1 92 ANATOMY OF THE GYMNOSPERMS 



matter representing the carbon residue of the original structure. 

 Such a condition is presented in various species of Cupress- 

 oxylon and Pityoxylon from the Permian and Cretaceous of 

 Kansas (45, 76-77). Eventually, as so commonly expressed in 

 specimens from the petrified forests of Arizona, the silica may 

 entirely replace the organic matter with an absolute obliteration 

 of all structural details. Such alterations are necessarily regional 

 and local, as determined by the more or less energetic action of 

 the fungus and the progress of infiltration, of which two factors 

 it is the necessary resultant. 



A peculiar combination of decay and infiltration may, under 

 other circumstances, give rise to a false structure which bears 

 no relation whatever to the normal. This has been recognized 

 by Penhallow on former occasions (57, 1 17, and 58, 25) in the case 

 of the so-called Celluloxylon primsevum of Dawson, which he 

 has shown to be nothing more than peculiarly altered forms of 

 Nematophycus Logani and N. crassus. No similar instance has 

 yet been recorded for the vascular plants, and it would almost 

 seem as if the alterations noted were to be specially identified 

 with the more resistant forms of marine algae. The peculiar 

 changes observed in these plants were of such a nature as to 

 give rise to tissuelike figures, which, under a low power, present 

 the precise aspects of a coarse parenchyma tissue in process 

 of decay. Such appearances led the late Sir William Dawson 

 to recognize in Celluloxylon primaevum a distinct type of plant, 

 though it was subsequently shown that the appearances were 

 wholly due to highly altered forms of Nematophycus. The effect 

 arises from the fact that the carbon residue, the decay of which 

 has already been carried to an extreme point, is in a finely gran- 

 ulated form, which admits of very ready redistribution. The 

 crystallization of the infiltrated silica at such a time supplies 

 exactly the necessary conditions for such redistribution. Under 

 these circumstances the carbon particles arrange themselves 

 upon the surfaces of the crystals without any reference to the 

 original lines of structure, but in such a way as to produce 

 definite figures of the size of the silicious crystals. 



