Links With the Past in the Plant World

CHAPTER VII

THE ARAUCARIA FAMILY

'And so the grandeur of the Forest-tree
Comes not by casting in a formal mould,
But from its own divine vitality.'

Wordsworth.

As an additional illustration of existing cone-bearing trees which form links with the past we may briefly consider the genera Araucaria and Agathis, the two members of the family Araucarieae. It is generally agreed that the branches of the genealogical tree of this family extended farther back into the past than in the case of the majority of Conifers. By some authors the surviving representatives of the Araucarian stock are considered to have a strong claim to be regarded as the most primitive as well as the oldest of cone-bearing trees, though this opinion, like many others, is not held by botanists as a whole. This is not the place to discuss matters of controversy, and I shall confine myself to a general consideration of Araucaria and Agathis from the point of view of their present distribution and the part they played in the vegetation of the Mesozoic and Tertiary epochs. In 1741 a plant from Amboyna, one of the Moluccas, was described under the name Dammara alba. For this tree, known as the Amboyna Fine, the English botanist Salisbury instituted the generic name Agathis, from a Greek word (a?a???) meaning a ball of string and probably suggested by the form of the cones, which is the designation usually adopted in botanical literature instead of the pre-Linnean term Dammara. The best known species of the genus is the Kauri Pine, probably the finest forest tree in New Zealand where it still flourishes from the North Cape to latitude 38° S., though the occurrence of sub-fossil trunks and pieces of buried resin shows that the Kauri forests are gradually dwindling. The stems of this species, Agathis australis, rise like massive grey columns to a height of 160 ft., terminating in a succession of spreading branches given off in tiers from the main trunk. The thick narrow lanceolate leaves, with several parallel veins, reach a length of 2 to 3 inches. The female shoots have the form of small and almost spherical cones consisting of a central axis bearing overlapping spiral series of broadly triangular scales (Fig. 16). Each scale carries a single seed with a large wing attached to one side which facilitates disposal by wind. Other species of Agathis occur in the Malay Archipelago, the Philippines, in Queensland, in the New Hebrides, New Caledonia, the Fiji Islands, and elsewhere. With the exception of the Australian Kauri (Agathis robusta), with leaves larger and broader than those of the New Zealand Kauri, the genus is essentially an island type. With the exception of some species of the southern hemisphere genus Podocarpus, there are no Conifers with foliage like that of Agathis. It is, however, the broad and thin single-seeded scales and the spherical cones, in some species six inches in length, which furnish the most trustworthy means of identifying the genus.

Fig. 16. A. Agathis robusta Muell. (much reduced). B. Agathis Moorei Lind. (¹/₂ nat. size).]

Fig. 17. Araucaria excelsa. The upper part of a small tree in the Cambridge Botanic Garden. (Much reduced.)]

The allied genus Araucaria, with the exception of two South American species, the familiar Monkey Puzzle, Araucaria imbricata, and a Brazilian tree, Araucaria brasiliana, is confined within the geographical area occupied by Agathis. The name Araucaria was first used by de Jussieu in 1789 for a plant previously referred to the genus Pinus and described as one of the most beautiful trees of Chili. This species, A. imbricata, introduced into England in 1796, grows on the southern slopes of the Andes and, as in the case of the Kauri forests of New Zealand, buried stems point to a wider extension of the forests in earlier days. The sharp and thick leaves of the Monkey Puzzle distinguish it from all other Conifers; its large almost spherical seed-bearing cones, more than half a foot in length, which may occasionally be seen on well-grown British trees, are unlike those of other genera. Each of the deep and narrow scales bears a single seed embedded in the substance of the scale and terminates distally in a narrow upturned process. Some species of Araucaria, differing considerably in the form of the leaves and in the shape and structure of the seed-scales from the Chilian species, are conveniently placed in a distinct sub-division of the genus Araucaria. Of this type the Norfolk Island Pine, Araucaria excelsa, is the best-known example (Fig. 17). It was introduced to Kew by Sir Joseph Banks in 1793, soon after its discovery by Captain Cook, who describes the stems of the Norfolk Island trees as resembling basaltic columns, and relates how on approaching the island everyone was satisfied that the columnar objects were trees, 'except our Philosophers, who still maintained they were basaltes.' The leaves are short, about half an inch long, laterally compressed and slightly spreading and sickle-shaped—sometimes shorter and broader and overlapping—arranged in crowded spirals. The scales of the broadly oval cones are single-seeded, but differ from those of Araucaria imbricata in having the seed exposed on the surface and in the greater breadth and thinner borders of the scales. In both Araucaria and Agathis the nature of the seed-scales constitutes a distinguishing feature. The leaves of Araucaria imbricata differ in form from those of other Conifers. The foliage shoots of Araucaria excelsa and other species, e.g. the very closely allied A. Cookii of the New Hebrides and New Caledonia, though not unlike the branches of a Japanese Conifer (Cryptomeria japonica), often cultivated in England, afford fairly trustworthy characters for identification purposes.

