GEOGRAPHY OF PLANTS. 473 



the actual state of meteorological knowledge. This distinguished observer 

 followed the vegetation of one variety of vine growing near Orange, in France, 

 from foliation to maturity, noted the minima of heat for each day in the shade, 

 and the maxima shown by the thermometer in the sunshine, but protected 

 by a slight covering of earth. The mean between these minima and maxima 

 give, according to Gasparin, a more satisfactory number than that derived from 

 other processes ; and when multiplied by the number of days during which 

 vegetation is influenced by this particular mean, results in a sum total of heat 

 which varies but little from year to year at the locality where the observations 

 were made. So nearly do these sums agree that the presumption is strength- 

 ened that the process may be the correct one, and deserving of much moi-e at- 

 tention than has been awarded to it. The result of de Gasparin's experiments 

 in 1844 was a sum of 4195°; in 1845, 4203°; in 1846, 4057°; and in 1847, 

 4100<^ centigrade. 



Among the contributions of these inquirers into the mystery of heat as 

 adapted to the wants of plants, none have hitherto elicited more interest than 

 those of Boussingault. His method, proposed by Reaumur, appears to be 

 logical and precise, giving results sutficieutly satisfactory for determining the 

 heat necessary for maturing annual plants, particularly the cereals in spring. 

 He has been very happy in determining the conditions of a good vintage in his 

 district of Alsace, and has shown that certain mean temperatures are necessary 

 during certain eras of vegetation, and especially that a definite amount of heat 

 is needed to insure the elaboration of a due proportion of sugar in the grape. 



The principle that we must combine the values of temperature and time in 

 our inquiries into this subject cannot be controverted, for all must perceive 

 that heat acts proportionally as regards its duration and its force. Boussin- 

 gault, therefore, asserts that if a plant has required twenty days to ripen its 

 seed, numbering from the period of flowering, and the mean temperature during 

 those twenty days has been fifty degrees, it will be found that the plant will 

 have received one thousand degrees of heat. The same number of degrees of 

 heat might have influenced the plant during a lesser number of days had the 

 mean temperature been proportionally higher. This is well illustrated by the 

 rapidity with which some annual plants germinate in Arctic regions on the re- 

 turn of midsummer heats. In these northern regions, where for a short time 

 plants are subjected to an intense heat, often as high as 109*^ Fahrenheit in the 

 shade, and which enjoy a longer continuance of the sun above the horizon 

 than in moi-e southern latitudes, the growth of some vegetables is said to be 

 so rapid under assiduous culture and in genial situations that their progress 

 may be traced from hour to hour. In Norway, in latitude 70° north, peas 

 grow at the rate of three and a half inches in twenty-f nir hours for many days 

 in summer, and some of the cereals, probably barley and oats, grow as much 

 as two and a half inches in the same time. Not only is the rapidity of growth 

 affected by the constant presence of the sun's heat and light, but those vege- 

 table secretions which owe their existence to the influence of the actinic force 

 on the leaves are all produced in far greater abundance than in more southern 

 climes ; hence the coloring matter is found in greater quantity, the tints of the 

 colored parts of vegetables are deeper, the flavoring and odoriferous matters 

 are more intense, though in saccharine properties the plants of Norway are not 

 equal to those of the south. 



The successful cultivation in northern countries, by artificial means, of plants 

 naturally demanding the high temperature and long seasons of more southern 

 latitudes do but combine the duration and degree of heat. Nor need it excite 

 our surprise that so well defined are the laws regulating the temperatures 

 necessarily accorded to each variety of plant undergoing these artificial cli- 

 matic conditions, and so accurately determined are they by practiced gardeners, 



