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Castilloa as a protection against drought does not mean that it 

 may not have other functions here and elsewhere. The prob- 

 lem 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 sahs 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 protection against dry weather. It is possible 

 even that the thickening of the milk might finally impede the cir- 

 culation 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 

 favourable for rubber cuhure 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 decrease formation of 

 rubber, and these factors would vary even in the same neighbor- 

 hood 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 not from another might mean merely that the former had access 

 to a larger supply of water and was thus able to maintain a greater 

 latex pressure. Such questions will need to be studied in detail 

 after uniform methods of tapping and pressure measurement have 

 been devised. This need not obscure the fact that unless tapping 

 be done at the most favourable date, the productiveness of rubber 

 trees and the localities in which they grow may be misjudged 

 very easily. 



CASTILI.OA IX NICARAGUA. 



The opinion that the production of rubber by Castilloa is favoured 

 by a dry season is based, as yet, only on observations made in 

 Guatemala and southern Mexico : other conditions and different 

 species of Castilloa may be found in the countries to the south- 