The minute structure of the wood of both Araucaria and Agathis constitutes an important distinguishing feature and enables us to recognise on microscopical examination even a fragment of wood of either of these genera. The small elongated cells or water-conducting elements of the wood of the Araucarieae are characterised by one or two, and occasionally as many as three or four, contiguous rows of pits on their radial walls, and these appear in surface view as flattened circles or polygonal areas.

These details have been mentioned in order to show that Araucaria and Agathis are sufficiently distinct in many respects from other Conifers to render their identification in a fossil state comparatively easy, at least much easier than the recognition of the majority of the members of the Coniferae. It would be going too far to state definitely that Araucarieae, as defined by reference to existing species, existed during the Palaeozoic period; on the other hand it would seem in a high degree probable that the vegetation of the Coal age and of the succeeding Permian period included trees in which certain Araucarian characters were clearly foreshadowed. The name Araucarioxylon was formerly applied to petrified wood, obtained from Palaeozoic as well as from later formations, which agrees anatomically with that of Araucaria and Agathis. It has been shown in recent years that much of the Palaeozoic wood of this type of structure belongs to the extinct genus Cordaites, a tree which played a prominent part in the earlier floras. Cordaites affords a good example of a generalised type: in its wood-structure it resembles very closely the existing Araucarieae; its long strap-like leaves are not unlike those of some species of Agathis; its male flowers have often been compared with those of the Maiden Hair tree, Ginkgo biloba, and certain anatomical features form connecting links between this Palaeozoic genus and the Cycads.

It is noteworthy that in another Palaeozoic genus, Walchia, the leaf-bearing branches are identical in appearance with those of the Norfolk Island Pine (Fig. 17) and some other species of Araucaria. Unfortunately our knowledge of the reproductive organs of Walchia is insufficient to warrant any definite statement as to the degree of consanguinity between this Permian and Upper Carboniferous plant and the Araucarieae; it is probable that in Walchia we have a type not far removed from the line of evolution which led to Araucaria. Petrified wood, identified as that of Walchia, and exhibiting the Araucarian type of structure, has been recorded from Permian rocks of the Vosges. Other instances might be quoted in support of the view that the Palaeozoic floras included a few plants with which the surviving Araucarieae may fairly claim relationship. Professor Zeiller of Paris has recently described some fossil shoots from Palaeozoic rocks in India under the name Araucarites Oldhami on the ground of the similarity of the leaves to those of Araucaria imbricata. Similarly, from Triassic rocks several fossils have been described as closely allied to Araucaria, in some cases because of anatomical resemblances and in others on the less satisfactory evidence furnished by a similarity in the foliage shoots. Professor Jeffrey of Harvard has recently given an account of a new type of stem (Woodworthia) from the petrified Triassic forest of Arizona possessing some Araucarian characters, though differing from existing species of Araucaria in certain structural features, a combination of characters regarded by this Author as an indication of relationship with the family of Conifers, which includes the Pines, Firs, Larches and other well-known northern genera.

It is, however, from the records of Jurassic rocks that we obtain the most satisfactory information as to the great antiquity and the very wide geographical range of the ancestors of the recent genus. The plant-beds of the Yorkshire coast afford clear evidence of the occurrence of Araucarian trees in the woodlands of the Jurassic period. Petrified wood has been found at Whitby, associated with jet, showing the minute structural characteristics of the surviving species of Araucarieae, and it is not improbable that some at least of the Whitby jet has been formed from the wood of Araucarian plants. The carbonised remains of leafy shoots preserved in the Jurassic shales near Scarborough and on other parts of the Yorkshire coast include twigs hardly distinguishable from those of Araucaria excelsa, though the resemblance of external form alone, especially in the case of foliage shoots, does not amount to proof of generic identity. We have, however, the much more trustworthy evidence of cones and seed-bearing scales in which the characteristic features of living species are clearly shown. Seed-bearing scales almost identical with those of Araucaria excelsa and other recent species have long been known from the Jurassic rocks of Yorkshire.

From other parts of England where samples of Jurassic floras are preserved, as at Stonesfield in Oxfordshire, in Northamptonshire and elsewhere, equally striking examples of undoubted Araucarias have been found.

Fig. 18 represents part of a large cone described in 1866 by Mr Carruthers from Jurassic rocks at Bruton in Somersetshire: this specimen, now in the British Museum, consists of one side of a spherical cone about 5 inches long and 5 inches broad; in size, as in the form of the seed-scales, it shows a striking likeness to the cones of the Australian species Araucaria Bidwillii, the Bunya Bunya of Queensland. Other equally convincing examples of Jurassic Araucarian cones and seeds may be seen in the museums of York and Northampton. On the north-east coast of Sutherland there is a narrow strip of Jurassic beds forming a low platform between the granitic and Old Red Sandstone hills and the sea. From these rocks Hugh Miller described several fossil plants in his Testimony of the Rocks, and an examination of a large collection obtained from this district by the late Dr Marcus Gunn shows that Miller was justified in speaking of Araucaria as a member of this northern flora.

