24. XXIII. Eucalyptus sepuleralis, F. v. Mueller.

The limitations of morphology, and record of oil constituents considered in regard to the determination of species of Eucalyptus  244 
Explanation of plates  251 

  ― 244 ―


E. sepulcralis, F.v.M.

In Dec. viii “Eucalyptographia” (1882).

I HAVE no specimens other than a portion of the type, and have nothing to add to the description as given in “Eucalyptographia.”

But I would point out that the precise position of this species is still unknown, and will remain so until seedlings are raised. I trust, therefore; that seeds will soon be again available.

Mueller places it next to E. buprestium while drawing attention to the anthers, which are indeed one of the connecting links between the Renantheræ and the Parallelantheræ. I would suggest that the true affinity may be with E. erythronema, Turcz., from which it is sharply separated by the fruits; but leaves, anthers, and even buds show resemblance.

Work to show the affinities of the species of this extensive and perplexing genus is much desired. At present many of the species have been described without due reference (often data were not available) to their congeners.

The limitations of Morphology and record of Oil-constituents considered in regard to the determination of species of Eucalyptus.

I THINK I have fairly shown that the present group of species, the Stringybarks, exhibits variation in a most marked degree. It is, therefore, opportune to againnote deal with the subject of variation in the genus. Darwin has uttered the dictum that species of the larger genera in any country vary more than the species of the smaller genera.note Experience with the large genus Eucalyptus certainly bears out the truth of this dictum. Hooker's papernote may be profitably studied in this connection.

Variation can be studied from three standpoints:

  • 1, Selection;
  • 2, Hybridisation, or crossing;
  • 3, Mutation;

and all of them, in my view, are operative in the genus Eucalyptus, accounting in varying degree for the innumerable variations so far observed.

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Selection.—This is Darwin's expression for what Herbert Spencer has termed the “survival of the fittest.” This selection can be aided by man, but most of the variations already noted in the genus refer to naturally-grown forms.

Darwin's view was that of all the causes which induce variability, excess of food is probably the most powerful.note

Hybridisation is a term implying the breeding together of members of different species. The word is derived from the Latin for a mongrel. In other words, two different species must be concerned. The word “crossing” is sometimes taken to imply “the mingling of strains within a species”; but, in view of the unequal relations of varieties and species as often defined, it seems convenient to take hybridisation and crossing together, at all events for our present purpose.

I have dealt with hybridisation as regards Eucalyptus in various publications,note and need not repeat the facts and inferences at this place.

Mutation or Saltation is the term applied to sudden changes of characters for which no immediate cause is apparent. The phenomena were first largely investigated and brought under notice by Hugo de Vries, of Amsterdam.

The resultant plants or sports are not hybrids, and are produced as the effect of various circumstances which disturb the conditions of a plant. The tendency to alteration is latent in the plant, and stimuli not always clear to us are sufficient to bring out these mutation-forms.

When we speak of the natural or innate tendency of a plant to differ from the remainder of the plants of similar origin,note we often refer to mutation-forms.

Variation in plants induced by environment.

Let me quote some references by eminent botanists to this subject:—

1. Pseudo-species of Botanists.—Dr. D. Mariano de la Paz Graells … adds the following remarks upon some of the many so-called species, which he shows are only modifications due to environment. Thus, of Pyrethrum sulphureum and Dianthus brachyanthus, he writes:—

The polymorphism which these plants acquire at different elevations has given rise to the formation of distinct species, i.e., admitted as such by botanists of note. Studying the original division of P. pulverulentum, of Lagasca, and of P. sulphureum, of Boissier and Renter, Willkom has united them into one single species, which he has called in his Prodromus Flora Hispanica, P. Hispanicum. In this, he recognises two well-defined groups, the “pinnatifid” and “laciniate” types, placing in the first group P. pulverulentum of Lagasca, and the P. radicans of Cavanilles; and in the second, P. sulphureum, Boiss. et Rent., which Asio had named Chrysanthemum Aragonense, and C. Bocconi or P. Bocconi, Wal. P. versicolor, Willkom; which turns out to be the P. sulphureum, var. P. alpinum, Boiss. et Rent.

