8 THE BOOK OP EVERGREENS. 



monocotyledonous plants have usually leaves with parallel 

 veins, while those of the other class have the veins of the 

 leaves more or less interwoven or netted. The stems of the 

 woody dicotyledonous plants show a regular annual 

 growth ; each year a layer of wood is deposited outside of 

 the older growth, and they are called Exogens, or outside 

 growers, while, as this order of growth is not observed in 

 the monocotyledonous plants, and the newer woody fibres 

 are mostly in the centre of the stem, they are called 

 Endogens, or inside growers. To the first of these classes, 

 the JExogens, or Dicotyledons, belongs the Coniferae, or 

 Pine Family, which presents peculiarities of structure 

 differing from other Exogens, and which will be described 

 farther along. 



To the Cryptogamous or flowerless plants belong the 

 Mosses, Ferns, Club-mosses, and Others still more simple 

 in their structure. 



Geological researches have shown that after the forma- 

 tion of this lower order of vegetation, the Coniferce came 

 into being. This period was coeval with the formation of 

 stone coal, which is indicated by the distinct and beauti- 

 ful impressions found in the various strata throughout the 

 coal regions. 



In the three great periods or epochs of geological his- 

 tory, we find by the vegetable remains that, during the 

 first period, their flora consisted principally of the crypto- 

 gaim'ge, or flowerless plants, with a few dicotyledonous 

 plants. These two classes were about equally divided in 

 the second period ; and in the third, the dicotyledonous 

 plants constituted much the larger portion, whilst the 

 cryptogamia were rare. 



Monocotyledonous plants existed in every period, al- 

 though by no means plentiful in any of them. 



In the first period the prevailing families were Ferns and 

 enormous Equisetums, or Horse-tails ; in the second epoch, 

 only one-third were Ferns, and the remaining two-thirds 



