INTRODUCTORY: THE LONGEVITY OF TREES, ETC.

'Believe me who have tried. Thou wilt find something more in woods than in books. Trees and rocks will teach what thou canst not hear from a master.' St Bernard.

The recent publication in the daily press of instances of human longevity under the heading 'Links with the Past' prompted a comparison between the length of time represented by the duration of a tree and the lifetime of a human being. The comparison of single lives suggested the further step of contrasting the antiquity of the oldest family-histories with the remoteness of the period to which it is possible to trace the ancestry of existing members of the plant kingdom.

My primary object in these pages is not to deal with familiar cases of longevity in trees, but to consider in the first place some of the problems connected with the origin of the present British flora, and then to describe a few examples of different types of plants whose ancestors flourished during periods of the earth's history long ages before the advent of the human race.

In dealing with plants of former ages we are confronted with the difficulty of forming an adequate conception of the length of time embraced by geological periods in comparison with the duration of the historic era. Some of the 'Selections from the Greek Papyri' recently edited by Dr Milligan (Cambridge 1910) refer to common-place events in terms familiar to us in modern letters: we forget the interval of 2000 years which has elapsed since they were written. Similarly, the close agreement between existing plants and species which lived in remote epochs speaks of continuity through the ages, and bridges across an extent of time too great to be expressed by ordinary standards of measurement. Terms of years when extended beyond the limits to which our minds are accustomed cease to have any definite meaning. While there is a certain academic interest in discussions as to the age of the earth as expressed in years, we are utterly unable to realise the significance of the chronology employed. After speaking of the futility of attempting to introduce chronological precision into periods so recent as those which come into the purview of archaeologists, Mr Rice Holmes suggests a method better adapted to our powers. He says—'Ascend the hill on which stands Dover Castle, and gaze upon Cape Grisnez, let the waters beneath you disappear; across the chalk that once spanned the channel like a bridge men walked from the white cliff that marks the horizon to where you stand. No arithmetical chronology can spur the imagination to flights like these(1).' On the other hand, the use in some country districts in Britain of spindles almost identical with instruments used in spinning by the ancient Egyptians, and similar survivals described by the author of a book entitled The Past in the Present(2), bring within the range of our vision an early phase of the historic era. The rude implements still fashioned by the flint-knappers of Brandon in Suffolk connect the present with the Palaeolithic age. Measured from the standpoint of historic reckoning, survivals from prehistoric days appeal to us as persistent types which have remained unchanged in a constantly changing world.

In one of his essays Weismann quotes an old German saying with regard to comparative longevity, which asserts that 'a wren lives three years, a dog three times as long as a wren' and so on in a regularly ascending series: the life of a deer is estimated at three times that of a crow and an oak three times that of a deer, which means that, computed on this basis, an oak lives nearly 20,000 years(3)! This fanciful illustration of the relative longevity of an oak is the expression of a truth, namely the superiority of trees over animals in regard to the duration of life. As a seventeenth-century translator of Pliny's Natural History writes, 'In old times trees were the very temples of the gods: and according to that antient manner, the plaine and simple peasants of the country, savouring still of antiquity, do at this day consecrate to one God or other, the goodliest and fairest trees that they can meet withal.' Oaks growing in Pliny's day in the Hercynian forest are said to have been there 'ever since the creation of the world(4).' Sir Joseph Hooker, in an account of some Palestine oaks, gives a drawing of a famous tree at Mamre, known as Abraham's Oak, which is supposed to mark the spot where the Patriarch pitched his tent(5). Examples such as these, though of no scientific value, serve to illustrate the well-founded belief in the extraordinary longevity of trees. In the absence of evidence to the contrary, it would be rash to deny the possibility that William the Conqueror's Oak in Windsor Forest, described by Loudon in his Arboretum Britannicum and mentioned by later writers, may be a survival from the reign of the king whose name it bears. Although it is seldom possible to state with confidence the exact age of old oaks and yews famed for length of days, there can be no doubt as to the enormous antiquity of many of our trees whose years are 'sacred with many a mystery.' The section of a trunk of one of the mammoth trees of California (Sequoia gigantea) exhibited in the Natural History department of the British Museum, shows on its polished surface 1335 concentric rings denoting successive increments of wood produced by the activity of a cylinder of cells situated between the hard woody tissue and the bark. It is generally assumed that each year a tree produces a single ring, though, as is well known, an estimate of age calculated on this assumption cannot be regarded as more than an approximation to the truth. If this giant tree, which was felled in 1890, was then 1335 years old, it had already reached an age of over two centuries when Charlemagne was crowned Emperor at Rome. The concentric rings on a tree trunk owe their existence to certain structural differences between the wood formed in the spring and in the late summer. In Sequoia, as in other members of the great class of cone-bearing trees, the wood is composed of comparatively narrow elements which serve to carry water from the roots to the branches and leaves. As spring succeeds winter the inactivity of the plant-machine is followed by a period of energetic life; opening buds and elongating shoots create a demand for a plentiful supply of ascending sap, and in response to this the tree produces a fresh cylinder of wood composed of relatively wide conducting tubes. After the first rush of life is succeeded by a phase of more uniform and gentler activity, the demand for water becomes less exacting and the wood which is formed during the rest of the growing season consists of narrower water-pipes. A period of rest ensues, until in the following spring new layers of larger tubes are laid down in juxtaposition to the narrower elements of the latest phase of the preceding summer. This alternation of larger and smaller tubes produces the appearance of concentric rings on a cross-section of a tree. It is not the pause in the active life of the plant which is responsible for the effect of rings, but the fact that the wood produced immediately before and immediately after the pause is not structurally identical. In trees grown under the more uniform conditions of certain tropical regions, the annual rings are either feebly developed or absent; for example, in some Indian oaks the wood shows no concentric rings of growth.

