Black Rubber, or Central American Rubber. 
The chief source of this rubber is Castilloa clastica Cerv. It 
is a native of the western slope of the Andes of Peru and Ecua- 
dor up through Central America to Mexico. The tree grows 
to the height of 40 to 60 feet in the open, but often reaches 
120 to 150 feet in height and 6 feet in diameter in the dense 
and hot tropical forests. Unlike Hevea it will not stand wet 
feet but requires good drainage. It is never found in wet or 
swampy localities and grows best at the lower elevations in hot, 
sheltered inland valleys. In the equatorial belt Castilloa oc- 
curs up to 2,500 feet but at 15° either side of the equator does 
not grow well above 1,600 feet. It requires rich alluvial well- 
drained soils, high temperatures and shelter from winds. 
The optimum mean annual temperatures for the growth of this 
tree range from 77° to 82° F. It grows in Mexico where the 
mean yearly temperature is 72° F. and will stand a much 
lower one, but as the temperature and humidity decrease the 
growth of the tree is slower and the yield of rubber rapidly 
diminishes. 
The leaves of Castilloa elastica are heart-shaped at the base, 
entire, bright-green and shining, those of young trees 20 inches 
long by 7 inches wide, those of old trees 6 to 12 inches long 
by 2 1-2 to 5 inches wide. The trunk of the tree is wing but- 
tressed at the base. Rubber collectors distinguish 3 varieties 
depending on the color of the bark and twigs, but botanically 
all are considered one species. 
Castilloa elastica has been widely planted. Five hundred 
thousand trees of this variety have been planted in the vicinity 
of Bluefields, Nicaragua, during the last seven years. The first 
of the planted Castilloa forests in Nicaragua has been tapped 
this year. Six thousand seven-year-old trees yielded 534 pounds' 
of rubber, an average of 1 1-3 ounces per tree. A few of the 
largest were tapped repeatedly at 2-week intervals without in- 
jury and yielded an average equal to that secured at the first 
tapping. In the regions climatically suited to it Castilloa is 
considered one of the surest and most reliable species in the 
cultivation of which it is safe to invest the large capital re- 
quired to plant and care for an artificial forest up to the time 
when dividends may be expected. However, it is doubtful 
whether its cultivation should be attempted on more than an 
experimental scale in Hawaii. Our mean annual temperatures 
and mean humidity are too low, and also the islands lie within 
the zone of the trade-winds, and lack the tropical climate of 
other island groups in equal latitudes. 
