257 



will enhance the yield of our crops and of the world's crops, but 

 the results already achieved prove that in time the gain will be 

 enormous. 



THE CULTURE OF THE CENTRAL AMERICAN 



RUBBER TREE, IV.* 



{Continued from Bulletin for October.) 



By O. F. Cook, Botanist in charge of Investigations in Tropical 



Agriculture, U. S. Department of Agriculture. 



DECKEASE OF MILK WITH ALTITUDE AND CONTINUOUS HUMIDITY. 



{Continued)) 



The fact that Castilloa yields little or no milk in elevated 

 situations is commonly recognized in Soconusco, though it is not 

 necessary to accept the popular impression that the difference is 

 due to the mere fact of elevation. The temperature being lower 

 and the atmosphere more humid, there is less rapid transpiration 

 of moisture and less need at once of means of resisting dryness 

 of maintaining the high pressure of fluids found in trees growing 

 near sea level. 



Decrease of temperature would also mean a decreased effect from 

 dryness. If this interpretation of the function of rubber be correct, 

 a region like the Isthmus of Tehuantepec, which might be suitable 

 for coffee at relatively low elevations, would not for this very 

 reason be favourable for rubber. It is also not to be assumed that 

 a region in which the rubber tree grows wild is favourable for the 

 production of rubber. The case is quite different from that of a 

 seed or fruit crop. A plant is not likely to become established in 

 a region where it can not ripen seed, but if rubber is an adaptation 

 against unfavourable conditions, it might be dispensed with where 

 the unfavourable conditions do not exist. That latex serves in 



♦ Extract from U. S. Department of Agriculture, Bull. No. 49, Bureau of Plaut 

 Industry. 



