PINE-APPLE. 



[ 600 ] PITTED STRUCTURES, 



1866, Brauu, Monatsb. Berl. Ak. 1870; 

 Sachs, Bot. 444. 



PINE- APPLE. See Bro>ieliace;e. 



PINNULAEIA , Ehr.— A genus of Uia- 

 tomacefe, family Naviculacese. 



C/mr. Frustules single, free, longer than 

 broad ; front view linear or oblong ; valves 

 navicular, elliptical, lanceolate, or oblong 

 (side view), with a median line, and a 

 nodule at the centre and at each end ; sur- 

 face exhibiting transverse or slightly radi- 

 ating striae or furrows (costse). 



This genus diifers from Navictda in the 

 strife not being resolvable into dots. They 

 ai'e mostly distinct under ordinary illumina- 

 tion. In some of the species tliey are absent 

 in the middle, leaving a transverse clear 

 space or band, resembling in appearance the 

 stauros of Staurmieis. Many species. 



P. nohilis (PI. 15. fig. 1, side view). 

 Valves linear, dilated in the middle and at 

 the rounded ends ; strige coarse. Fresh- 

 water and fossil; length 1-100 to 1-70". 



P. viridis (PI. 15. fig. 2, side view). 

 Valves elliptical, somewhat turgid, ends 

 obtuse ; freshwater ; length 1-500 to 

 1-220"; common. 



j3. Strise parallel, absent from a transverse 

 band. 



P. ohlonga (PI. 15. fig. 8, side view). 

 Valves linear-oblong, ends rounded ; fresh- 

 water and fossil ; length 1-120" ; common. 



P. radiosa (PI. 15. fig. 4, side view ; fig. o, 

 front view) . Valves lanceolate, ends some- 

 what obtuse ; fi-esh water ; length 1-500" : 

 common. 



BiBL. Smith, Br. Died. i. 54, ii. 95 ; Ehr. 

 Abh. 1840, 20; Donkin, Mic. Jn. 1861, 8; 

 Pritchard, Inf. 899; Kutz. 8p. Alg. 81; 

 Gregory, Diat. of Ckjde, 7 ; Q. Micr. Jn. 

 ii. 28, 98 ; Griin, Wien. Verh. 18G0, 524 ; 

 Rabenh. Alg. iii. 209 ; Pfitzer and O'Meara, 

 Qu. 3Iic. Jn. 1872, 386. 



PI'NUS, L.— A genus of Coniferaj (Gym- 

 nospermous Flowering Plants), presenting 

 many interesting points of structure. The 

 most familiar example is the Scotch Fir 

 (P. sylvestris) ; but a great number of other 

 species are cultivated in this country. For 

 the microscope they yield instructi^ e objects: 

 — in the wood (PI. 48. fig. 1), composed of 

 peculiarly pitted cells (see CoNiFERiE) and 

 traversed by turpentine-reservoirs ; in the 

 Babk, which has a kind of false corli ; in 

 the development of the Gymnospermous 

 Ovules, and in the structure of the Pol- 

 LEN-grains. 



The wood of species of the genus Pinus 



frequently occurs in a fossil condition (Pl. 

 25. figs. 29-33). 



Bibl. See the articles above cited. 



PISOM YX' A, Cordni Brgucladimti, Ktz.). 

 — A genus of Mucorini (Phycomycetous 



Fig. 579. 



Pisomyxa racodioides. 

 Magnified 200 diameters. 



Fungi), growing upon leaves. A species 

 has occm'red in Ceylon on the leaves of 

 A7}wmum. 



Bibl. Fries, Swn. Veg. 406 ; Corda, Ic. 

 Fung. i. pl. 6. fig. 292 ; Berk, and Broome, 

 Linn. Jn. xiv. 139, t. x. f. 54. 



PISTIL.— The parts of a flower included 

 in the terms ovary, style, and stigma. It 

 is in theory composed of modified leaves 

 or carpels. 



PISTILLA'RIA, Fr.— A genus of Cla- 

 variei (Hymenomycetous Fungi), consisting 

 of small club-shaped heads which are con- 

 fluent with the ste-m. There are about six 

 species indigenous to this country, of which 

 that on fern-stems is the most common. 



Bibl. Fr. Ep. 586; Berk. Outl. 286; 

 Cooke, Handb. 342. 



PISTILLID'lUM=AHcnEGONiuM, the 

 female reproductive organ of the higher 

 Crvptogamia. 



PITH. SeeMEDtTLLA. 



PITHOPH'ORA, Wittr.— An obscure 

 genus of Algse. (Wittrock, Qu. M. Jn. 

 1877, xvii. 293; Moore, Jn. Bot. 1877; 

 M. M. Jn. xviii. 40.) 



PITTED STRUCTURES of Plants. 

 — The secondary deposits of cellulose which 

 form the layers of thickening of the walls of 

 vegetable cells are seldom uniform or homo- 

 geneous in character. In most, if not in all, 

 cases some special microscopic structure 

 may be distinguished, either by mere in- 

 spectiDU or on the application of reagents. 

 These layers, .spoken of more particularly as 

 to their nature under Secondary depo- 

 sits, may be divided into two classes, com- 



