5CO EPIRRHEOLOGY. TEMPERATURE. 



pole to the other from the summit of the highest mountains to the 

 bosom of the ocean. Palms, Bananas, Tree Ferns, and Orchideous 

 Epiphytes, are chiefly confined to the tropics ; Cruciferous anc 1 - Um- 

 belliferous plants are found in temperate regions; some Coniferous and 

 Amentaceous plants flourish in more northern countries, while Saxi- 

 frages and Lichens extend to the arctic regions. In warm regions are 

 found those fruits which are so necessary for the well-being of the in- 

 habitants ; in temperate climates chiefly, occur the cereal grains for 

 the food of man, and the green pastures for the nourishment of cattle; 

 and hi the arctic regions, the Lichen, on which the reindeer feeds, 

 grows luxuriantly. 



1133. The number of known species of plants amounts to about 

 100,000. The following is an estimate of the known species of plants 

 on the globe at different dates : 



Linnaeus, 1753 



Persoon 1807 



Steudel, 1824 



Steudel, 1841 



Steudel, 1844 



5,323 Phaneroa;. 615 Cryptog. 5,938 

 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, 939 Genera, 8.394 Species. 



Acrogens 310 4,086 



Khizogens, 21 53 



Endogens 1,420 13,684 



Dictyogens, 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, New Holland, and South America. Meyen conjectures that 

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



1134. The distribution of species over different quarters of the glebe 

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

 Epirrheology (tirtppia, I influence). These agents are chiefly tem- 

 perature and moisture, and the nature of the soil. The effects pro- 

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

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



1. EFFECTS OE TEMPERATURE. 



1135. 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, 



