76 BULLETIN 1380, U. S. DEPARTMENT OF AGRICULTURE 
cork tissue in small isolated patches on different parts of the stem. 
The cork development took th,e form of small tubercles 2 to 4 milli- 
meters in diameter and projected about 5 millimeters above the sur- 
rounding green bark. These tubercles could be rubbed off with the 
hand, leaving small brown depressions. Since the formation of the 
outer cork layer is uniform over the entire surface of the stem this 
phenomenon was noticeable. 
ABNORMAL PODS AND SEEDS 
The surface of Hevea pods is normally smooth. Occasionally 
pods with the surface raised in wartlike tubercles or ridges were ob- 
served. In all such pods the seeds were normal. 
Seeds with calluslike ridges or outgrowths at the pointed ends 
were observed in a few instances. 
SOIL AND MOISTURE RELATIONS 
The direct influence of the different kinds and conditions of soils 
in the Amazon Valley on the vigor and disposition to disease in 
Hevea was observed in but few cases. In general, red or yellow 
subsoils predominate over the entire region. These are covered 
either with a rich forest mold or alluvial deposits. In some regions 
these subsoils are covered with a layer of sand with little or no top- 
soil. The sandy soils occur frequently in the lower Amazon region, 
and when above inundations they become very hard and dry. The 
effect on Hevea is a stagnation of growth and a general loss in vigor. 
The taproot in some cases may not be able to penetrate the harder 
strata beneath, and this results in early decay. This condition was 
observed where trees had been blown over by the wind. A short 
distance away from the banks of the streams in some regions the 
ground is low and swampy. The soil is usually a light-colored clay, 
without much forest mold or alluvial deposits. The few Heveas 
observed growing in such situations were of low vigor and showed 
evidences of root decay. 
The finest rubber is found wherever the yellow or red clay soils 
predominate on the higher levels where they are porous and well 
drained and have a good cover of humus. Little disease of any kind 
was observed in such locations. Such sites would no doubt afford the 
best opportunity for growing rubber. 
A few Heveas were found growing in a permanently wet or Avater- 
covered swamp, notably on the flats between the high bluffs and the 
banks of the Rio Solimoes. Such trees were of considerable height 
but of small diameter. There was root-rot, and the leaves were much 
attacked by fungi. 
The floods transport and deposit the seeds of Hevea on all types 
of soils in the low lands. The migration of the tree from the high 
lands of the upper drainage to the lower Amazon was no doubt 
brought about in this manner. Observations show that on land 
where the roots of Hevea are permanently below the water level or 
when established in poor, compact, un drained soils the trees do not 
attain as great a girth and state of vigor as when growing on the 
higher levels. 
