592 CHARACTERS AND ARRANGEMENT OF FOSSIL PLANTS. 



tion as to furnish distinct characters. It is chiefly from the frag- 

 ments of stems, and the impressions of leaves, and some fruits, that 

 the fossil botanist can draw conclusions. Sometimes the internal 

 structure of the stem can be traced, and by examination under the 

 microscope, the nature of its woody tissue may be determined. In 

 this way, some fossilized woods have been referred to the Coniferous 

 tribe, in consequence of the presence of punctated woody tissue (fig. 

 47). Fossil woods have been shown by chemical tests to contain por- 

 tions of vegetable tissue, cemented into a mass by silica. In some 

 cases the vessels and cells are separately silicified without being united 

 into a compact mass. In these instances the wood breaks down easily. 

 At times the internal structure is obliterated, and it is only from the ex- 

 ternal configuration, the nature of the outer covering, and the scars of 

 the leaves, that any conclusions can be drawn. The leaves often furnish 

 important and valuable characters, and, in the case of fossil ferns, their 

 form, divisions, and venation, supply distinguishing marks. The leaves, 

 however, are generally isolated, and are rarely found in connection with 

 the stems. Thus, the separation of the different parts of the plant, and 

 in most cases, their imperfect state of preservation are great obstacles 

 in preventing the determination of fossil plants by a comparison with 

 those which now exist on the earth. These difficulties are increased 

 as we go back to the earliest geological epochs, for the further they 

 are removed from the present state of things, the greater are the 

 differences between the fossil and living plants. Dr. Hooker remarks, 

 that the knowledge of recent botany required to throw light upon the 

 study of fossil plants, and the origin of coal must be both varied and 

 extended. " Some acquaintance with systematic botany is the first 

 requisite ; through this alone can any approximation to the living 

 affinities of the fossil be obtained. It should embrace not only a 

 knowledge of the principal groups, or natural orders under which all 

 plants are arranged, but a familiarity with vegetable anatomy; for 

 when the stem or trunk alone is preserved, which is often the case, a 

 minute examination of its tissues is the sole method of determining 

 its position hi the natural series. There must also be some general 

 ideas of the vegetation both of the tropics and cooler latitudes, of 

 mountain-chains, table-lands, vallies and estuaries ; more especially of 

 countries characterized by equable, and by excessive or extreme 

 climates, as compared with continents, and of humid and desert dis- 

 tricts ; in short, of all the complex associations with, or dependence of 

 botanical characters upon, surface, soil and climate, which the globe 

 presents." 



1172. Many of the fossil plants of the tertiary or recent strata may 

 be referred to genera at present existing, and merely present specific 

 differences ; such as pines, elms, beeches, maples, &c. Those of the 

 secondary strata may, in general, be referred to known families, but 



