330 Barber . — Cupressinoxylon vec tense. 
in former periods than at present, and the comparatively late 
appearance of the Dicotyledons in geological time. 
Unfortunately the conditions required for the preservation 
of the minute anatomy of pieces of stems and roots of fossil 
plants differ widely from those under which the leaf- and 
bark- impressions have been handed down to us, and the 
two are therefore hardly ever found together 1 . This is not 
entirely due to the phenomenon of leaf-fall. The imprinting 
of the delicate venation of decaying leaves is largely 
a mechanical process, and pictures to the mind still lagoons 
and slowly-flowing streams ; whereas the fragments of wood 
frequently bear the marks of much tossing to and fro before 
they were subjected to the chemical replacement of their cell- 
walls by silica, lime, or iron pyrites. 
It is comparatively rare that the bark or cortex is left in 
fossils, and it thus happens that the sole guide to their 
systematic position is to be found in the minute structure 
of the wood. The uniformity of the elements in Coniferous 
wood has already been noted, and the problem here presented 
is seen to be of no common difficulty. Some of the best 
workers on the anatomy of plants have devoted their energies 
to its elucidation. Unger, Schleiden, Schacht, Mohl, Goppert, 
T. Hartig, Mercklin, Cramer, Kraus, Stenzel, Schenk, Con- 
wentz, Felix, and many others, have produced a mass of 
monographs and treatises, almost all of them originating in 
an attempt to describe fossil woods, the necessity soon arising 
of extending their researches to an exhaustive study of recent 
Conifers. 
In spite of these laborious investigations, the results have 
frequently been anything but satisfactory. Much labour was 
wasted by the earlier writers in attempting to unite each 
fossil wood with leaf or fruit remains occurring in strata 
of the same age. It was however soon demonstrated that 
classifications founded upon the structure of the wood could 
not be made to agree with those recognizing the natural 
1 Schenk, in Zittel, Palaeophytologie, p. 873, mentions Sequoia Couttsiae among 
Coniferae. See also Elate austriaca in Unger, Chloris Protogaea, 1847. 
