42 CENTRAL AMERICAN RUBBER TREE. 



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 

 favorable for rubber. It is also not to be assumed that a region in 

 which the rubber tree grows wild is favorable 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 unfavorable 

 conditions, it might be dispensed with where the unfavorable con- 

 ditions do not exist. That latex serves in Castilla as a protection 

 against drought does not mean that it may not have other functions 

 here and elsewhere. The problem of rubber culture is to encourage 

 the formation of latex by placing the tree under suitable conditions. 



In a dry atmosphere the transpiration — that is, the moisture given 

 off by the leaves — is much greater, and as this water is taken up 

 from the soil the amount of salts and other soluble substances taken 

 into the plant with the water is also much increased. It is by no 

 means impossible that substances obtained in this way are used in the 

 formation of rubber, and if this be the case the tree would have, as it 

 were, an automatic protective device; the drier the weather the greater 

 the quantity of rubber-forming materials and the greater the protec- 

 tion against dry weather. It is possible even that the thickening of 

 the milk might finally impede the circulation of water and be itself 

 the cause of the falling of the leaves, as Parkin observed with the 

 leaves of Hevea. The falling of the leaves in the dry season would 

 thus be an indication of conditions favorable for rubber culture rather 

 than the reverse, as some have supposed. It is not at all impossible 

 that a rubber tree might grow best in a region where it would not 

 yield the maximum quantity of rubber, and, conversely, it may be 

 found that the most rapid growth of the trees does not insure the 

 largest yield of rubber. If it be true that rubber is a dry-weather 

 product, the limitations of rubber culture on this side are in securing 

 enough rain to permit rapid growth. One problem would be to find 

 out how much of a dry season is necessary for best results. Too 

 much dry weather would mean slow growth, too much rain decreased 

 formation of rubber, and these factors would vary even in the same 

 neighborhood and with different seasons. The prospects of particular 

 localities for rubber can not be ascertained by the tapping of a few 

 trees in each at the same date or in the same month. A tree in which 

 the pressure in the milk tubes was too low or the milk too thick to 

 flow out in the dry season might yield abundantly at the beginning of 

 the rains, while in a more humid locality the fluctuations would be 

 much less. 



That rubber could be obtained from one tree in the dry season and 



