similis, H. pauciflora, H. confusa, H. nitida, H. viridis, H. 
Kunthiana; he arranged these series into two groups based on 
characters of the disk of the pistillate flower, with the last three 
(incompletely known species) in a grouping which he called 
Incertae sedis. 
Huber maintained that Section Eu/hevea is ‘‘very natural and 
well characterized’’. While quite distinct from Euhevea, 
Bisiphonia is, he confessed, **not very homogeneous and does 
not have a rational subdivision’’ — for which reason he set up 
his three Series. 
As late as 1913, Huber still continued to maintain these two 
Sections and the three Series in Bisiphonia, believing that, in 
general, this treatment represented natural trends. He did state 
of Series Luteae, nonetheless, that ‘‘species in the Linnean 
sense seem almost non-existent in this group. ... With the 
present state of our understanding, all appear to be in move- 
ment and fluctuation, and we must be satisfied if we arrive at a 
rational grouping of small, provisional species.” 
In 1910, Pax used the division of Hevea into Sections 
Euhevea and Bisiphonia, separating the two solely on the basis 
of the number and placement of the anthers. Of the 17 species 
that he accepted, he grouped three in Euhevea (H. guianensis , 
AH. nigra, H. collina) and 14 in Bisiphonia (H. Benthamiana, 
H. Duckei, H. nitida, H. paludosa, H. brasiliensis, H. lutea, 
A. rigidifolia, H. spruceana ,H. similis ,H. discolor,H. minor. 
H. microphylla, H. pauciflora, H. membranacea). He pointed 
out that the flowers of Hevea exhibit few sharp characteristics 
of use in separating species and that the fruits and seeds, which 
might provide good differentiating characters, were not known 
for some species. He further pointed out that the differentiating 
character employed for Section Bisiphonia were not sharp, 
noting that he could find intermediates in the anthers of H. 
guianensis and H. lutea. 
It is now clear that Pax’s infrageneric classification, as well 
as those attempts that preceded his, were far from natural. Pax, 
a specialist in the Euphorbiaceae, was at a great disadvantage 
in not having seen Hevea growing in the natural state. 
In 1929, Ducke wrote that ‘‘the natural system of the Hevea 
is still to be made; the species are very difficult to group 
246 
