not critical to this discussion, has been purposely left out of the 
drawing, in order to impart greater clarity to the model. 
Figure (b) represents the beginning stage of wood silicifica- 
tion. The upper figures differ from the lower only in the extent 
of mineral deposition, the quantity being greater, - essentially 
filling the entire pit chamber, in the latter case. The distinction 
is drawn now for reasons which will become clear shortly. In 
this early stage of the process, silica deposition occurs primar- 
ily upon the more accessible cellular surfaces in woody tissue, 
particularly on the inner cell wall forming the perimeter of the 
lumen and on the lining of the pit chamber. Growth of the 
deposit is directed outwards with further mineral input. If 
deposition is limited to the extent shown in part (b), and the 
specimen is macerated, i.e., all wood substance is oxidatively 
removed and the siliceous lithomorphs of the tracheids sepa- 
rated from one another, the structures pictured in part (c) will 
result. It might be noted here that the ease with which silicified 
vascular plant tissue can be macerated is a reflection of the 
extent to which silica has penetrated across cell walls and into 
the intracellular organic substance. 
With the model just described, a basis is provided for inter- 
preting the morphological characters observed in siliceous 
lithomorphs, both synthetic and natural, in terms of their de- 
velopment. For example, the siliceous discs appearing in the 
micrographs of the petrified specimens of the Taxodiaceae 
(Plates 2 and 3) are infillings formed within the pit chamber. 
The biconvex lenses in Figures | through 5 in Plate 3 corre- 
spond to the lower figures of the model in Plate 7, where 
deposition of silica has essentially filled the entire cavity of the 
pit chamber. Upon maceration, with the separation of indi- 
vidual tracheids, these near-solid lenses remain intact. De- 
pending upon the exact point of detachment of the lens from the 
wall in the region of the pit aperture, the lens will assume a 
superimposed architecture of either a short protruding knoll or 
knob (Fig. 3 in Plate 3) or a shallow hollow or cavity (Fig. 4 in 
Plate 3). Because the attachment of the lens to each adjoining 
cell wall replica is rather tenuous’, owing to the relatively small 
diameter of the pit apertures and, hence, of the mineral depo- 
*When silicification proceeds to more advanced stages, with considerable mineral 
impregnation of wall substance, extensions of silica, other than that through the pit 
aperture, probably exist between the wall and pit chamber, permitting more firm 
attachment. 
17 
