PER 



PER 



8. Peri-gonium (70V//, generation). A 

 terra synonymous with perianthium, 

 and denoting the parts which surround 

 the organs of generation in plants, 

 viz. the floral envelopes. 



9. Peri-gynium {'^vvrj, a woman). A 

 terra which has been variously employed 

 by botanical writers. Link applied it to 

 the disk found in certain plants. With 

 sorae, it is synonymous with urceolus ; 

 with others, it relates to the hypogynous 

 setcB found at the base of the ovary of 

 cyperaceae. 



10. Peri-gynous {jwrj, a woman). That 

 condition of the stamens of a plant in 

 which they contract adhesion to the sides 

 of the calyx, as in the rose. 



11. Peri-helion {riXioi, the sun). An 

 astronomical term, denoting the greatest 

 approximation of a planet to the sun. See 

 Aphelion. 



12. Peri-meter {/jLerpov, a measure). 

 The bounding line of any plane figure, of 

 whatever parts or shapes that line may 

 consist. The bounding line of a circle, or 

 perhaps of any curve which returns upon 

 itself, is termed the circumference or 

 periphery. 



13. Peri-ceci {o'Ikos, a habitation). 

 Those who live under the same parallel 

 of latitude, whether north or south, but 

 on opposite meridians. 



14 Peri-ostracum {6<npaKov, a shell). 

 The epidermis, or membrane analogous 

 to scarf-skin, which covers shell, 



15. Peri-phery {(pepm, to carry). The 

 line which bounds a circular body; a 

 circumference, as that of a circle, an 

 ellipse, or any other curvilinear figure. 



16. Peri-phyllia {^vWov, a leaf.) The 

 term applied by Link to the minute 

 hypogynous scales found within the 

 paleae of grasses. These little organs 

 have disturbed the nomenclature of 

 botany not a little; for while some call 

 them corolla, others call them squamulcB, 

 Linnaeus nectarium, Richard glumella, 

 De Candolle glumellula, and De Beauvois 

 lodicula. 



17. Peri-scii {cKia, shadow). The inha- 

 bitants of the polar circles; those whose 

 shadows make complete revolutions, in 

 consequence of the sun being present for 

 twenty-four hours together. See Hete- 

 roscii. 



18. Peri-sperm {anepfxa, a seed). An- 

 other name for the albumen, or the sub- 

 stance lying between tbe integuments 

 and the embryo of the seed. By Richard, 

 the term is applied to the testa or sper- 

 moderm of other writers. 



254 



19. Peri-sporum {<nr6po9, a seed, a 

 spore). The name given by some French 

 writers to the hypogynous setce found at 

 the base of the ovary of Cyperaceae. They 

 have also been termed perigynium. 



20. Peri-stomians {ar6ij.a, the mouth). 

 The name given by Lamarck to a family 

 of Gasteropods, consisting of the genera 

 Valvata, Paludina, and Ampullaria. 

 Cuvier comprises these under his tro- 

 choid pectinibranchians. 



21. Peri-stomium {a-rofxa, the mouth). 

 The membrane, or series of tooth-like 

 processes, which closes the orifice of the 

 theca of mosses. Strictly speaking, it 

 consists of two membranes, and hence 

 we hear of an inner and an outer 

 peristomium. The organ is highly hy- 

 grometrical. 



22. Peri-thecium {6i]Kf], a theca, or 

 case). The case which contains the re- 

 productive organs of certain fungi. Peri- 

 dium is also a kind of covering of spori- 

 dia ; peridiolum is its diminutive. 



23. Peri-trema (rprj^a, a hole). The 

 raised margin which surrounds the 

 breathing holes of scorpions. 



24. Peri-tropal (xpeTrcy, to turn). A 

 botanical term applied to the embryo of 

 the seed, when it is directed from the 

 axis to the horizon. 



PE'RICLINE. A felspathic substance, 

 united by some mineralogists with ice- 

 spar, and found in the Tyrol, St. Go- 

 thard, &c. 



PERI DOTE. Chrysolite. A silicate 

 of magnesia, constituting olivine in its 

 purer state. 



PERIOD. The time occupied by a 

 planet in making a revolution round the 

 sun ; or the duration of its course till it 

 returns to the same point of its orbit. 

 In Arithmetic, a period is the recurring 

 part of a circulating decimal. For its 

 chronological meaning, see Cycle. 



PERTO'DIC ACID. An acid consist- 

 ing of iodine and oxygen. 



PERIO'DIC FUNCTIONS. Trigono- 

 metry has for its subject periodic magni- 

 tude, i. e. magnitude which varies in 

 such a manner as to go through stated 

 cycles of changes, each cycle being a 

 reiteration of the preceding one. Peri- 

 odic functions are those which, performed 

 any given number of times on a variable, 

 reproduce the simple variable itself. 

 Thus 1 — X and — x are periodic func- 

 tions of the second order, since 



\—{\—x)=x, — {—x)=x. 



PERIODICAL STARS. Stars which 

 undergo a regular periodical increase 



