19 



at the time the tree is tapped. The indications are that pressure 

 attains its greatest intensity in trees which are exposed for a part 

 of the time to a relatively dry atmosphere and which are accustom- 

 ed, as it were, to pump water rapidly to supply the leaves. Such 

 trees may, on the contrary, yield no milk at all when the water 

 supply is deficient. It may be expected, therefore that open culture 

 will require much more careful attention to the time of tapping. 

 This may prove a disadvantage if it requires all the trees of a 

 large plantation to be tapped on the same day or in the same 

 week, but this is not likely. On the other hand, tapping at the 

 right time would mean the drawing of a larger amount of milk from 

 a smaller cut, a saving of labour, and a lessening of injury to the 

 trees. 



The above considerations make it easy to understand also that 

 writers acquainted with humid districts commonly refer to the 

 rubber harvest as occurring in the dry season, while in the drier 

 regions, as in Soconusco, the beginning of the rainy season is the 

 recognized time, when the tree's demand for water is largest and 

 the internal pressure highest. 



I.EGUMINOrs SHADE I'UEES lO HE I'REFEHKEl). 



Where the policy of thinning out the forest is followed the 

 question arises as to which trees are to be left and which cut down. 

 A study of coffee and cacao culture has revealed the probability 

 that much of the benefit ascribed to shade is due in reality to the 

 nitrogen furnished by the bacteria of the root tubercles of the 

 leguminous trees which are preferred in all countries where the 

 shade culture of coffee has become popular. If shade trees are to 

 be planted with rubber, they must be different from the species of 

 Inga which are preferred for coffee shade in Mexico and Central 

 America, for the reason that Castilloa grows faster than Inga. 

 Some leguminous trees, however, grow with great rapidity and may 

 be able to outstrip the rubber. No comparative experiments seem 

 to have been made. If, as suggested above, shade trees are more 

 useful as windbreaks than for the shadow they cast on the rubber, 

 the planting of fruit trees like the mango or other useful species 

 in rows or hedges would be preferable to scattering them amongst 

 the rubber. . 



DISTANCE BETWEEN TREES. 



As yet there have been no experiments yielding any definite in- 

 formation on the above point, but the recent trend of opinion 

 among planters seems to be distinctly in the direction of closer 

 planting. There has been a gradual decline from 20 feet and up- 

 ward between trees to 12 feet and under. 



The questions of shade and of distance between trees are closely 

 related and need to be considered together because several of the 

 arguments for shade can be met, wholly or partially, by close plant- 

 ing. The first of these is that of the greater expense incidental to 

 open culture. The frequency with which the land rcciuires to be 

 cleaned and the period of years during which it would be necessary 



