THE HEVEA RUBBER TREE IN THE AMAZON VALLEY 43 
so very different from other trees in the region and because a general 
tendency was observed in the rubber country to give far too low an 
estimate on the age of planted trees. Even if correct, the data from 
these few trees can not counteract the general impression that bark 
development in the whole area is as backward as growth in general. 
The. younger trees at Seringal Miry have an average bark thickness 
of 6 millimeters and an average of 9.6 rows of latex vessels. The old 
trees from Purus seeds have means of 5.5 millimeters and 17.5 for 
bark thickness and rows of laticif erous vessels, respectively. 
The 12-year-old trees at CacLoeira Grande, near Manaos, have a 
mean bark thickness of 5.3 millimeters and an average of 12 rows of 
latex vessels. 
The lower Amazon has a great advantage as a plantation site for 
the rubber industry on account of its proximity to the sea and 
transportation facilities to both New York and Europe ; but the evi- 
dences available do not point to it as a favorable location for planta- 
tions, because the trees do not appear to thrive there. Neither has 
the region ever been noted as a producer of high-grade rubber. At best 
one can only hazard a guess as to the reason for this, but it is possibly 
one of soil. A great deal of this land is permanently water-logged; 
other areas are better drained but are not fertile. The uplands, such 
as one sees in the direction of Braganca, do not appear to be especially 
well suited to rubber. The temperature throughout the rubber 
country of Brazil and Bolivia is well suited to rubber production in 
that the thermometer never reaches a dangerously low point. Rain- 
fall, too, is probably adequate in most regions, though the dry season 
is rather long for the best growth of the trees. 
THE VALLEY OF THE MADEIRA RIVER AND ITS TRIBUTARIES 
As on the Amazon, not many trees of known age are to be found in 
the Madeira River region. In the aggregate doubtless a fairly large 
number have been planted, but these are mostly in lots of 10 or fewer 
around dwellings, and it is not certain that the ages of even these are 
correctly recorded. Larger plantings are extremely rare. Data 
from a number of plantings and several groups of old jungle trees are 
shown in Table 10. 
In general one feels that the planted trees here are rather smaller 
than they should be for their age but not so backward as the trees 
on the Amazon. The girth of the trees is somewhat greater, and 
the condition of the bark is better. The bark is thicker, much 
softer, and has a larger number of latex vessels. Conditions of 
planting and culture are about the same as on the Amazon, but in 
general the soil on which these trees were planted is better than that 
of the lower Amazon. Most of these trees are growing either on 
terra firma, permanently above the flood level of the rivers, or on 
varzea alta, which is alluvial soil well drained and subject to only 
occasional floods. These conditions allow better root development 
than is possible in water-logged soils. According to a preliminary 
report by C. F. Marbut, who accompanied the expedition as a 
member of the party from the Department of Commerce, the soils 
of the Madeira, especially the upper Madeira, are superior to those 
of the down-river areas. 
