THE REDWOOD AND MAMMOTH TREES OF CALIFORNIA

Since their introduction into England about the middle of the nineteenth century, the two Californian species Sequoia sempervirens (the Redwood) and Sequoia gigantea (the Mammoth tree) have become familiar as cultivated trees. The name Sequoia, said to be taken from Sequoiah, the inventor of the Cherokee alphabet, was instituted in 1847, while the name Wellingtonia, often used in horticulture though discarded by botanists in favour of the older designation Sequoia, was proposed in 1853. Both species are now confined to a comparatively small area in California: their restricted geographical range, considered as an isolated fact, might be regarded as a sign of recent origin. The records of the rocks, however, afford ample proof that rarity in this as in many other instances is the precursor of extinction. The famous groves of Mariposa and Calaveras represent the last resting-place of giant survivors of a race which formerly held its own in Europe and in other parts of the world.

The Redwood, Sequoia sempervirens, occupies a narrow belt of country, rarely more than 20 or 30 miles from the coast, three hundred miles long from Monterey in the south to the frontiers of Oregon; it has a stronger hold on existence than Sequoia gigantea. In Northern California it still forms pure forests on the sides of ravines and on the banks of streams. The tapering trunk, rising from a broad base to over 300 ft., gives off short horizontal branches thickly set with narrow spirally disposed leaves ¹/₄-¹/₂ inch in length arranged in two ranks like the similar leaves of the Yew. The lower edge of each leaf is decurrent, that is it runs a short distance down the axis of the branch instead of terminating at the point of attachment. It is by paying attention to such details as this as well as to more important features, that we are able to connect fragmentary fossil twigs with those of existing species. The female 'flowers' have the form of oblong cones from ³/₄ to 1 inch long: each consists of a central axis bearing crowded wedge-shaped, woody appendages or cone-scales, which gradually increase in breadth towards the exposed distal end characterised by its four sloping sides and by a median transverse groove. Several small seeds are borne on the upper surface of the cone-scales. The smaller and short-lived male flowers need not be described.

The other and better known species Sequoia gigantea (Fig. 13) has an even more restricted range and is confined to groves on the western slopes of the Sierra Nevada between 3000 to 9000 ft. above sea-level.

This tree is at once distinguished from the Redwood by its ovate, sharply pointed and stiffer leaves which retain their spiral disposition and closely surround the axis of the twigs like obliquely-set needles. The cones are of the same type as those of Sequoia sempervirens, but are broader and may attain a length of 3¹/₂ inches (9·5 cm.) (Fig. 14).

Reference has already been made to Sequoia as a striking illustration of longevity. It is also selected as an equally impressive example of a type verging on extinction, which played a prominent part in the vegetation of both west and east during the Cretaceous and Tertiary periods.

Scraps of branches with leaves hardly distinguishable from those of the existing Californian trees are frequently met with in Tertiary and Mesozoic sediments, and with them occasionally occur cones too imperfectly preserved to afford satisfactory evidence of more than superficial agreement with those of the recent species. The task of deciphering the past history of plants, particularly of the Conifers, is accompanied by many difficulties and insidious temptations. It is clear from a critical examination of many of the recorded instances of fossil Sequoias that the generic name has been frequently used by writers without adequate grounds. The fragmentary specimens available to the botanical historian cannot as a rule be subjected to microscopical investigation, and even a partial acquaintance with the similarity of the foliage of different types of living Conifers is sufficient to convince the student of the need of self-control in the identification of the fossils. It is, however, easy to point out obvious pitfalls, though difficult to maintain a judicial attitude in the excitement of endeavouring to interpret documents which are too incomplete to be identified with certainty. If we put on one side all records of supposed fossil Sequoias not based on satisfactory data, there remains a wealth of material testifying to the antiquity of the surviving species.

It is by no means improbable that Conifers closely allied to the Redwoods and Mammoth trees of California were represented in Jurassic floras; but hitherto no proof has been obtained of the occurrence of a Sequoia among the rich material afforded by the Jurassic plant-beds of Yorkshire and by beds of the same age in other countries. A small cone has recently been described from strata near Boulogne belonging to the latest phase of the Jurassic period, which presents a strong resemblance in shape and size and in the form of the cone-scales to those of the recent species. This specimen, though not conclusive, is the most satisfactory indication of a Jurassic Sequoia so far discovered. From Lower Jurassic rocks in Madagascar similar cones have been recorded in association with foliage-shoots like those of Sequoia gigantea, but here too the evidence is not beyond suspicion. In plant-bearing strata of Wealden age, such as are exposed in the cliff near Hastings and in deposits of the same age in North Germany, Portugal, and elsewhere, twigs and cones have been found which may be those of trees nearly allied to the genus Sequoia.

