658 EPIEEHEOLOGY. 



features, the number of species being found to increase as we approach 

 the equator, and to decrease as we retire from it. Palms, Bananas, 

 Tree Ferns, and Orchideous Epiphytes, are chiefly confined to the 

 tropics ; Cruciferous and Umbelliferous plants are found in temperate 

 regions ; some Coniferous and Amentiferous plants flourish in more 

 northern countries ; while Saxifrages and Lichens extend to the arctic 

 regions. In warm regions are found those fruits which are so neces- 

 sary for the well-being of the inhabitants ; in temperate climates 

 chiefly, occur the cereal grains for the food of man, and the green 

 pastures for the nourishment of cattle ; and in the arctic regions, the 

 Lichen, on which the reindeer feeds, grows luxuriantly. 



The number of known species of plants amounts to upwards of 

 100,000, including about 10,000 genera. ■ The following is an esti- 

 mate of the known species of plants on the globe at different dates : — 



LinniEus 



Persoon 



Steudel 



Steudel 



Steudel 



1753 

 1807 

 1824 

 1841 

 1844 



5,323 Phanerog. 615 Cryptog. 6,938 



19,949 — 6,000 — 25,949 



39,684 — 10,965 — 50,649 



78,000 — 13,000 — 91,000 



80,000 — 15,000 — 95,000 



In 1846, Lindley gave the following estimate of known genera 

 and species : — 



Thallogens 



Acrogens 

 Ehizogens 



Dictyogens 



939 Genera, 8,394 Species. 



310 — 4,086 



21 — • 53 



1,420 — 13,684 



17 — 268 



Gymnogens . 37 — 210 



Exogens .. . 6,191 — 66,225 



Total . . 8,935 92,920 



Much yet remains to be done in regard to the Floras of India, China, 

 Africa, Australia, and South America. Meyen conjectures that the 

 total vegetation of the globe may be about 200,000 species. 



The distribution of species over different quarters of the globe is 

 regulated by various external agents, the study of which is termed 

 Epirrheology (siriggiu, I flow on the surface). These agents are chiefly 

 temperature and moisture, and the nature of the soil. The effects 

 produced on plants by increase or decrease of light, and by changes in 

 the state of the atmosphere, have not been sufficiently determined. 



1. — Effects of Tbmpeeatuke. 



The effects of this agent must be considered both as regards its 

 latitudinal and its altitudinal ranges. In proceeding from the equa- 

 tor to the poles, or in ascending from the surface of the ocean to the 

 summit of a lofty mountain, there is a gradual decrease of temperature, 



