308 PHYSIOGNOMY OF PLANTS. 



family, diminishes as the cold zones are approached; 

 it has its true home in such or such a latitude ; it is a 

 southern form ; it predominates in the temperate zone ;" 

 ' care should always be taken to state expressly whether the 

 writer is speaking of the absolute number of species, and 

 its increase or decrease with the change of latitude ; or 

 whether he means that the family in question prevails 

 over other families of plants as compared with the entire 

 number of phanerogainse of which a Plora consists. The 

 impression of prevalence as conveyed by the eye depends 

 on relative quantity. 



Terrestrial physics have their numerical elements, as has 

 the System of the Universe, or Celestial Physics, and by 

 the united labours of botanical travellers we may expect to 

 arrive gradually at a true knowledge of the laws which 

 determine the geographical and climatic distribution of 

 vegetable forms. I have already remarked that in the 

 temperate zone the Composite (Synantherese), and the Glu- 

 macese (including under this latter name the three families 

 of Grasses, Cyperoidae and Juncacese), make up the fourth 

 part of all phsenogamous plants. The following numerical 

 ratios are the results of my investigations for 7 great families 

 of the vegetable kingdom in the same temperate zone. 



Glumacese % (Grasses alone -^) 



Compositse 



Leguminosse $ 



Labiatse -^ 



Umbelliferae ^ 



Amentacese (Cupuliferse, Betulinese, and Salicinese) ^ 



Cruciferae 