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The same thing has happened with Dianthus brachyanthus, Boiss. et Rent., which Xatar and Maill took to be D. attenuatus, Benth., in the Pyrenees, and Koch for D. virgineus; such mistakes being due to the modifications produced by varying elevations. In some cases the very same organs become atrophied or disappear, while in others they become much more developed than usual.note

2. The especially characteristic features of alpine plants, as compared with similar or allied plants growing at lower levels, are a dwarfing in size and compactness of growth, sometimes giving rise to a moss-like appearance; a more intense green colour in the leaves, and greater brilliancy and size in the flowers; an increased hairiness of the leaves, and occasionally a certain degree of fleshiness of the tissues.note Now, by growing lowland plants at high altitudes, Bonnier,note Flahault,note and others have shown that such characters as these may be rapidly acquired. For instance, Bonnier made observations on Teucrium Scorodonia for no less than eight years, and he found that this plant, when sown at a high situation in the Pyrenees, produced very short aërial stems, with more hairy and darker green leaves, and more compact inflorescence. On the other hand, seeds gathered from plants growing at high altitudes, and sown in Paris, after three years produced elongated stems, with less hairy and brighter green leaves, or plants very similar to those from seeds obtained in the neighbourhood of Paris.note

3. Existing floras exhibit only one moment in the history of the earth's vegetation. A transformation which is sometimes rapid, sometimes slow, but always continuous, is wrought by the reciprocal action of the innate variability of plants, and of the variability of the external factors.note

And again:—

4. There are, further, some species—and this fact is as important to the systematist as to the physiologist—which adapt themselves to the varying conditions of humidity so completely that their extreme forms appear to belong to different species, but these by a change in the supply of moisture may pass over into one another.note

5. Every plant .… occupies its place in the order of nature by the action of two forces—the inherent constitutional life-force with all its acquired habits, the sum of which is heredity; and the numerous complicated external forces or environment. To guide the interaction of these two forces .… is, and must be, the sole object of the breeder, whether of plants or animals.note

6. The combination and interaction of these innumerable forces embraced in heredity and environment, have given us all our bewildering species and varieties, none of which ever did or ever will remain constant.note

7. Bringing a species into a new environment disturbs its fixity. Rich soil especially gives rise to variations in growth which seems to be new, and by repetition become inherently fixed. Sometimes ancestral states are brought about by good soil; sometimes (perhaps oftener), also by starvation; new variations oftenest by rich soil and general prosperity.note

Variation is going on now.

Authors sometimes argue in a circle when they state that important organs never vary; for these same authors practically rank that character as important (as some few naturalists have honestly confessed) which does not vary; and, under this point of view, no instance of an important part varying will ever be found; but under any other point of view many instances assuredly can be given.note

And again:—

I will add another remark: Naturalists continually assert that no important organ varies; but in saying this they unconsciously argue in a vicious circle; for if an organ, let it be what it may, is highly variable, it is regarded as unimportant, and under a systematic point of view this is quite correct. But as long as constancy is thus taken as a criterion of importance, it will, indeed, be long before an important organ can be shown to be inconstant.note

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Reputed constancy of characters in Eucalyptus.

The following statements have been made:—

1. “Comparative constancy of specific characters of Eucalyptus species … but it is individual species that we maintain show a comparative constancy of specific characters throughout their known geographical distribution.” (Messrs. Baker and Smith.)

And again:—

2. “The reputed or supposed great variation of individual Eucalyptus species has probably arisen by the attempts of botanists to found species on morphological characters alone.” (The italics are not mine.)

Two points are involved in this statement:

  • (a) The reputed invariability (or in other words, the “comparative constancy”) of species of Eucalyptus;
  • (b) The founding of species on morphological characters alone.

Real genetic relationships take cognisance of all the characters.

Some of the items in regard to which I always endeavour to obtain information as regards any particular species of Eucalyptus are as follows:—

Shape of juvenile leaves; venation and mature leaves; principal constituents of oil; anthers; fruit; bark; timber; kino; habit; any other character.

I attach great importance to studying the trees in the field. In this way habitat, habit, size, bark, timber, can best be studied.

In these researches I may be pardoned for saying that I have travelled more or less in every. State of the Commonwealth, covering thousands of miles on foot in pursuit of this study alone, in contradistinction to mere herbarium work.

This is one way of learning what are “natural” species, and affinities and dissimilarities can be largely learned in this way. I have, indeed, inaugurated on a comprehensive scale the study of genetic relationships in Eucalypts, and have always deprecated the study of this genus from herbarium specimens or “morphological characters alone.”

