1837.] Extra-Tropical Plants within the Tropics. 



297 



less influence on the mean temperature of the whole year, than on the 

 distribution of the same quantity of heat among the different parts of 

 the year. It is a striking spectacle, to see the grain of Europe culti- 

 vated from the equator as far as Lapland, in the latitude of 69°, in re- 

 gions where the mean heat is from 22° to 2o in every place where 

 the temperature of summer is above 9° or 10°. We know the minimum 

 of heat requisite to ripen wheat, barley, and oats ; we are less certain 

 in respect to the maximum, which these species of grain, accommodat- 

 ing as they are, can support. We are even ignorant of all the circum- 

 stances, which favour the culture of corn between the tropics at very 

 small heights. La Victoria and the neighbouring village of San Ma- 

 theoyield'an annual produce of four thousand quintals of wheat. It 

 is sown in the month of December, and the harvest is reaped on the 

 seventieth or seventy-fifth day. The grain is large, white, and abound- 

 ing in gluten : its pellicle is thinner and not so hard as that of the wheat 

 of the very cold table-lands of Mexico. An acre near Victoria general- 

 ly yields from three thousand to three thousand two hundred pounds 

 weight of wheat. The average produce is consequently here, as at 

 Buenos Ayres, three or four times as much as that of northern countries. 

 Nearly sixteen times the quantity of the seed is reaped ; while, according 

 to Lavoisier, the surface of France yields on a mean only five or six for 

 one ; or from one thousand to twelve hundred pounds per acre. Not- 

 withstanding this fecundity of the soil, and this happy influence of the 

 climate, the culture of the sugar-cane is more productive in the valleys 

 of Aragua, tljan that of corn. 



" Near San Matheo we find the last fields of wheat, and the last 

 mills with horizontal hydraulic wheels. A harvest of twenty for one 

 was expected : and, as if the produce were but moderate, I was asked, 

 whether corn produced more in Prussia and in Poland. It is an error 

 that generally prevails under the tropics, to consider grain as plants 

 which degenerate in advancing toward the equator ; and to believe, 

 that the harvests are more abundant in the northern climates. Since 

 calculations have been made on the progress of agriculture in the differ- 

 ent zones, and the temperatures under the influence of which corn will 

 flourish, it has been found, that, beyond the latitude of 45° the pro- 

 duce of wheat is no where so considerable, as on the northern coasts 

 of Africa, and on the table-lands of New Grenada, Peru, and Mexico. 

 Without comparing the mean temperature of the whole year, but only 

 the mean temperature of the season which embraces the cycle of vege- 

 tation of corn, we find for three months of summer, in the north of 

 Europe, from 15° to 19© ; in Barbary, and in Egypt, from 27° to 29° j 

 within the tropics, between fourteen and six hundred toises of height 

 from 14° to 25.5° of the centigrade thermometer, 



