sits extended within, both attachments are sometimes broken, 
causing total detachment of the lens from the rest of the cell. 
Note the absence of several lenses in a number of the micro- 
graphs, especially in Figure 5 of Plate 3. 
The single concave lenses appearing in Figure 4 of Plate 2 
correspond to the upper figures of the model, in which silica did 
not build away appreciably from the pit chamber surfaces, 
leaving their centers empty. Upon maceration, these hollow 
discs become divided, each symmetric half remaining with its 
adjacent wall replica. 
Even in the earliest stages of silicification, however, it ap- 
pears that some silica does penetrate the cell wall and does 
deposit within the wall in intimate association with organic 
substance. For example, the degree of petrifaction achieved 
with the ethyl silicate technique does go somewhat beyond that 
depicted in part (c) of the model. In some cases, appreciable 
deposition occurs in the middle lamella region (Fig. 3 in Plate 
5). Furthermore, the micrographs of the organic-free speci- 
mens in Plates 5 and 6 appear to indicate some silica did deposit 
within the secondary layer of the cell wall. Some of the lumen 
plugs, upon contracting, tore away portions of the wall, 
suggesting penetration and firm attachment with wall sub- 
stance. Since void space in the relatively densely packed or- 
ganic substance of anessentially undergraded cell wallis small, 
the silica deposited in this condition can be expected to be 
somewhat limited, fragile, and discontinuous — hence, subject 
to facile disruption and loss from the lithomorph upon removal 
of all organic matter during maceration. This might account for 
the total absence of silica in the former wall space of the 
deorganicized ray parenchyma cells appearing in Figure 4 of 
Plate 4. 
The foregoing discussion was concerned largely with wood 
in only its beginning stages of petrifaction, when wood sub- 
stance is still present and, though probably having undergone 
some chemical alteration, is still structurally intact. While in 
this condition, the bulk of the silica deposition occurs on the 
exposed cell wall surfaces, from which it builds outwards to fill 
the void spaces, particularly the lumina. The outline of the 
siliceous lumen casts shown in the figures of Plates 5 and 6 
indicate deposition began on the surface of the lumen perimeter 
and then grew concentrically into the lumen interior with 
further input of silica. But, upon setting, the lumen casts 
18 