The extracts from the writings of eminent botanists are pertinent in this connection:—

1. It is clear that at present the question (relation of plants to one another) is very far from settled; indeed, hardly more than a beginning has been made in the establishment of a system which can be said to represent real genetic relationships.note

2. Rather there is an increasing tendency to the view that the solution of plant-affinities, as Linnæus long ago affirmed, must be sought in a comparative study of all the characters.note

3. The idea that morphology has nothing to do with the function of organs has been acquired entirely because the fact has been overlooked that the transformations seen in organs are conditioned by a change of function. Their functions, therefore, have been treated as subordinate in determining the characters of organs; external relations alone have been taken as the chief points for consideration. But the relationships of mere form are by no means the permanent ones in ‘the tide of phenomena.’ They also change. The determination of this change, that is to say, of the alterations which have taken place, and are believed to take place in the formation of organs of a natural group, is one of the weightiest tasks of organography. If we separate function from form we are at once led into altogether unfruitful speculations.note

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I repudiate any suggestion that the taxonomic evidence afforded by the morphology of plants can be intelligently studied except in connection with such physiological evidence as may be available.

At the same time, the classifactory characters must be mainly morphological.

“But in his clear definition of the principles which must guide the worker who is seeking a true natural system, De Candolle did work of far greater value. He pointed out that characters which are of the utmost importance to the life-functions of the plant are useless from a systematic point of view. In a word, it is to morphology, and not to physiology, that we must look for aid in establishing relationships.”note

Oil an accessory or adaptive character.

The essential oils are accessory substances, and, may I repeat it, variable like everything else connected with Eucalyptus.

1. The chemical qualities, odours, and tissues of plants are often modified by a change which seems to us slight. The Hemlock is said not to yield conicine in Scotland. The root of the Aconitum napellus becomes innocuous in frigid climates. The medicinal properties of the Digitalis are easily affected by culture. The Rhubarb flourishes in England, but does not produce the medicinal substance which makes the plant so valuable in Chinese Tartary. As the Pistacia lentiscus grows abundantly in the south of France, the climate must suit it, but it yields no mastic. The Laurus sassafras in Europe loses the odour proper to it in North America. Many similar facts could be given, and they are remarkable because it might have been thought that definite chemical compounds would have been little liable to change either in quality or quantity.note

2. Just as the presence and quantity of opium, hasheesh, aconitine, &c., secreted by plants, vary greatly with the climate, so it is reasonable, in the absence of strict investigations, to assume that these oils are in an excess through the intense heat, and other conditions of the climate of deserts.note

3. Interesting as is this correlation of morphology and constituents in the Eucalyptus species, it may be pointed out that a knowledge of the constituents of a plant is never likely to play such an important part in systematic botany as the authors appear to believe, since there are already known numerous instances of plants which, grown under different climatic conditions, show no morphological change, yet exhibit remarkable variation in constituents, and, on the other hand, plants which are not at all closely related frequently contain the same colouring matters, alkaloids, etc., so that the necessary specific constancy of constituents, which alone would make such criteria useful, is wanting. The authors lay stress on observations made by them as to the absence of marked variation in the composition of oils yielded by the same Eucalyptus species grown in different districts of Australia; but the evidence of constancy in this respect would be greatly strengthened if it could be shown to hold for the same species outside Australia. For an investigation of its kind ample material now exists in foreign plantations.note

The cases of the Peppermint and Lavender, both plants yielding essential oils, are notorious. Science has not yet established a connection between morphological characters and oil-yields in these cases.

It is only necessary to consult any good work on essential oils, say Die ætherischen Oele, E. Gildemeister and Fr. Hoffmann (Julius Springer, Berlin), of which the authorised translation is The Volatile Oils, by Edward Kremers (Pharm. Review Co., Milwaukee, U.S.A.), and especially Schimmel's Semi-annual Reports

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(Berichte) to be satisfied that the chemical composition and physical properties of essential oils of ascertained botanical origin often vary considerably for the same species.

Oil determinations are usually difficult to apply for purposes of diagnosis. Similarly, in Radlköfer's Monograph of the Sapindaceæ, I find it difficult to accept such characters for the genera as, “fruit contains saponine; fruit without saponine.”

In Eucalyptus in a given species there is variation in regard to the constituents of the oil. For example, as regards E. Wilkinsoniana, R. T. Baker:

At many different times of the year the oil contains small quantities of Eucalyptol; at other times, however, some phellandrene; it contains, moreover, a small amount of ester.

It is an invidious task to be the judge as to the amount of chemical variation which will be admitted as evidence of the validity of a botanical species.

