PITTED STRUCTURES. [ 602 ] PITTED STRUCTURES. 



broad rim (fig. 583 ; PI. 48. figs. 1, 4, 5) ; tlie 

 portion within the rim projec-ts somewliat 

 into the cavity of the cell, and appears like 

 a lenticular body attached on the wall; hence 

 tiie markings were formerly termed the 

 " glands " of Coniferous wood. In reality, 

 however, while the pits themselves resemble 

 ordinary pits, the broad rim, or rather the 

 circular line outside the pit, depend* on a 

 condition of the cell-wa'.l outside the mem- 

 brane, and is merely the outline of a lenti- 



Fig. 584. 



r.iu 



pt. 



Section of Pine wood at right angles to the pitted 

 walls, p.f, walls of a pitted cell ; c.j, cavity of a cell ; 

 c. I , lenticular cavity between two adjacent pits ; r. in, 

 cells of a medullary ray, the pits have no rim here. 



Magnified 400 diameters. 



cular cavity existing between two adjacent 

 cells, the boundary of which is visible 

 through the wall on account of the trans- 

 parency of the latter : the nature of this 

 structure is very evident in sections made at 

 right angles to those which show the bor- 

 dered pits in face (fig. 584 ; PL 48. fig. 1 b). 

 In most of the ConiferaB the wood is ex- 

 clusively composed of large elongated pros- 

 enchym'atous cells, with bordered pits of 

 this character on the side walls (that is, on 

 the walls standing radially or perpendicular 

 to the bark) ; the pits, however, which lie 

 on parts of the wall adjoining the cells of 



medullary rays, ai'e generally devoid of the 

 rim. 



Similar bordered pits occur very generally 

 on the walls of the pitted ducts of Dico- 

 tyledons ; but as the wood is here of mixed 

 composition, and the ducts adjoin cells as 

 well as other ducts, independently of the 

 medullary rays, we often find a greater va- 

 riety of conditions on the wall of the same 

 duct, which may have bordered pits when 

 adjoining another duct, and simple pits, or 

 pits with a double outline, when adjoining 

 cells. The pits with a double outline (PL 48. 

 figs. 15 b, ic 20) are of difiereut nature from 

 the bordered pits (PL 48. figs. 13, 14, 15 a, 

 16, 18), the double outline depending simply 

 on the fact that the later or more internal 

 layers of thickening do not reach the edge 

 of the orifice in the earlier secondary de- 

 posits, so that the pit is conical, or rather 

 has sloping edges, the circumference at the 

 primary membrane being rather less than 

 that of the margin next the cell-cavity. A 

 peculiar modification of this unequal mode 

 of deposit is seen in company with the true 

 rim or border in many cases (PL 48. figs. 14, 

 1(3, 18), where the central spot or ongiual 

 pit appears in the middle of a sUt running 

 across .the circle indicating the border ; this 

 slit indicates the alteration of the shape of 

 the gap in the secondary deposits in the 

 successive layers, and corresponds to the 

 inner margin of the pit, where this has the 

 form of an elongated groove or slit, gi-adu- 

 ally diminishing- to a small round hole 

 towards the primarv cell-membrane (PL 48. 

 fig. 18«). Sometimes (PL 48. fig. 18 «r, b) 

 the two or more slits formed in this way on 

 contiguous pits become confluent. The last 

 condition indicates a transition to the more 

 sparing form of the secondary' deposit where 

 it appears as a modification of a spiral fibre 

 or fibres; and the later secondary deposits of 

 pitted ducts do sometimes actually assume 

 this Ibrni, and produce a spiral-!ibruus layer 

 of thickening inside the layers perforated by 

 pits. This is the case in Taxus (PL 48. 

 figs. 4), in the Lime (PL 48. fig. 13), and 

 Mezereon (PL 48. fig. 19 b), &c. 



Ilartig and Mohl have described a pecu- 

 liar kind of pitted tissue formed of cells, 

 which the former calls Siebrohren, or sieve- 

 tubes, the latter clathrafe cells. They are 

 thin-walled cells occurring associated with 

 the prosenchymatous liber-cells of Dicotyle- 

 dons, and forming part of the vasa j)ropria 

 of Monocotylednns; having their walls 

 mi.rked with large shiUow pits, the mem- 



