PINUS. 



[ 508 ] PITTED STRUCTURES. 



P. nobilis (PI. 11. fig. 1, side view). 

 Valves linear, dilated in the middle and at 

 the rounded ends ; striae coarse. Aquatic 

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



P. viridis (PI. 11. fig. 2, side view). 

 Valves elliptical, somewhat turgid, ends 

 obtuse. Aquatic; length 1-500 to 1-220". 

 Common. 



/3. Striae parallel, absent from a transverse 

 band. 



P. oblonga (PI. 11. fig. 3, side view). 

 Valves linear- oblong, ends rounded. Aquatic 

 and fossil; length 1-120". Common. 



P. radiosa (PL 11. fig. 4, side view; fig. 5, 

 front view). Valves lanceolate, ends some- 

 what obtuse. Aquatic ; length 1-500". 

 Common. 



BIBL. Smith, Brit. Diatom, i. p. 54. 



PINUS, L. A genus of Coniferae (Gym- 

 uospermous Flowering plants), presenting 

 many interesting points of structure. The 

 most familiar example is the Scotch Fir (P. 

 Abies), but a great number of other species 

 are cultivated in this country. For the mi- 

 croscope they yield instructive objects in 

 the wood (PI. 39. fig. 1), composed of pecu- 

 liarly pitted vessels (see CONIFERAE) and tra- 

 versed by turpentine reservoirs, in the BARK, 

 which has a kind of false cork, in the deve- 

 lopment of the Gymnospermous OVULES, 

 and in the structure of the PoLLEN-grains. 



The wood of species of the genus Pinus 

 frequently occurs in a fossil condition, both 

 in coal and silicified (PI. 19. figs. 29-33). 



BIBL. See the articles above cited. 



PISOMYXA, Corda (Bryocladium, Kze.). 

 A genus of Antennarei (Physornycetous 



Fig. 584. 



Pisomyxa racodioides. 

 Magnified 200 diameters . 



Fungi), growing upon leaves. Not British, 

 as stated under the family. 



BIBL. Fries, Summa Veg. p. 406 j Corda, 

 Icones Fung. i. pi. 6. fig. 292. 



PITH. See MEDULLA. 



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 inspec- 

 tion 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- 

 prehending pretty accurately all the varied 

 conditions, namely, the Spiral deposits, 

 where the secondary layers assume the 

 aspect of fibres applied upon the inside of 

 the cell- wall ; and Pitted, or as they are often 

 termed, Porous deposits, where layers are 

 applied over the whole internal surface of 

 the cell, which layers present orifices of dif- 

 ferent characters, leaving the primary mem- 

 brane bare, and forming in this way a pit as 

 viewed from the inside of the cell. When 

 the secondary layers are comparatively thin, 

 their presence is often overlooked, and the 

 pits have thus often been mistaken for ori- 

 fices or pores (figs. 585, 586) in the primary 



Fig. 585. 



Fig. 586. 



Pitted cells of elder pith. 

 Magnified 250 diameters. 



membrane ; but such pores are never origi- 

 nally present ; the closure of the pit by the 

 layer of primary membrane may always be 

 demonstrated in young structures ; and when 

 orifices really do occur in cell-walls, these 

 arise from the absorption of the primary 

 cell-membrane converting the pit into a 

 pore. The best way of demonstrating that 

 young spotted cell-walls are only pitted and 

 not perforated, is to apply sulphuric acid 

 and iodine for the production of the blue 

 colour in the primary cell-wall. 



Simple pits, of no great depth, occur on 

 the slightly thickened walls of most perma- 

 nent parenchymatous cells; they may be 

 seen in the cells of herbaceous stems, in 

 pith, bark, in the cells of the parenchyma of 

 leaves, &c. (figs. 585, 586. PI. 38. fig. 14). 



In most prosenchymatous wood-cells, or 

 liber-cells, and in the woody cells of the 

 stones or shells of fruits and seeds, the pits 

 are far more clearly evident, and become 

 more and more distinct (PI. 39. fig. 3) as 

 the layers of thickening increase in number, 

 since by the successive application of these, 



