PITTED STRUCTURES. [ 603 ] 



PLAGIOCHILA. 



brane of the pits being again very finely 

 punctate or reticulated ; Ilartig regards the 

 fine punctuations as holes. 



For the guidance of microscopic observers, 

 we may furnish a series of examples (in ad- 

 dition to those of the CONIFER^E, PL 48. 

 figs. 1, 4, 5), of the different kinds of mark- 

 ing on pitted cells and ducts. 



A. Forms where there is no spiral-fibrous 

 secondary deposit. 



a. Bordered pits uniformly distributed, 

 without reference to adjacent structures ; 

 Elcpaunm acuminata, Clematis Vitalba (PI. 

 48. rig. 18). 



b. Bordered pits fewer on the walls ad- 

 joining cells: Acacia lophantha, Sophora 

 Japonica. 



c. Bordered pits on the walls adjoining 

 ducts, 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 : Cassijtha ghibella 

 (PL 48. fig. 14), Bombaxpentandrum\\A&. 

 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 : Chilianthus arboretts 

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

 manner). Erynyium maritimum (PI. 48. 

 fig. 21) exhibits a condition approaching this. 



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

 walled cells with round, oval, or elongated 

 thinner places (pits) on their Avails, 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, Asparagus, &c. 



B. Forms where a spiral-fibrotis structure is 

 added after the pits. 



g. All the ducts with bordered pits, but 

 the larger ducts with smooth walls, the 

 smaller with a spiral fibre : Clematis Vitalba, 

 Ulmus campestriSj Morus alba. 



h. All the ducts closely pitted, with 

 slender fibres between the rows of pits : 

 Hakea oleifolia. 



i. The larger ducts with pits, the smaller 



without ; both kinds with spiral fibres on 

 the internal surface: Dap/me Mezereum 

 (PI. 48. fig. 19), Passer ina Jlliformis, Genista 

 canariensis. 



j. The walls adjacent to other ducts 

 pitted, those next cells with veiy distant 

 pits, or devoid of them ; all the walls with 

 fibres : the lime, horse-chestnut, sycamore, 

 cornel, holly, hawthorn, Prunus Padus, P. 

 virginiana, &c. 



The last set of forms allies these struc- 

 tures to those characterized peculiarly by 

 the SpiRAL-fibrous STRUCTURES : and, as 

 will be indicated there and under S BCOND AR Y 

 DEPOSITS, the smooth layers of thickening, 

 such as those between the pits of P t nus, 

 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 CELL, Vegetable. 



BIBL. Works on Structural Botany, and 

 the Bill, of SPIRAL STRUCTURES. 



PLACENTA OF PLANTS. The region 

 of the carpel whence ovules arise. Stronu 

 would be a preferable term. 



PLACO'DIUM. A genus of Placodei 

 (Lichenaceous Lichens) j 13 species, on 

 rocks and walls. (Leighton, Lich. Fl. ICO ) 



PLACOPSIL'INA , D'Orbigny. Lituolee 

 of irregular growth and attached, belong to 

 this subgenus. 



BIBL. Carpenter, For. 143. 



PLA'CUS, Cohn. A genus of Holo- 

 trichous Infusoria. Free, ovate ; surface 

 indurated, reticular, mouth inferior ; move- 

 ment rotatory ; length 1-780" : salt water. 

 (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. 



PLAGIOCHI'LA, Nees and Montagne. 

 A genus of Jungermannieae (Hepaticae), 

 containing a number of British species, viz.P. 

 (Jungermannia,RQok.}asplenioides,spinidosa, 

 decipiens, resupinata, undulata (tig. 321), 

 planifolia, nemorosa, and umbrosa, some of 

 which, especially P. aspknioides (fig. 585), 

 are among the most frequent and finest 



