8 BULLETIN 1422, U. S. DEPARTMENT OF AGRICULTURE 
membranacea, and H. lutea, a total of eight species. The district 
south of the Equator bears H. guianensis, H. collina, H. nigra, H. 
duckei, H. cuneata, H. nitida, H. paludosa, H. brasiliensis, H. viridis, 
H. discolor, H. spruceana, and H. similis, a total of 12 species. 
Huber divides the area inhabited by the genus as follows : 
The Rio Negro region: H. lutea, II. apiculata, H. benthamiana, H. rigidifolia, 
H. minor, and H. microphylla. The region west of the Rio Negro: H. duckei and 
H. paludosa. The eastern region to the coast of Guiana: H. guianensis, H. 
membranacea, and H. pauciflora. The upper central Amazon basin: H. brasili- 
ensis, H. spruceana, H. similis, H. discolor, H. nitida, and H. viridis. The region 
south of the Amazon: H. cuneata, H. nigra, and H. collina. 
Work on the distribution of Hevea is difficult and time consuming. 
Travel on the rivers is very slow, and vast distances must be traversed 
before adequate knowledge of this subject can be obtained. The rea- 
son why the Rio Negro region is apparently so much richer in species 
may well be that that region has been better explored; perhaps, also 
as Huber quotes from Ule (11, p. 227), "bei dem niederen und 
schwaecheren Wuchs der Waelder am Rio Negro bluhendes Material 
von Kautschukbaeumen leichter zu sammeln ist." With conditions 
as they are at present in the Amazon basin our knowledge of the 
distribution of Hevea is likely to remain imperfect for some time. 
RUBBER-BEARING SPECIES 
Of the species of Hevea which bear rubber, H. brasiliensis is un- 
questionably the most important. , Not only is it the most exten- 
sively exploited tree of the Amazon basin, but it is also the species 
which, of all rubber-bearing plants, has succeeded best in cultivation. 
The rubber produced from this tree is superior both in quantity and 
quality to that from any other known plant. 
For convenience the other species may be listed in the following 
three groups: 
Species yielding fair to good rubber . 
Species yielding poor rubber, rarely collected. 
Species not yielding rubber, or at least not known to yield rubber. _ 
benthamiana. 
discolor. 
rigidifolia. 
foxii. 
pauciflora. 
minor. 
I cuneata. 
'guianensis. 
nigra. 
lutea. 
apiculata. 
duckei. 
paludosa. 
microphylla. 
spruceana. 
viridis. 
membranacea. 
frandiana. 
similis. 
confusa. 
nitida. 
glabrescens. 
Hevea benthamiana is one of the leading producers, if not indeed 
the most important producer, of rubber in the area north of the 
Amazon. H. discolor is another species of importance; according to 
TJle (42) it is the principal rubber producer of the Rio Negro. Still 
