Ternifolia Group of Macadamia — STOREY 
513 
grow together (Hamilton and Fukunaga, 1959: 
7). Despite their putative interspecific hybrid 
origin, such trees are highly fertile. 
Although the natural range of M. ternifolia 
lies completely within that of M. integrifolia, 
the two species are not found growing together. 
Nothing which might be construed to be a 
natural hybrid between them has been reported. 
I know of no attempt which has been made up 
to the present to hybridize M. ternifolia with 
either M. integrifolia or M. tetraphylla. Conse- 
quently its cross fertility with those species and 
the fertility of any hybrids which might be pro- 
duced is unknown. 
graft compatibility 
Recent experiments in grafting have shown 
that the three species are mutually graft-com- 
patible in any combination, whether used as 
rootstocks or scions (Storey and Frolich, 1964: 
54-58). The degree of compatibility is so 
high that there is no observed tendency of any 
one species to overgrow the other at the graft 
union. 
All attempts to graft M. integrifolia and M. 
tetraphylla on M. prealta and M. whelani have 
failed. Attempts to graft those species on Gre- 
villea robusta A. Cunn. and G. banksii R. Br., 
closely related species in the family Proteaceae, 
also have failed. 
SUMMARY AND CONCLUSIONS 
This paper enumerates the ten species which 
comprise the genus Macadamia as it is pres- 
ently understood. These fall into four clear-cut 
intrageneric groups. Species identification and 
typification in three of the groups have given 
no difficulty to systematic botanists and horti- 
culturists in the past. The fourth group, how- 
ever, which consists of the three species M. 
ternifolia, M. integrifolia, and M. tetraphylla 
and is commonly referred to as the "ternifolia 
group,” was a source of confusion taxonomi- 
cally for almost a hundred years. 
Much of the confusion among the species un- 
doubtedly resulted from the fact that every- 
thing that was collected in the field in the early 
years was placed under M. ternifolia in the 
herbarium. It is understandable how botanists 
working on such material in the herbarium 
came to regard Macadamia as a single highly 
polymorphic species, especially when one con- 
siders the overlapping ranges, the similarity of 
juvenile shoots and leaves, the transitional 
forms of leaves between the juvenile and the 
adult, and other factors, added to which is the 
possibility that the collections included some 
interspecific hybrid material. It is noteworthy, 
however, that most of the confusion and un- 
certainty one experiences from the study of 
pressed herbarium specimens is dispelled when 
he sees the trees in the field, for the species are 
so different in size and aspect that there is little 
doubt about their being distinct taxons. 
That the three species are indeed closely re- 
lated in a group set apart from the other spe- 
cies of Macadamia is attested by their high 
degree of mutual graft compatibility, by identi- 
cal chromosome numbers, by the genetic cross- 
compatibility between two of the species, and 
by the high order of fertility of the interspecific 
hybrid. One can speculate that they arose from 
a single ancestral intrageneric prototype, and 
that the characters which distinguish one from 
another represent accumulations of gene changes 
over a long period of time which have not, 
however, affected chromosome homology. The 
occurrence of fertile hybrids is of interest, be- 
cause it opens the way to genetic studies and 
to the possible improvement of horticultural 
varieties in a plant breeding program. 
Superficially, the natural habitats of the spe- 
cies appear to be identical, and their natural 
requirements with respect to rainfall and soil 
factors to be the same. M. integrifolia and M. 
tetraphylla are often grown side by side in or- 
chards in Australia, Hawaii, California, and 
elsewhere. In Australia cultivated trees of the 
former are to be seen as far south as Sydney. 
Trees of the latter are to be seen as far north 
in Queensland as Rockhampton. A matter for 
conjecture, therefore, is why the natural range 
of each is restricted to a comparatively small 
region, and why the ranges end so abruptly 
where they come together immediately north of 
the McPherson Range. This should make an 
interesting study in plant ecology. 
