This section is from the book "A Research On The Eucalypts Especially In Regard To Their Essential Oils", by Richard T. Baker, Henry G. Smith. Also available from Amazon: A Research On The Eucalypts And Their Essential Oils.
Perhaps one of the most interesting results brought to light by what appears to us the natural system of classification here adopted, is the affinity shown to exist between the members of the several groups of Eucalypts in their morphological and other physical characters, chemical constituents, venation of their leaves, and the nature of their barks and timbers. And not only is this affinity shown between the species of this important genus, but a close connection is also found to be well marked between Eucalyptus and the cognate genus Angophora, which, therefore, appears to us to be more closely connected to the former-both morphologically and chemically - than does the allied genus Tristania.
Since the above was written in 1902, Dr. Cuthbert Hall has published in the Proceedings of the Linnean Society, N.S.W., 1914, and in the Report of the British Association, 1915, the results of his work on the seedlings and cotyledons of a large number of Eucalypts. Photographs are also given. He there shows the close affinity of these two genera in the following words :- " The original Eucalypts represented now by those of the E. corymbosa class had large entire reniform cotyledons; these are practically identical with those of the nearly-related genus Angophora."
If we deal with the affinity between the Angophoras and the Eucalypts, it will be found that the former have apparently only one feature - the absence of a calycine lid - which removes them generically from the Genus Eucalyptus. Some Eucalypts, however, show a tendency seemingly to develop petals, for in the case of E. tetrodonta, F.v.M., E. terminalis, F.v.M., E. tessellaris, F.v.M., and a few others, it is found that the calyx is irregularly ruptured, rather than circumcised by a clearly-defined sutural line; for, as stated by Baron von Mueller in his "Eucalyptographia," under E. tetrodonta, " the strongly-toothed calyx demonstrates some transit towards Angophora, although the lid is in no way dissolved into petals as in that genus."
In continuing this connection between these genera, it will be noted that in the nature of the bark, timber, calyx tube, inflorescence, kinos, and particularly in the chemical constituents of the oil, and venation of the leaves, the alliance between certain species is very striking.
The oils yielded by those Eucalypts known vernacularly as "Bloodwoods," such as E. calophylla, E. corymbosa, E. trachyphloia, E. eximia, etc, and the allied species, E. botryoides, etc, invariably contain a large proportion of pinene; phellandrene is always absent, and cineol only occurs in traces, if at all. .
The venation of the lanceolate leaves of all this group of species was found to correspond with that of the leaves of the Angophoras, and this feature is illustrated in plates 1, 11, and 111.
It was this affinity in venation which first led us to inquire if the oils, were also in agreement. For this purpose oil was distilled from the leaves of
Angophora lanceohtta, ( Cav. and was found to contain identical constituents to those obtained from the "Bloodwoods" just mentioned. The sesquiterpene (aromadendrene) which occurs in some quantity in all this group of Eucalypts, was also present in the oil of Angophora lanceolata, as it gave the characteristic colour reaction for that constituent with bromine. (For the essential oils of the Angophoras, see paper by one of us, Roy. Soc, N.S.W., Aug., 1913.)
In August, 1896, a paper was read by one of us (Roy. Soc, N.S.W.) on a crystalline substance obtained from the exudation, or kino, of the " Red Gum " (E. calophylla), sent to the Museum by the Bureau of Agriculture, Western Australia; this substance was named aromadendrin. In the description of a new Angophora (A. melanoyxlon), by one of us (Proc. Linn. Soc, N.S.W., 1900), the announcement was made that the kino of this tree contained aromadendrin, it being chemically identical with that described from E. calophylla. The chemical evidence relating to the affinity of these genera which has since accumulated, shows the connection to be somewhat complete.
Assuming the Angophora to be the older genus, we have endeavoured to formulate, on data similar to the above, a table or " tree," showing the supposed line of origin of the various groups of Eucalypts from the apparent initiation of the Genus. The table will be found at the end of this article, and a map of Australia indicating more fully the territorial distribution in connection with this evolution table is also added. The table includes the majority of the species of Eastern and Southern Australia, but when those of Northern and Western Australia shall have been more fully investigated on similar lines, a more complete tabulation will be possible, as then, no doubt, many of the connecting links which are at present felt to be wanting, will be forthcoming. We do not think that the investigation of those species will interfere materially with the main principle of evolution as here laid down.
Proceeding from the "Bloodwoods," it is possible, from the evidence available, that the line of descent was through E. saligna and E. botryoides, the venations (plate III), together with the chemical constituents of the oils, being very closely allied to those of the ."Bloodwoods." In the oil of E. saligna cineol is making its appearance, for, although present but in small amount, it could be detected quite satisfactorily.
If botanical features are considered in conjunction with those of the chemical, it is seen that as the characteristic constituents of the oils vary in amount and change their character, so do the trees form well-defined groups.
For instance those seceding from the "Bloodwoods" apparently pass in three directions, one through the "Stringybarks" to the group of "Peppermints," another through the "Ironbarks " to that large group which includes the cineol-pinene oils generally, or those in which the terpene phellandrene is absent, and thirdly through one section of the "Stringybarks "-particularly E. obliqua-to the other large group which includes the typical "Boxes" and their associated "Mallees." The Genus may thus be considered as embracing four large groups which may be indicated, chemically, as follows: -