permanence to the siliceous lithomorph. Further induration 
could be achieved if water content were lowered and some 
semblance of crystallographic order imparted, without effect- 
ing excessive contraction and disaggregation in the lithomorph 
—1i.e., by converting the silica to opal-CT or, better, opal-A, 
without any appreciable distortion or destruction in form. 
The use of ethyl silicate as a source of silica for laboratory 
studies of the petrifaction process provides several distinct 
advantages over the use of other silica sources such as colloidal 
silica suspensions, alkaline solutions of silicate ions, and satu- 
rated solutions of silicic acid. 
First of all, the form of soluble silica released from ethyl 
silicate upon decomposition is monomolecular silicic acid, the 
most probable silicifying agent operative in natural processes. 
This is the only common form of soluble silica found in natural 
waters, both fresh and marine (Siever, 1972). Furthermore, 
being of small size and in true solution, these molecules can 
penetrate cellular openings too small to accommodate passage 
of species of colloidal-sized dimensions. 
Secondly, ethyl silicate is an unusually rich source of silicic 
acid. A saturated aqueous solution of monomolecular silicic 
acid contains only about 1.4 x 10? ug. of the acid per ml of 
solution (equilibrium solubility). The same volume of ethyl 
silicate, on the other hand, has the potential for releasing 4.3 x 
10° ug. of the acid, a three thousand-fold increase. Moreover, 
the silicic acid is released inside the wood within cellular in- 
terstices where it can immediately interact with the woody 
chemical constituents. 
Thirdly, the ethyl silicate system is free of alkali, permitting 
experiments to be performed under near neutral conditions 
and, therefore, more in keeping with conditions inferred for the 
natural process. In contrast, even very weak solutions of 
sodium metasilicate are strongly alkaline. 
Fourthly, the use of an organic fluid helps to prevent pit 
aspiration, thereby maintaining communication among cells. 
Aqueous solutions, on the other hand, promote adhesion be- 
tween the torus and pit border, thereby tending to obstruct 
fluid diffusion through the xylem tissue (Comstock and Coté, 
1962). 
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