
Pinna (a feather); pl. pinne. One of the primary divisions of a pin- 
nate leaf, as in ferns. 
Pinnate. Having the veins or the divisions of the leaf arranged in 
rows on each side of the midrib, as in black locust (Robinia). 
Pinnule (a little feather). One of the divisions of a pinna. 
Pistil (a pestle). A simple or compound carpel in spermatophytes. 
Placenta; pl. placenta. That portion of the ovary which bears the 
ovules. 
Plerome (that which fills). A name given to that part of the meri- 
stem, near the growing points of the plant axis, which forms a central 
shaft or cylinder and develops into the axial tissues. 
Plumule (a little feather). The terminal bud of the embryo above 
the cotyledons. 
Pod. A dry, several-seeded, dehiscent fruit. 
Pollen. The spores developed in the anther. 
Pollen tube. The structure that develops from the wall of the micro- 
spore of spermatophytes and carries male cells to the egg. 
Pollination. The transfer of pollen to the stigma. 
Polypetalous (many petals). Applied to flowers that have their petals 
free from one another. 
Prosenchyma. Tissue composed of elongated cells, with tapering ends 
which overlap. 
Prothallium (a forerunning shoot); pl. prothallia. The small, usually 
short-lived plant which develops from the spore and bears the sex organs. 
Protonema (that which is first sent out); pl. protonemata. In mosses, 
the filamentous growth which is produced by the spores, and from which 
the leafy moss plant is developed. 
Protoplasm (that which is first formed). The living matter of cells. 
Pubescent. Downy, with fine hairs. 
Pyrenoid (kernel formed). Minute colorless bodies embedded i in the 
chlorophyll structures of some lower plants. 
Receptacle. That portion of an axis or pedicel (usually broadened) 
which forms a common support for a cluster of organs, either sex organs 
or sporophylls. 
Respiration. The series of processes by which plants obtain energy 
through breaking down of protoplasm or food. Usually oxygen is used 
and carbon dioxide is formed as a result of the process. 
Reticulated (net-like). Having a net-like appearance. 
Rhizoid. Root-like; a name applied to the rootlike hairs found in 
bryophytes and pteridophytes. 
