EXPLANATION OF THE ILLUSTRATION 
Plate 7. Model of mineralization of wood (white) with silica (black), 
schematically relating silica deposition to vascular tissue structure. 
a. Unmineralized wood. Wood prior to mineralization, showing two adja- 
cent tracheid walls and their bordered pit pair interconnection. 
b. Petrified wood. Wood during early stages of impregnation with silica, 
with silica coating cell wall rimming the lumen and partial (upper) or complete 
(lower) infilling of pit chamber. 
c. Lithomorph. Silica replicated structure, devoid of woody substance. 
Tracheid separated by maceration. 
This model is discussed in further detail in the text. 
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