PITTED STRUCTURES. [ 003 ] 



PLAGIOCIIILA. 



Itrane of the pits beinp: ag-iin vovv finely 

 punctate or reticulated ; llartig regards the 

 line punctuations as holes. 



For tlie guidance of microscopic observers, 

 •we may furnish a series of examples (in ad- 

 dition to those of the Conifku.^:, PL 48. 

 fiffs. 1, 4, 5), of the dilferent kinds of mark- 

 ing- on pitti'd cells and ducts. 



A. Forms wliere there is no spiral-Jihrous 

 secundiin/ deposit. 



a. Bordered pits uniformly distributed, 

 ■without reference to adjacent structures ; 

 E!cea(inm acicninata, Clematis Vilalha (PI. 

 4± lig. 18). 



b. Jiordered pits fewer on the walls ad- 

 joining cells: Acacia lophantha, Sophora 

 Japotiica. 



c. Bordered pits on the walls adjoining 

 duets, while the walls adjoining wood-cells 

 have few or no bordered pits, and those 

 next the medullary rays have pits without 

 a border : elder, beechj hazel, poplar, alder, 

 plane, apple, &c. 



d. Bordered pits on the walls adjoining 

 ducts, but with large pits devoid of a bor- 

 der where adjoining cells : Cassi/tha (jlahella 

 (PL 48. tig. 14), Bombax pentandruyn (PL 48. 

 tig. 15). 



e. A modification of the last, where the 

 bordered pits have the form of slits as wide 

 as the ducts when adjoining ducts, while 

 the walls adjoining cells have large pits 

 without a border : Chilinnthiis arboreus 

 (PI. 48. fig. 17); the vine (in a less striking 

 manner). Urt/nc/ium maritimiim (PL 48. 

 fig. 21) exhibits a <;onditiou approaching this. 



/. Sieve-tubes or clathrate cells, large thin- 

 walled cells with round, oval, or elongated 

 thinner places (pits) on their walls, the mem- 

 brane of the pit being finely reticulated or 

 perforated like a sieve. These are found in 

 the liber of Dicotyledons, as in Biynonia, 

 the lime, the vine, elder, pear, &c., and in 

 the central part of the vascular bundles of 

 Monocotyledons, as Musa, Asparayus, &c. 



B. Forms where a spiral-Jibrotis structure is 

 added after the jjits. 



g. All the ducts with bordered pits, but 

 the larger ducts with smooth walls, the 

 smaller with a spiral fibre : Clematis Vitalba, 

 Uimus campestris, Mortis alba. 



h. All the ducts closely pitted, with 

 slender fibres between the rows of yjita : 

 Hakea oUifolia. 



i. The larger ducts with pits, the smaller 



without ; both kinds with spiral fibres on 

 the internal surface: J)ii/)/<nr Mrzereinn 

 (PL 48. fig. 19), PasserinajViformis, Genista 

 canariensis. 



J. The walls adjacent to other ducts 

 pitted, those next ells with very distant 

 pits, or devoid of them ; all the walls with 

 fibres : the lime, horse-chestnut, sycamore, 

 cornel, holly, hawthorn, Pruiuis Padus, P. 

 virjiiniana, &c. 



The last set of forms allies these struc- 

 tures to those characterized pecidiarly by 

 the SpiRAL-fibrous Structurks : and, as 

 will be indicated there and under S R( :ond ary 

 DEPOSITS, the smooth layers of thickening, 

 such as those between the pits of Pmas, 

 may be made to show a spiral structure by 

 the action of reagents. 



For the micro-chemical conditions of 

 these objects, their development and rela- 

 tions, see Secondary deposits ; Tissues, 



Vegetable ; and Cei.l, 



B table. 



BiBL. Works on Structural Bcjtany, and 

 the Bihl. of Spiral structures. 



PLACENTA OF Plants.— The region 

 of the carpel whence ovules arise. Stroma 

 would be a preferable term. 



PLACO'DIUM.— A genus of Placode! 

 (Lichenaceous Lichens) ; 13 species, on 

 rocks and Avails. (Leighton, Lick. Fl. K'O.) 

 PLACOPSIL'INA , B'OiVignj.—Litvolre 

 of irregular growth and attached, belong to 

 this subgenus. 



BiBL. Carpenter, For. 143. 

 PLA'CUS, C'Ohn. — A genus of Holo- 

 trichous Infusoria. Free, ovate ; surface 

 indurated, reticular, mouth inferior ; move- 

 ment rotatory ; length 1-780"; saltwater. 

 (Kent, Inf. 489.) 



PLAGIACAN'THA, Claparede.— A ge- 

 nus of Acanthometrina (Rhizopoda). 



Char. The spicula, which are branched 

 and without a central canal, do not unite 

 in the centre of the body, but on one of the 

 sides, so as to form a scaffolding on which 

 the sarcode rests; pseudopodia elongating 

 either at the ends or sides of the spicula to 

 which they are attached, and which they 

 unite more or less together. 



BiBL. Clap, et Lach. Etudes, 461. 

 PLAGIOCIU'LA, Nees and Montague. 

 — A genus of Jungermannieaj (llepaticfe), 

 containing a number of British species, viz. P. 

 [Ju7i(fermannia,ilook.)aspleni(iides,spinnlosa, 

 decipiens, resupinata, undiduta (fig. 321), 

 planifolia, nemorosa, and umbrosa, some of 

 which, especially P. a-:plenioides (fig. 585), 

 are acong the most frequent and finest 



