PETALONEMA. 



[ 499 ] 



PEZIZA. 



PETALONEMA, Berk. (Arthrosiphon, 

 Kiitz.). A genus of Oscillatoriaceae(Confer- 

 void Algae), presenting a very remarkable 

 mode of growth . The filaments are branched 

 and cylindrical, with a very evident terete, 

 gelatinous, duplicate sheath (PI. 4. fig. 21). 

 The inner is thin and follows the filament, 

 the outer presents oblique striae indicating 

 the interposition of lengths of the outer 

 sheaths, one inside another, like a series of 

 nested funnels or conical cups. This appear- 

 ance is produced by the bursting and expan- 

 sion of each length of the sheath at the apex 

 alone, to make room for the growth of the 

 new cells of the filament formed at the apex. 

 This structure is analogous to that occurring 

 in UROCOCCUS, when each parent-cell mem- 

 brane bursts at one side only to allow the 

 new one to emerge, thus at length forming 

 a jointed pedicel. The edges of the " fun- 

 nels " of Petalonema sometimes become de- 

 composed into curled filamentous processes. 



The filament of P. alatum is green and 

 striated, about 1 -3000" in diameter, the inner 

 sheath is yellowish, the outer colourless and 

 1-400" in diameter. It forms a brownish 

 stratum on rocks and stones. 



BIBL. Berkeley, Gleanings, p. 23. pi. 7 ; 

 Greville, Sc. Crypt. Fl. pi. 222; Hassall, 

 Brit. Fr. Alg. p. 237. pi. 68. fig. 6; Kiitzing, 

 Spec. Alg. p. 311, Tab. Phyc. ii. 28; Al. 

 Braun, Rejuvenescence, fyc., Ray Soc. Vol. 

 1853. p. 178. 



PETALS. Thepetalsof Flowering Plants 

 afford many interesting microscopic objects, 

 in the epidermis, glandular and other hairs, 

 the colour-cells and the veins composed of 

 spiral vessels. Entire petals of small size 

 and delicate character form good objects 

 when dried and mounted in Canada balsam. 

 Those of the smaller Caryophyllacese, the 

 ligulate corollas of Compositae, Sec., are well 

 suited for this. The larger kinds are studied 

 by means of sections, like LEAVES. 



PETROBIUS, Leach. A genus of In- 

 sects, of the order Thysanura, and family 

 Lepismenae. 



P. maritimus has a general resemblance to 

 Lepisma saccharina; but it exercises a leap- 

 ing movement. The antennas are longer 

 than the body ; of the setae at the tail, the 

 middle one is longest. The insect is of a 

 blackish-brown colour, and is covered with 

 scales; the legs are yellowish, and the caudal 

 setae ringed with white ; the abdomen is 

 furnished with gill-like processes. 



It is found upon the rocky sea-coast. 



The scales have been used as test-objects. 



BIBL. Gervais, Walckenaer's Apt. iii. 

 p. 447 ; Guerin, Iconogr. Ins. pi. 2. fig. 1 /. 

 and Ann. d. Sc. nat. 2 ser. v. p. 374. 



PETER'S GLANDS. See INTESTINES 

 (p. 366). 



PEYSSONELIA, Dene. A genus of 

 Cryptonemiaceae (Florideous Algae), consist- 

 ing of small plants with a depressed lobed 

 thallus (fig. 565), growing over stones, 



Fig. 565. 



Frond. Nat. size. 



Fig. 566. 



Peyssonelia squamosa. 



Vertical section of a portion through two warts. 

 Magnified 25 diameters. 



shells, &c., and attached by the whole under 

 surface which produces jointed radical hairs 

 (fig. 566), especially at the thin margins. The 

 thallus is composed of several rows of com- 

 pact parenchymatous cells, and bears on 

 the concentrically-marked surface, warts 

 composed of radiating rows of cells, among 

 which occur crucially-divided tetraspores. 

 P. Dubyi is not uncommon on British shores; 

 it is 1 to 2" in diameter, roundish at first, 

 ultimately irregularly lobed, colour dull 

 brownish. Thuret has observed antheridia 

 on distinct plants of P. squamosa, a Medi- 

 terranean form ; they are jointed filaments 

 collected into wart-like bodies, like those 

 containing the tetraspores. The spores are 

 not described. 



BIBL. Harvey, Brit. Mar. Alg. p. 144. 

 pi. 14 D ; Phyc. Brit. pi. 71 ; Thuret, Ann. 

 des Sc. nat. 4 ser. iii. p. 23. pi. 4. 



PEZIZA, Dill.^-A genus of Helvellacei 

 (Ascomycetous Fungi), containing numerous 

 species, a large number of which grow upon 

 2K2 



