38 GEOLOGICAL SURVEY 
layers. If a cross section of the specimen under examination, instead of a concentric structure, 
presents the woody fibres disposed in irregular spots—it belongs to a monocotyledonous plant. If 
opaque circular segments, with the ends turned outwards, be observed, the plant belongs to the 
arborescent forms. Persons unacquainted with the structure of plants affect to know, from external 
appearance, even the species to which silicified trees belong. 'The facts stated above are all that 
could be determined with certainty, from the strueture of the wood, even with the aid of the 
microscope. 
Fossil plants in general present, particularly in the older rocks, other characters, upon which 
generic and specific distinctions are founded. 
Vasculares.—We find among the dicotyledonous fossil plants of this class numerous common 
genera at present existing, principally confined to the Tertiary system, such as Nympheea, Ulmus, 
Carpinus, Castanea, Betula, Salix, Populus, Comptonia, Juglans, and Acer. Dicotyledonous plants 
are known, however, to exist in the coal formation, where it seems they have, in this country at 
least, played an important part in the formation of that mineral, as was ascertained by Professor 
Bailey. 
Of the coniferous plants we have, in the fossil state, the well known living genera Pinus, Abies, 
Podocarpus, and Thuya. ‘To this division also belong the interesting genera Zamia and Cycas, so 
beautifully characteristic of the Oolitic and Liassic formations. 
Sigillaria, which was once considered as belonging to the cellular plants, must, according to 
more recent researches, be placed here. And if the observations of Mr. Dawson, made in Nova 
Scotia, and of Mr. Binny, in Lancashire, be correct, which go to show that Stigmaria is the root 
of a tree, probably of Sigillaria, that fossil must also be referred to this class. 
Mr. Logan has established the interesting fact that Stigmaria is confined to the bed of clay that 
underlies the coal, whilst Sigillaria is almost as constantly found in the sandstones above. 'This 
would show that the two may stand to each other in the relation of trunk and root. They are 
found in the coal fields of Alabama, occupying the same relative position. 
Sigillaria is distinguished by having the stem deeply furrowed, and not jointed; scars, when 
present, are placed between the furrows. 
Stigmaria may be known by its external surface being marked by round tubercles ; and before 
the specimen is detached from the rock, rootlets may be seen extending from each tubercle. I 
mention these genera more particularly because they will be met with by the student more fre- 
quently than almost any other. 
Endogenous Plants.—There are numerous fossil plants of this division of the Vasculares ; 
among these the most remarkable are the Palms, which seem to be divided between the ‘Tertiary 
and coal formation. Some fruits or seed vessels are also placed here. 
The absence of all traces of the family of grasses is quite remarkable. Before the existence of 
herbivorous animals they may not have been necessary in the economy of nature ; but after the 
appearance of these animals we are naturally led to look for some evidence of the existence of 
those plants which at present constitute so large a portion of their food. 
Prof. Lindley has indeed shown that the relative number of the various species of fossil plants 
does not depend so much upon the number in which they once grew as upon their relative power 
of resisting decomposition when first enclosed in the mud in which they were embedded. And it 
