METEOROLOGY. (iKOWTH OF PLANTS. 4fc. 



Jakoustk wheat and rye sometimes yield a retiivn of 15 for 1, al- 

 though at the depth of a yar<l tlie soil which grows them is con- 

 stantly frozen.* 



The limit of perpetual snow being much lower upon the mountains 

 of Europe than in tropical countries, agriculture ceases at a much 

 less elevation. At a height of 6560 feet above the level ot the sea, 

 the vetretables of the plain have almost entirely disappeared. In 

 Northern Switzerland the vine docs not grow at an elevation of 

 more than 1800 feet above the sea-line ; maize scarcely ripens at ar 

 elevation of 2850 feet, while in the Andes it stdl aflords abundant 

 harvests at an elevation of 8260 feet. On the plateau or table land 

 of Los Pastes, fields of barley are seen at upwards of 10,000 feet 

 above the level of the sea ; but on the northern slope of Monte kosa, 

 m Switzerland, barley fails at an elevation of about 4260 feet ; on 

 the southern slope, indeed, it reaches a height of about 6500 teet ; 

 and this great variation in the ultimate limit of barley is frequently 

 observed with reference to the same plant grown upon opposite as- 

 pects of a mountain range. The difference is ascribed to local in- 

 fluences ; thus, it is a well-ascertained fact, that on the mountains 

 of the northern hemisphere vegetation reaches a much higher lati- 

 tude upon southern than upon northern exposures ; but a general 

 law, and one applicable to every latitude, is, that the higher we rise 

 above the level of the sea, the scantier does vegetation become, the 

 later do harvests reach maturity : but as the heat of the atmosphere 

 increases with the elevation, it follows that there is an obvious rela- 

 tion between the time a crop is upon the ground and the mean tem- 

 perature of the place or season where it grows. We have still to 

 examine this relationship. 



^ III. METEOROLOGICAL CIRCUMSTANCES UNDER WHICH CERTAIN 

 PLANTS GROW IN DIFFERENT CLIMATES. 



In discussing the conditions of temperature under which the va- 

 rious plants that are common in our European agriculture come to 

 maturity, we are led to conclusions which are not without interest. 

 A knovvledo-e of the mean temperature of a place situated between 

 the tropics ''suffices of itself to give us an idea of the nature of its 

 ao-riculture ; in fact, the temperature of each day differs little trom 

 that of the entire year, during which vegetable life proceeds without 

 interruption. It is altogether different with regard to countries sit- 

 uated beyond the limits of the torrid zone. The mean annual tem- 

 perature is not then a datum sufficient to enable us to appreciate the 

 agricultural importance of a country. In order to know what the 

 earth will produce, the temperature proper to the different seasons 

 of the year must be knowm ; in a word, it is the mean temperature 

 of the cycle in which vegetation begins and ends that it iniports us 

 to ascertain, in order to learn what the useful plants are which may 

 be required of the soil. 



In examining the question which now engages us, we first inquire 

 what time elapses between the sprouting of a plant and its maturity, 



* Humboldt's Centr.i! Asia, voL iii. p. 49. 



41 



