110 PHYSIOGNOMY OF PLANTS. 



torrid zone (from to 10 of latitude) at -^ we obtain 

 for the part of the temperate zone which is between 45 

 and 52 latitude -yV, and for the frigid zone (lat. 67 to 

 70) only -^. The direction followed by the great family 

 of Leguminosae (increase on approaching the equator), is 

 also that of the Rubiaceae, the Euphorbiaceae, and especially 

 the Malvaceae. On the contrary, the Grasses and Juncacese 

 (the latter still more than the former), diminish in approach- 

 ing the equator, as do also the Ericese and Amentacese. 

 The Compositse, Labiatae, Umbelliferse, and Cruciferae, de- 

 crease in proceeding from the temperate zone, either towards 

 the pole or towards the equator, the Umbelliferae and Cruci- 

 ferae decreasing most rapidly in the last-named direction; 

 while at the same time in the temperate zone the Cruciferae 

 are three times more numerous in Europe than in the United 

 States of North America. On reaching Greenland the 

 Labiatae have entirely disappeared with the exception of 

 one, and thellmbelliferse with the exception of two species; 

 the entire number of phsenogamous species, still amounting, 

 according to Hornemann, to 315 species. 



It must be remarked at tfce same time that the develop- 

 ment of plants of different families, and the distribution of 

 vegetable forms, does not depend exclusively on geographical, 

 or even on isothermal latitude ; the quotients are not always 

 on the same isothermal line in the temperate zone, for 

 example, in the. plains of North America and those of the 

 Old Continent. Within the tropics there is a very sensible 

 difference between America, India, and the West Coast of 

 Africa. The distribution of organic beings over the surface 

 of the earth does not depend wholly on thermic or climatic 



