244 



northward we have a greater amount of possible sunshine during the 

 growing period, ahhough the actual sunshine is very materially 

 diminished by the quantity of cloud and fog. Tisserand calls atten- 

 tion to the maximum possible duration of sunshine as given in the 

 following table for the season of spring wheat from sowing to 

 ripening: 



These numbers of possible hours of sunshine should be diminished 

 to actual hours of sunshine on account of cloudiness. Moreover, 

 actual actinometric observations would have shown that owing to the 

 atmospheric absorption the efficiency of the sunshine is less at low 

 altitudes and, therefore, at high latitudes. But in the absence of 

 fundumental climatic data Tisserand is probably correct in conclud- 

 ing that the temperature of the air has apparently little to do, in and 

 of itself, with the duration of the time from sowing to ripening, but 

 that this depends principally on the sunshine, so that at northern 

 latitudes the wheat ripens best in localities that have the least cloudi- 

 ness or the sunniest exposure. On the other hand, the temperature 

 of the air does appear to materially affect the chemical constitution 

 of the grain, since the northern crops are richer in hydrocarbons, 

 and the proportion and quality of the starchy principle increases 

 and the nitrogenous compounds diminish as the locality ap^'oaches 

 the equator. 



The acclimatization of plants is accompanied b}' notable changes 

 in their nature; frequently the leaves increase in size relatively to 

 the rest of the plant, and their colors are more pronounced, as if the 

 plant sought to supplement the low temperature by a more complete 

 absorption of the solar rays. A similar change as to the leaves and 

 colors takes place in the flora of high mountains as compared with 

 that of the plains below. The aromatic principles of plants are also 

 developed in a remarkable manner in high latitudes. Thus the beans 

 have a more decided flavor in Norway in proportion as we go north- 

 ward, and at Alten (lat. 70° N.) the most aromatic cumin {Cuminum 

 cyminum) of all Europe is cultivated. 



The incident sunshine seems to be the productive climatic element 

 in effecting the growth of plants ; it furnishes the total vis viva, or 



