133 



If tropic air reaches 25°C. with the same saturation (80 per cent.) it con- 

 tains 18.48 g. water vapor and ehminates 1.18 g. water per cubic meter when 

 cooled down to 5°C. This amount of precipitation therefore is more than 

 three times that of air at 9°C. when influenced by the same decrease in tem- 

 perature on the North Sea coasts. Thus are explained the heavy tropic 

 rains and especially the heavy fall of dezv which, in places, must suffice for 

 a certain period in hot climates as the only source of water. 



Just as in cultivation experiments, soil analyses and mean temperature 

 offer no sufficient insight into a possible utilization of food substances on 

 the part of cultivated plants, just so httle can the annual rain fall indicate 

 the moisture conditions of a region. For it depends essentially upon the soil 

 conditions and the distribution of the precipitation in the different months. 

 Over a greater part of the desert of Sahara (see Fesca) the same or a greater 

 amount of rain falls than that sufficient for Germany's agriculture (60 cm.) 

 without its having there any essential effect. For, on a highly heated soil, 

 moisture exaporates immediately. The most desirable distribution of rain 

 in the tropics is not the one extending uniformly throughout the whole year, 

 but, viz., at the beginning of the vegetative period an abundant precipitation 

 and then a time of dryness. The abundant clouds in the rainy season con- 

 tribute essentially to the production of a cooler temperature which is es- 

 pecially favorable for the development of the vegetative organs. 



Along the coast the climate is cloudier than it is inland. In regions of 

 great atmospheric dryness, as in the Mediterranean basin, often there is 

 only 20 per cent, cloudiness as an annual average : in the dryest months often 

 only 10 per cent., — in the moist tropics not infrequently more than 80 per 

 cent. Since, howeven the cloudiness decreases the taking up and giving off 

 of heat, the temperature of the lower latitudes is less and that of the higher, 

 greater. Many cultivated plants require these lower temperatures and 

 cloudiness. We believe, with Zimmerman^, that many diseases in coffee plan- 

 tations, especially the excessive production of fruit, may be due to insuf- 

 ficient shading. In the same way it may be that the great susceptibility to 

 fungous diseases which has appeared in the last 15 years- since tea has been 

 cultivated in the Caucasus, has been due in part to the difference of the 

 Caucasian climate from that of the regions from which tea was introduced. 



The development of the plant body is of course adaptable to the climatic 

 conditions and factors of growth. The more recent biology takes these cir- 

 cumstances into consideration as is shown by the work of Hansgirg". He 

 speaks of stenophyllus wind leaves (as in the willow type) ; of leather 

 (coriaceous) and wind leaves (palm type) ; of xerophyllus leather leaves 

 (Myrtus, Laurus), of dew leaf types (BromeHaceae, Pandaneae) ; thick 



1 Zimmermann, Sonderberichte iiber Land- und Forstwirtschaft in Deutsch- 

 Ostafrika. Vol. I, Part 5, 1903. 



2 Speschnew, Travaux du jardin bot. de Tiflis VII, 1 Verhandl. d. Internat. 

 landwirtsch. Congresses in Rom 1903. 



3 Hansgirg-, A., Phyllobiologie nebst Uebersicht der biologischen Blatttypen etc. 

 Leipzig, Borntrager, 1903. 