E. rostrata, Schlecht., var. borealis, Baker and Smith, and E. lævopinea, R. T. Baker, var. minor, R. T. Baker (E. Wilkinsoniana, R. T. Baker) have been founded “on chemical evidence alone,” or mainly. I do not say that morphological evidence will not be forthcoming to justify this nomenclature, but it is not available yet, and it has been specially sought for. There are two ways of looking at this matter; one way is to endeavour to ascertain the position of a plant by morphological methods, and then to ascertain if the evidence supplied by physiology supports the view arrived at; or physiological evidence (e.g., based on examination of essential oil) may first be examined, and the morphological facts then brought under review.

Is the oil character the one invariable?

In a case such as the Stringybarks (to choose one group out of many) where there is an infinite gradation of forms, the result of environment, hybridisation, and perhaps other causes, the suggestion that in the oil there is a master-key to the limitation of species seems to be based on a shifting foundation.

The suggestion that we have at length discovered a kind of philosopher's stone is indeed alluring; that in all the manifold changes of Eucalyptus that we have at length obtained a test by which we can diagnose a species is tempting, but the test will only be found to be general in application, like those applied to the timbers.

As to the variation in timber in the genus, that is the experience of any man who has much to do with it. I do not wish to quote the views of the bush-worker, who is often ignorant and empirical; at the same time, many of them are very shrewd. The timber-inspector and the timber-merchant, who have broader views, however, both share this view.

The composition of the oil is not the only character other than that usually employed to aid the botanist in the diagnosis of Eucalypts. For example, I restored E. exserta, F.v.M., to specific rank, following up a clue given to me by the timber.

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That there are limitations in regard to the interpretation of morphological characters for the purposes of taxonomic research, I at once admit. Following is a very good example of such searching after the light:—

“As soon as three Orchidean forms (Monochanthus, Myanthus, and Catasetum) which had previously been ranked as three distinct genera, were known to be sometimes produced on the same spike, they were immediately included as a single species.”note

A botanist often has to work on incomplete material, and to unduly disinter the mistakes of morphologists, some of which (as regards Eucalyptus) have been made before the era of exhaustive field investigations on the genus inaugurated by myself, would be as unnecessary as to point out the mistakes of scientific discoverers in any other branch of science, who, by their work, have paved the way to research on higher planes.

Variation in oil.

The variation in oil constituents within the same species has already been referred to. To consider another aspect of the question, a man, when he makes a distillation, which costs much trouble and expense (which is certainly the case with Eucalyptus oil) is apt to stereotype its results; whereas, if he could make a hundred times as many distillations he could take a broader view of the variability of the oils.

Thousands of distillations require to be made before oil results can be based on material as varied as that on which the systematists referred to as “morphologists” base their conclusions.

The distillations of Eucalyptus oils from material of authenticated botanical origin, were inaugurated by me with the view (inter alia) of obtaining authentic Eucalyptus oils for therapeutic and other purposes, and also of ascertaining to what extent the oils could be used as aids in the diagnosis of species.

Researches on Eucalyptus oils may, however, be employed for two ends:—

  • 1. Acquiescence in the naming of existing species (e.g., E. saligna and E. botryoides).
  • 2. For the naming of new species.

As regards (1) if the premises be wrong,—if it should prove that E. saligna and E. botryoides are identical species, what becomes of the statement that the oils show them to be distinct?

If the answer be that there is some innate principle in these two trees that morphology does not reveal, then we require very strong evidence that the alleged oil differences are real and not apparent. But I have already touched upon this point.

It seems to me that an important difference between the morphological and the oil-system determination is this:—

In morphology you have a fixed standard termed the type.

In oils you have no fixed standard, the oil-constituents being variable within limits not yet determined and perhaps indeterminable.

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This brings one to ask:—What variation in amount of a constituent, or what constituent must be present or absent in any particular case to constitute a valid species?

Classification on oils alone associates dissimilar species.

We should reflect when we find it stated that the oils from E. bicolor, A. Cunn., are similar to those attributed to “E. bicolor,” and which are really the product of E. Bosistoana, F.v.M. If this should be founded on fact, then it proves that species with different genetic relationships resemble each other in oil-constituents.

The classification based on oils alone places certain species and their varieties in different groups, e.g.

E. sideroxylon, A. Cunn.

E. Stuartiana, F.v.M.

E. tereticornis, Sm.

E. rostrata, Schlecht.

It also brings together species which perhaps every other classification shows us do not exhibit close affinity. Instances of this can be quoted at any time.

To sum up, I think that characters based on the essential oils are subject to variation. They do not escape the interacting laws of change any more than morphological characters do.