Fig. 18. Araucarites sphaerocarpus Carr. From Jurassic rocks at Bruton, Somersetshire. (British Museum, ²/₃ nat. size.)]

There is abundant evidence pointing to the existence in Britain during the Jurassic period, and in the early days of the Cretaceous epoch, of Araucarian trees which differed but slightly from the modern species confined to the southern hemisphere. In several localities in France, Germany, and other parts of the continent, Araucarian fossils have been recognised in Jurassic rocks. It is almost certain that some foliage shoots and imperfectly preserved cones described by Dr Nathorst from Upper Jurassic rocks in Spitzbergen were borne by a species of Araucaria. Cone-scales very similar to those from Yorkshire have been discovered in Wealden beds in Cape Colony, and Araucarian wood of Jurassic and Cretaceous age has been found in Madagascar. From Jurassic strata in India and Victoria (Australia), as well as from Upper Jurassic and Lower Cretaceous rocks in Virginia and elsewhere in the eastern United States, well preserved Araucarian fossils are recorded. In a collection of Jurassic plants, obtained a few years ago by the members of a Swedish Antarctic Expedition in Graham's Land, Dr Nathorst has recognised some cone-scales of Araucaria, which demonstrate a former extension of the family beyond the southern limits of South America.

It is interesting to find that when we ascend higher in the geological series and pass beyond the Wealden strata to the Middle and Upper sub-divisions of the Cretaceous period, evidence of the wide geographical distribution of the Araucarieae is still abundant. Araucarian wood has been obtained in rocks classed as Upper Cretaceous in Egypt, in East Africa, in Dakota, and elsewhere. In the sedimentary rocks of the Tertiary period undoubted examples of Araucaria are less common, though there can be no doubt that the genus was much more widely spread then than it is at the present day. The well-known Tertiary plant-beds of Bournemouth have afforded specimens of foliage shoots which have been described as a species of Araucaria, though in the absence of well-preserved cones or petrified wood we must admit that the data are inconclusive. It is, however, legitimate to regard the striking similarity of the Bournemouth twigs to those of Araucaria excelsa and A. Cookii as constituting a fairly strong case in favour of the persistence of Araucaria in Western Europe up to the earlier stage of the Tertiary period. Araucarian wood of Tertiary age is recorded from India, while branches with broad leaves like those of Araucaria imbricata have been found in Seymour Island and the Magellan Straits, and specimens of Tertiary wood are described from Patagonia. At the other end of the world. Tertiary rocks on the west coast of Greenland have yielded fragments which may be referred with some hesitation to the genus Araucaria.

A few words must be added in regard to the recent discovery by Professor Jeffrey and Dr Hollick of some very interesting Cretaceous specimens in New Jersey of well-preserved cone-scales and foliage shoots of extinct plants closely related to the existing species of Agathis(51). The American fossils are particularly valuable because their preservation admits of microscopical examination of the tissues. In Cretaceous rocks of Staten Island and in other localities on the eastern border of the northern United States, kite-shaped seed-bearing scales almost identical in form with those of recent species of Agathis are fairly common fossils. Similar specimens have long been known from Tertiary rocks in western Greenland. In the case of some of the American examples each scale bore three seeds instead of a single seed in living species: on account of this difference Prof. Jeffrey and Dr Hollick have adopted a distinct generic name, Protodammara.

The foregoing sketch is necessarily far from complete, but it may serve as an illustration of the light which is thrown on the past history of recent plants by the investigation of the relics of ancient floras. The family Araucarieae now represented by a small number of species which, with the exception of the Andian and Brazilian Araucarias, are restricted to a small region in the southern hemisphere, was one of the most widely spread sections of the seed-bearing plants during the Mesozoic era. Ancestors of Araucaria must have been common trees in the European vegetation in Jurassic and Lower Cretaceous periods, and even as late as the Tertiary period there is evidence that representatives of the family still lingered in the north. One conclusion which seems almost unavoidable is that the species of Araucaria and Agathis that survive, in some cases only in one or two small islands in the South Pacific, have in the course of successive ages wandered from the other end of the world. Their migrations can be partially traced by the fragments embedded in Jurassic and later sediments, but we can only speculate as to the causes which have contributed to the changes in the fortunes of the family; how much influence may have been exerted by changes in physical conditions in the environment, and to what extent the production of more successful types may have been the dominant cause of the decline, it is impossible to say. One thing at least is certain, that few existing plants are better entitled to veneration as survivals from the past than are the living species of Araucaria.

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