MONOCOTYLEDONS AND DICOTYLEDONS 



255 



are evidently derived from them and become the most 

 highly organized of all plants. The two groups each con- 

 tain about forty thousand species, but the Archichlamydese 

 contain about one hundred and sixty families, and the 

 Sympetalae about fifty. 



To present over two hundred families, containing about 

 eighty thousand species, is clearly impossible, and a very 

 few of the prominent ones will be selected for illustrations. 



Arch icli lamydece 



138. Poplars and their allies. — This great alliance repre- 

 sents nearly five thousand species, and seems to form an 

 isolated group. It is a notable tree assemblage, and appar- 

 ently the most primitive and ancient group of Dicotyledons, 

 containing the most important deciduous forest forms of 





Fig. 238. An oak in winter condition.— From "Plant Relations." 