It is, however, in the sedimentary rocks of Cretaceous age, rather higher in the series than those in the Hastings cliffs, and in the succeeding Tertiary rocks, that undoubted Sequoias are met with in abundance. At Bovey Tracey in Devonshire there is a basin-shaped depression in the granitic rocks of Dartmoor filled with clay, gravel and sand—the flood-deposits of a Tertiary lake containing waifs and strays of the vegetation on the surrounding hills. Among the commonest plants is one to which the late Oswald Heer gave the name Sequoia Couttsiae, and his reference of the specimens to the genus Sequoia has been confirmed by the recent researches of Air and Mrs Clement Reid(50). This Tertiary (Oligocene) species is represented by slender twigs almost identical with those of Sequoia gigantea and by well-preserved cone-scales and seeds (Fig. 15). Moreover, it has been possible to examine microscopically the structure of the carbonised outer skin of the leaves and to demonstrate its agreement with that of the superficial tissue in the leaves of the Mammoth tree. With the Bovey Tracey Sequoia are associated fragments of Magnolia, Vitis, and Taxodium distichum, the swamp Cypress of North America, together with other types which have long ceased to exist in Western Europe. Other British examples of Sequoia have been described from Tertiary beds at Bournemouth, the Isle of Wight, Sheppey, and Antrim, but the material from these localities is inferior in preservation and cannot be identified with the same degree of certainty as in the case of the Devonshire specimens. The occurrence of twigs and cones of several species of Sequoia in both Cretaceous and Tertiary rocks in Austria, Germany, Italy, France, and elsewhere, shows that the ancestors of the Californian trees were common in the European region.

The exploration of Cretaceous and Tertiary rocks in Arctic Europe has revealed the former existence in Greenland, Spitzbergen, and other more or less ice-covered lands of plants which clearly denote a mild climate. Cones and branches of Sequoias have been found in abundance in Lower Tertiary beds on Disco Island off the west coast of Greenland, and similar evidence of the northern extension of the genus has been obtained from Spitzbergen. Dr Nathorst of Stockholm speaks of twigs of Sequoia in the Tertiary clays of Ellesmere Land almost as perfect as herbarium specimens. In Tertiary beds on the banks of the Mackenzie River, in Alaska, Saghalien Island and Vancouver Island, and in Upper Cretaceous rocks in the Queen Charlotte Islands, remains of Sequoia have been discovered. One of the most remarkable instances of the preservation of trees of a bygone age is supplied by the volcanic deposits of Lower Tertiary age exposed on the slopes of Amethyst mountain in the Yellowstone Park district. At different levels in the volcanic and sedimentary material, which is piled up to a height of over 2000 ft. above the valley, as many as fifteen forests are represented by erect and prostrate limbs of petrified trees (Fig. 6). The microscopical examination of some of these trees has shown that they bear a striking resemblance to Sequoia sempervirens. In a photograph of these petrified forests by the U.S. Geological Survey (36, 2) one sees living Conifers side by side with the lichen-covered and weathered trunks of the fossil species (Sequoia magnifica), living and extinct being at a distance hardly distinguishable. (Frontispiece.)

In concluding this brief survey of the fossil records of Sequoia, reference may be made to the discovery of petrified wood in Cretaceous rocks in South Nevada, possessing the anatomical features of Sequoia gigantea, which shows that close to the present home of the big trees their ancestors flourished during a period of the earth's history too remote to be measured by human reckoning.

The distribution of the Tertiary and Cretaceous Sequoias would appear to have been mainly in the northern hemisphere, extending well within the Arctic circle. It is, however, by no means improbable that the ancestors of Sequoia flourished far south of the equator. Reference has been made to Jurassic fossils from Madagascar which have been compared with the existing species, and from Lower Tertiary beds in New Zealand the late Baron Ettingshausen described some cones and twigs as Sequoia novae zeelandicae which bear a close resemblance to the existing type. The available evidence would seem to point to a northern origin of the genus, though allowance must be made for erroneous conclusions based on negative evidence. Further research may well extend the past distribution of Sequoia in southern lands, but the data to hand point to the conclusion that the Californian trees represent the survivors of a type which flourished in the Cretaceous and Tertiary periods over a wide area in North America and in what we now call the Continent of Europe.