PITTED STRUCTURES. [ 601 ] PITTED STRUCTURES. 



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. 580, 581) in the primary 



Fig. 580. 



Fig. 581. 



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. 580, 581 j PI. 47. 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. 48. fig. 3), as 

 the layers of thickening increase in number, 

 since, by the successive application of these, 

 the pits are deepened (with the contraction 

 of the cavity of the cell) until they become 

 canals^ or tubular passages radiating from 

 the central cavity (PI. 47. fig. 23). In these 

 cases it is evidently seen that the pits of 

 adjacent 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 internal deposits, so that the in- 

 ternally simple orifice leads out to several 

 branches corresponding 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. 47. fig. 28). 



Pits of the above kinds occur on the 

 structures called ducts (see TISSUES, VEGE- 



Fig. 582. Fig. 583. 



Fig. 582. Pitted ducta of Clematis. Magn. 100 diams. 



Fig. 583. Side wall of a cell of Pine, with bordv i ed 



pits. Magnified 200 diameters. 



TABLE), formed of cells applied end to 

 end and confluent (fig. 181, page 271). 

 These large pitted tubes (which occur abun- 

 dantly in most woods, with the exception 

 of that of the Coniferae) are sometimes 

 termed bothrenchyma, 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 

 wood-cells of many plants, especially of the 

 Coniferse, 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 by a 



