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Volume 60, 1930
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The Changes produced by Oxidation in the Pitted Tracheids of certain New Zealand Forest Trees and their Significance in the Study of Coals.

[Read before the Wellington Philosophical Society, 28th August, 1929; received by Editor, 9th September, 1929; issued separately, 30th November, 1929.]

Plates 51–59.

The experiments outlined in this paper were made in the hope that they would throw some light upon the origin of the spiral markings so often noticed in the tracheids of wood fragments occurring in our lignitic coals.* The results obtained seemed definite enough to warrant their being placed briefly on record.

The woods used for most of the work were Agathis australis (kauri), and Podocarpus ferrugineus (miro); these being chosen because their bordered pits could be photographed fairly well in an untouched natural fracture. Similar results were also obtained with Dacrydium cupressinum (rimu), Podocarpus totara (totara), and Notofagus fusca (red beech).

Thin strips of wood (approx. 20 × 5 mm.) were split out along radial and tangential planes and their larger faces examined to see whether the pits were sufficiently well shown and free from debris. A short length of a satisfactory strip was mounted on a hard glass slip by means of a heat-resisting cement and then slowly stroked through the flame of a Méker burner, the surface under attack being examined and photographed at intervals until it had become completely charred. In a few cases the first stage of the oxidation was effected by means of the ordinary chromic-sulphuric-acid mixture at temperatures varying from 25° to 75° C. The photomicrographs were taken on orthochromatic process plates, using a small concave metallic mirror in the vertical-illuminator, so as to obtain a maximum of contrast with a minimum of haze. The process of oxidation, however, produced thin films which often rendered the whole surface so cloudy that no clear negatives could be obtained.

[Footnote] *Incidentally, the experiments recorded in this paper may perhaps be taken as affording some evidence bearing indirectly upon that remarkable “forest-fire hypothesis” which, in effect, postulates the protective action of a preliminary charring for all those vegetable tissues which even at the present day are still able to reveal their minute structural details to the worker examining the coals derived from them. They show that such a forest-fire, while necessarily obliterating all finer detail in the outer portions of any wood fragment subjected to it, need have no such effect upon the inner portions, and might well tend to preserve detail in the tissues under the surface layers. It is hard, however, to understand why the supporters of the hypothesis demand this particular form of protection when equally good detail may be observed in cases where such charring is quite out of the question; and still harder to understand how the protection assumed could possibly have been given so generally as regards either geographic distribution or geologic time.

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Little need be said of the few individual photographs reproduced as examples. They are mostly of kauri surfaces and it is hoped that they may be able to speak for themselves. Nearly all that the writer can say definitely about them will be found in the conclusions following. Attention, however, may be drawn to the fact that the action was often selective, a few pits being completely destroyed while their neighbours were left almost untouched, though all would appear to have been equally stroked by the flame.

Conclusions:

1. In all the woods examined, oxidation tends to split up the secondary walls of the tracheid into narrow bands arranged spirally about the longer axis of the tracheid, and, in many cases, the spirals so produced may easily be mistaken for tertiary thickenings. In the case of Agathis australis the spiral structure is so well marked that the various turns can actually be laid quite open by carefully controlled oxidation.

Great caution is therefore necessary when drawing conclusions as to the presence of tertiary thickenings in the woody fragments observed in coals, especially if these fragments be of coniferous origin.

In general, the spiral banding of the wood tracheids found in a coal is simply evidence that the secondary wall has been split up along the bounding lines of its original spirals, either by enzyme action, by prolonged exposure to moist air, or by fire.

2. The cellulosic (?) substances composing the bordered pits themselves and those portions of the tracheid wall in their immediate neighbourhood appear to be more resistant to the action of oxidizing agents than those which make up the remainder of the spiral bands in which the pits are situated. In many cases the pits (though perhaps in greatly altered form) remain supported apparently by the skeletons of the spirals on either side of them. This apparent persistance of the pit-frame may, of course, be due to the minute fibrils, which build up the spiral bands, being crowded together as they pass the pits: such crowding might well give greater resisting power.

3. In Agathis australis oxidation appears to begin near the edges of the compressed pit-funnels, the characteristic star-like appearance of the pits soon giving place to simple, more or less circular holes. Almost at the same time the remainder of the spiral containing the pits is attacked generally, the holes-that-were-pits being left in such positions as to lend further support to the view* that the secondary walls were laid down spirally.

[Footnote] *It should hardly be necessary to explain that this is not a novel view put forward by the writer. It has been held by nearly all botanists for many years past. All that can be claimed for the present paper is that it records a method of direct attack which, so far as the writer is aware, is new.

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Agathis australis, radial surfaces.
A (× 375).—A singly-pitted tracheid, very slightly oxidized. Note that the light has penetrated the surface sufficiently to show both pit-openings, thus giving the characteristic cross.
B (× 255).—Several strongly-pitted tracheids, slightly oxidized.
C (× 510).—A part of same specimen. The elevation of the pit borders is gradually becoming more accentuated.

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Prodccaipus ferrugincus, radial surfaces, showing early stages in the oxidation process.
A (× 230).—Very slightly oxidized; the tracheids are not much altered but the rays are being freely attacked.
B & C (× 460).—Early successive stages, the latter showing elevation of pits and also suggesting that the “bars of Sanio” are very definite folds possessing high resisting power.
D (× 230).—A late stage in the oxidation of A.

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(× 230) Agathis [ unclear: ] australis, radial surfaces, showing comparatively late stages. Note that while some tracheids still show pits clearly, others are being broken up and are losing the bands which contained the pits. The thin-walled rays have been almost completely destroyed.

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(× 230) Agathis australis, radial surfaces, showing comparatively late stages. Note that while some tracheids still show pits clearly, others are being broken up and are losing the bands which contained the pits. The thin-walled rays have been almost completely destroyed.

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Agathis australis, radial surfaces, showing late stages.
(A and B, × 115; C and D, × 230).—The upper funnels near the centre of D are intact while those in the lower tracheids have been destroyed.

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Agathis australis, radial surfaces, showing still later stages. (A and B, × 230; C, × 510).

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Agathis australis, tangential surfaces (× 230), showing how rapidly the ray cells are destroyed and the ray passages enlarged. In A alternating pitting is shown in the tangential wall of the tracheid.

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Agathis australis, radial surfaces, very slowly oxidized. (× 510). The spiral structure is very clearly shown in A, a singly-pitted tracheid. B and C originally showed double (alternating) and treble-pitting.

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Diagram Showing Some of the Changes Occurring During the Oxidation of The Pitted Tracheids of Agathis Australis.
N shows a conventional view of a normally-pitted tracheid wall. Parts 1 to 7 show changes in such a tracheid, the dark portions of the diagram representing those portions of N which are being attacked most slowly. (The sharp lines used in the diagram to separate the various parts of the bordered pits have no real existence. The comparatively sharp separation visible in a photomicrograph is due to the different positions, curvatures, and probably refractive indices of the parts).