234 



of some of our more valuable and generally-known species of trees, and 

 to'tbe preponderance and tlie facies of tbeir families at different times, 

 may i)rove of general interest, and be the more accejjtable, as it does 

 not require for its comprehension any peculiar acquaintance with botani- 

 cal science. The few details necessary for the understanding of the 

 whole may be explained in a few words. 



The great primary divisions of the vegetable kingdom, as they are gen- 

 erally admitted now, are : 1st. The Thallogens, plants with mere cellular 

 tissue, like the Algse or Sea- Weeds, the Lichens, the Fungi. 2d. The 

 Acrogens, flowerless plants, like the former, but composed of a woody 

 tissue and of vessels. To this class belong the Ferns, known by every- 

 body, the Lycopods or Glub-Mosses, the Equiseta, generally named 

 Horsetail. 3d. TheGymnosperms, or Cone-bearing plants, like the Coni- 

 fers and the Cycadecc. 4th. The Endogens, or Monocotyledonous plants, 

 which grow and increase from the inside, like the Palms, the Grasses, 

 the Lilies, etc. 5th. The jE/^o^iew.s, or Dico/^/fe^ows, whose representatives 

 mostly compose our present arborescent vegetation. 



In regard to their character, these classes are generally admitted as 

 of a gradually more complex and complete organization in ascending 

 from the lowest division, the Thallogen ; and it is also a well-established 

 opinion that, in the geological succession of their representatives, the 

 plants of the lowest order appeared first, and were followed by species 

 of the other divisions, in the succession indicated above. The facts, 

 however, in support of this assertion are not yet sufficiently ascertained. 



The original or first crust of the earth's surface is composed of crystal- 

 line rocks, metamorphic or changed by fire, where no forms of organized 

 bodies can be recognized. Vegetable life, however, seems manifested 

 in these primitive rocks by deposits of graphite, a metamorphic car- 

 bonaceous substance, whose origin, like that of all the other combustible 

 minerals of this world, is attributable to the plants, and to animals also, 

 of which carbon is a compound. The AlgiE of the simplest structure 

 consist of single isolated cells ; like the iJiatomaceoi and DesmidacecBy 

 which are the smallest vegetables known, and which, simple as they 

 are, represent, however, most beautiful and innumerable forms, appreci- 

 able only by microscopical investigation. These kinds of Algce live 

 everywhere, in the hottest springs and the snow of the glaciers, on sand 

 or mud, on every kind of substance; they multiply rapidly and in enor- 

 mous proportions. In some localities, the water of the sea is colored to 

 a depth of many feet, and over a surface of wide extent, by the presence 

 of the Diatomacece ; plants so minute in size that a million of them may 

 live in a drop of water. The peculiar nature of these unicellular plants 

 confirms the idea that they have appeared with the first consolidated 

 strata of the globe. 



To their life, therefore, the origin of the deposits of bitumen or carbon, 

 as represented by*graphite in the primitive rocks, is probably due. The 

 temperature of these rocks, originally in a state of fusion, has been con- 

 sidered as an objection to this opinion. But even at the present epoch, 

 Algfe, of a higher degree of organism, fill, by their thread-like filaments, 

 basins of thermal water whose temperature reaches 100° C, as in the 

 Hot Springs of Arkansas or in the Geysers of the Yellowstone. 



Immediately above the crystalline rocks, and from the beginning of 

 the stratified deposits of the Lower Silurian, whose substance has been 

 derived from the disintegration and the removal of primitive materials, 

 vegetable fossil remains are recognizable. 



They represent marine plants, of course, mostly of indefinite forms, 

 like crushed bundles of filaments, whose contours are obliterated in a 



