132 



dition, a mealy one appears, together with a decreasing specific weight and 

 decreasing nitrogen content. 



Finally an important observation bearing on the exchange of seed is 

 the fact that w^inter grain coming from regions above the 45th parallel 

 of latitude and cultivated by us in the spring, does not produce shoots, while 

 on the other hand, that taken from lower latitudes behaved with us like 

 summer grain. 



Because of the great interest on all sides in the colonies, it is necessary 

 to take tropical conditions into consideration. Here the differences of tem- 

 perature on the land and between land and sea attain a greater significance. 

 Thus, for example, Fesca^ reports, in regard to the great warming of the 

 land in direct sunlight as compared with that of the sea, that the tempera- 

 ture of the tropical ocean rarely exceeds 30°C. w^hile the rock is heated up 

 to 60° to 70°C. Pechuel-Loesche observed a soil temperature above 75°C. 

 on the west coast of Africa in the 5th parallel of south latitude, not less 

 than 36 times between January ist and March 4th. In contrast to this, 

 however, stands the nightly cooling down to I5°C. and less. Daily fluctua- 

 tions of the soil temperature from 30° to 40°C. are very frequent in the 

 tropics while, on the other hand, the daily fluctuations of the sea might at 

 most reach i°C. 



As a result of the differences in the morning quality of land and sea, 

 a low barometric pressure must be produced on land in the day with the in- 

 tensive sunhght, so that the air from the sea streams in that direction and, 

 conversely at night. These sea and land breezes are considerably more in- 

 tensive in the tropics and sub-tropics with the stronger contrasts in warm- 

 ing land and water and form a factor to be reckoned with. According to 

 Saito^ the air above the sea is almost free from mould fungi, bacteria and 

 yeast germs, while the air above the land (street and garden air in Tokyo 

 was investigated) was especially rich in germs in wet and warm periods. 

 Thus the sea breezes act as purifiers of the air. The sea breezes decrease 

 towards the poles, since the sea gradually assumes a higher mean heat than 

 the land and also because the daily fluctuations of the soil are less. 



For the same reason the changing annual wdnds, the monsoons, corres- 

 pond to the periodic daily winds in the strong warming of the great conti- 

 nents to which vegetation must adapt itself. 



The amount of precipitation occurring as rain depends also on the re- 

 lation to the sea and the temperature and, accordingly, it is most abundant 

 in a warm sea climate, scantiest in a continental one. An annual mean of 

 9°C. approximately holds for all the German North Sea coasts. With an 

 80 per cent, saturation, the air would contain 7.26 g. water vapor in a cubic 

 meter. If the air cools down to 4°C. it can hold only 6.9 g. water vapor per 

 cubic meter and the difference must therefore be eliminated as precipitation. 



1 Pflanzenbau in den Tropen und Subtropen, p. 23. 



- Saito, Untersuchungen liber die Atmospharischen Pilzkeime. Journ. College of 

 Science, Tokyo. Vol. XVIII. 



