EFFECTS OF CONTINUOUS HUMIDITY. 41 



be useless with Castilla, since the milk flowed out as a liquid only in 

 the first few drops and soon turned into a pulpy mass, which remained 

 in the grooves and had to be wiped out with the finger. Dr. Preuss 



says: 



In a Castilla plantation near San Salvador the manager stated, on my inquiry, tnat 

 there are hule trees the milk of which is completely liquid and others of which 

 the milk is thick and does not run down. I had both kinds of trees pointed out to 

 me, but could recognize no difference in trunk, leaves, or fruits. All the trees, 

 which I tapped later, always showed the thick milk. 



In Guatemala, however, trees were pointed out to me on two plantations which, 

 with exactly the same appearance in leaves, fruits, habits, etc., still had a com- 

 pletely different behavior. On tapping there flowed out in abundance a thinly 

 liquid milk, which, however, contained no rubber, or only very small traces of it. 

 Of such trees there were many on both plantations. They had been specially 

 marked, and were never tapped; naturally their seeds were also not sown for new 

 plantations. The statement that the milk of Castilla, that from which good rubber 

 can be obtained, runs down the trunk into vessels, I have often heard asserted with 

 positive assurance. I have never been able to convince myself of it, and can only 

 suppose that it is a case of two different varieties, with one of which I have not 

 become acquainted. 



DECREASE OP MILK WITH ALTITUDE AND CONTINUOUS HUMIDITY. 



That rubber milk is obtained with greater freedom on the drier 

 western coast shows that continuous humidity is at least not indispen- 

 sable, but it does not prove that the larger production is due to the 

 drier climate. There may be, and probably are, differences in the 

 trees of the two regions, though these have not been detected. But 

 that there is a climatic element even on the west coast is made plain 

 by the fact that as the coastal plain is left behind and the slopes 

 increase in altitude and humidity the production of rubber gradually 

 declines. At an altitude of about 1,800 feet on the Esmeralda coffee 

 estate, only a few miles from La Zacualpa, wild Castilla trees appar- 

 ent^ normal in other respects yielded milk very sparingly, while at 

 an elevation of 2,500 feet no milk dropped from the cuts. Castilla 

 trees grew vigorously and attained a diameter of 15 inches in twelve 

 years at "Quien Sabe," in the coffee district above Tapachula. The 

 trees grow naturally up to 1,500 feet and beyond. Above 1,000 feet 

 the rubber gatherers do not expect to find much rubber. Trees 

 planted at an altitude of 2,000 feet from seed brought from the coast 

 do not yield rubber. 



The fact that Castilla 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 and of maintaining the 

 high pressure of fluids found in trees growing near sea level. 



