PITTED STRUCTURES. [ 509 ] PITTED STRUCTURES. 



the pits are deepened (with the contraction 

 of the ca%aty of the cell) until they become 

 canals or tubular passages radiating from 

 the central cavity (PL 38. fig. 23). In these 

 cases it is evidently seen that the pits of ad- 

 jacent cells and ducts correspond to each 

 other at their outer extremity ; and in old 

 tissues, when the primary cell-walls have been 

 absorbed, these coincident pits form tubular 

 canals leading from one cell to another. It 

 has been observed that two or more pits 

 sometimes become confluent in the later in- 

 ternal deposits, so that the internally simple 

 orifice leads out to several branches corre- 

 sponding to the original pits on the wall of 

 the cell. In rare cases, simple pits occur 

 on the outer walls of epidermal cells, as in 

 Cycas (PI. 38. fig. 28). 



Pits of the above kinds occur on the 

 structures called ducts (see Tissues, vege- 

 table), formed of cells applied end to 

 end and confluent (fig. 184, page 216). 

 These large pitted tubes, which occur abun- 

 dantly in most woods, wdtli the excep- 

 tion of that of the Coniferae, are sometimes 

 termed hothrenchyma, signifying pitted tis- 

 sue ; but the character not being exclusively 

 applicable to them, the name is bad. 



In many pitted ducts, and in the pitted 



Fig. 587. 



Fig. 588. 



Fig. 587. Pitted ducts of Clematis. Blagn. 100 diam. 



Fig. 588. Side wall of a cell of Pine, with bordered 



pits. Magnified 200 diameters. 



wood-cells of many plants, especially of the 



Couiferse, the pits present a greater degree 

 of complication. The markings on the walls 

 of the wood-cells of most of the Coniferae, 

 for example, consist of pits surrounded bv a 

 broad rim (fig. 588. PI. 39. figs. 1.4. 5); the 



Fig. 589. 



— r.ra 



p.t: 



Section of Pine wood at right angles to the pitted walls. 

 p. f, walls of a pitted cell ; c.f, canity of a cell ; c. I, len- 

 ticular cavity between two adjacent pits ; r. in, cells of a 

 medullary ray ; the pits have no rim here. 



Magnified 400 diameters. 



portion within the rim projects somewhat 

 into the cavity of the cell, and appears like 

 a lenticular body attached on the wall ; hence 

 the 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, depends on a 

 condition of the cell-wall outside the mem- 

 brane, and is merely the outline of a lenti- 

 cular cavity existing between two adjacent 

 cells, the boundary of which is visible through 

 the wall on account of the transparency of 

 the latter : the nature of this structure is 

 very evident in sections made at right angles 

 to those w^hich show the bordered pits in 

 face (fig. 589. PI. 39. fig. 1 b). In most of 

 the Coniferae the wood is exclusively com- 

 posed of large elongated prosenchymatous 

 cells, with bordered pits of this character on 

 the side-walls, that is, on the wall standing 

 radially or perpendicular to the bark ; the 



