300 PHYSIOGNOMY OF PLANTS. 



different systems of vegetation, what harvest may still remain to be 

 reaped in the several families. 



The comparison of the numerical ratios of families in different 

 already well-explored zones, has conducted me to the recognition of 

 laws according to which, in proceeding from the Equator to the Poles, 

 the vegetable forms constituting a natural family decrease or increase 

 as compared with the whole mass of phanerogamse belonging to each 

 zone. We have here to regard not only the direction of the change 

 (whether an increase or a decrease), but also its rapidity or measure. 

 We see the denominator of the fraction which expresses the ratio 

 increase or decrease : let us take as our example the beautiful family 

 of Leguminosse, which decreases in going from the equinoctial zone 

 towards the North Pole. If we find its proportion or ratio for the 

 torrid zone (from to 10 of latitude) at T 4 ^, we obtain for the part 

 of the temperate zone which is between 45 and 52 latitude T ' T , 

 and for the frigid zone (lat. 67 to 70) only ^ T . The direction 

 followed by the great family of Leguminosae (increase on approach- 

 ing 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 ap- 

 proaching the Equator, as do also the Ericeae and Arnentaceaa. The 

 Compositae, Labiatae, Umbelliferae, and Cruciferae, decrease in pro- 

 ceeding from the temperate zone, either towards the Pole or towards 

 the Equator, the Umbelliferae and Cruciferae 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 the Umbelliferae with the exception of two species ; the entire 

 number of phaenogamous species, still amounting, according to 

 Hornemann, to 315 species. 



It must be remarked at the same time that the development of 

 plants of different families, and the distribution of vegetable forms, 

 do 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 



