In the specimens treated with ethyl silicate, wood structure Is 
faithfully replicated in silica without any appreciable con- 
comitant loss of wood substance. Also, in many natural speci- 
mens which are thoroughly silicified — such as the specimen of 
Callixylon wood mentioned earlier, much of the original woody 
tissue is still present and can be recovered by dissolution of the 
silica with concentrated hydrofluoric acid. Furthermore, a 
molecular replacement process involving such chemically dis- 
similar molecules as those in the quite diverse ligno- 
holocellulosic groups with those of soluble silica is theoreti- 
cally unsound and chemically untenable. 
IV. A CHEMICAL HYPOTHESIS OF THE 
SILICIFICATION PROCESS 
The emplacement of silica on vascular tissue of plants during 
petrifaction may involve the establishment of actual chemical 
bonds, probably hydrogen bonds, between the two materials. 
The principal chemical components of vascular tissue are (a) 
the holocelluloses — a group of polysaccharides, including 
cellulose, and (b) the lignins — complex polymers composed of 
phenylpropane units. Both materials, as well as their degrada- 
tion products, contain abundant functional groups, particularly 
the hydroxyl group, that are capable of forming hydrogen 
bonds. 
Molecular silicic acid is the most probable silicifying agent 
involved in petrifaction. Suspended colloidal particles do not 
exist in solutions dilute in silica and, furthermore, are too large 
in size to penetrate into the finer cellular interstices — a requi- 
site for faithful reproduction of histological detail. The silicate 
ion is also an unlikely agent as it forms only in solutions of pH 
above about 9 — a degree of alkalinity seldom attained in 
natural situations. Silicic acid, on the other hand, is the only 
common form of soluble silica found in nature. It is the form of 
silica released in volcanic ash devitrification and in clay min- 
eral diagenesis (Siever, 1957). Silicified woods almost always 
have an environmental history which includes either former 
burial in ash or occurrence in altered sediments — especially 
for former event. With four hydroxyl groups per molecule and 
molecular dimensions of relatively small size, the probability 
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