Stated in general terms, rings of growth in the wood of a tree are the expression of a power possessed by the plant of regulating the structure of its component elements in response to the varying nature of the external stimuli. In certain circumstances, for example after the destruction of the young buds by caterpillars, the tree makes a special effort to repair the loss by producing a new set of shoots. This may be recorded by the occurrence of two concentric rings in one season. An extreme instance of departure from the normal has recently been described(6) in which a tree of Theobroma cacao (the cocoa tree), planted in Ceylon in the summer of 1893 and felled in January 1901, after a life of just over 7 years, was found to have 22 rings in its stem. In this case the tree shed its leaves three times a year, and each break in the uniformity of its vital activities was registered by the apposition of what under ordinary conditions are spoken of as spring and late-summer wood. At Aden trees stated by natives to be very old showed only five or six rings of wood, a fact connected with the almost complete lack of rain and with the uniform conditions of existence.

The degree of accuracy to be allowed to estimates of age founded on the number of 'annual' rings is, however, of secondary importance in comparison with the enormously greater hold on life possessed by trees as contrasted with the higher animals. Early in the nineteenth century the Swiss botanist A. P. de Candolle expressed the opinion that trees do not die from senile decay, but only as the result of injury or disease. Trees are constructed on a plan fundamentally different from that underlying the structure of the highly complex human organism, and are thus endowed with a sort of potential immortality. It has been suggested that some of the large corals in the Red Sea which are still tenanted by living polyps may have been growing in the days of the Pharaohs. The coral polyp represents the growing portion of a lifeless mass of rock which is constantly extended by the activity of the organism at the summit of each branch. Between a coral-reef and a tree there are many essential differences, but a rough analogy may be recognised. A tree, unlike the higher animals, does not reach a stage at which the whole of its substance attains a condition of permanence and fixity. It consists of a complex branching-system in which each shoot increases in length by virtue of the youthful vigour of its apex: to a large extent the tree as a whole consists of lifeless material incapable of further growth, as is the case of the older portions of a coral-reef; but the regular increase in girth of the trunk and its branches demonstrates that this comparison is only partially true, and that the power of growth in a tree is not confined to the extremities of the youngest shoots. The tip of every twig is composed of minute cells endowed with a potentiality of development like that which characterises the embryonic tissues of a seedling just emerged from the seed. In the course of its growth, each branch, by means of its living and dividing cells, contributes to the several parts of the complex mechanism of the tree. While the greater number of cells acquire a permanent form and lose the power of further development, there remains a cylinder of cells endowed with perpetual youth. This cylinder of living cells, known as the cambium, extends between the wood and bark from one end of the tree to the other: by its periodic activity it adds new layers of tissue each year and thus, by increasing the amount of conducting tubes for the transport of water and for the distribution of elaborated food, it enables the tree to respond to the increasing demands which are the necessary accompaniment of increasing size. It has already been pointed out that in the spring when the sap flows most vigorously the cambial cylinder produces larger tubes, and afterwards when the tree settles down to its normal life, these are succeeded by narrower and stronger tubes. These later formed elements serve also an important mechanical purpose; by the strength of their walls they increase the supporting power of the tree and enable it to sustain the added burden of the annual increase in the weight and extent of its spreading branches.

It is the persistence of permanently juvenile tissue in certain regions of a tree, together with the remarkable power of repairing injuries and shedding effete parts, that constitute some of the most striking contrasts between the higher animals and plants. The embryo oak in the earlier stages of development consists entirely of actively growing cells; by degrees differentiation of the embryonic tissues results in the localisation of regions of cell-production at the tips of the elongating stem and root. These apical groups of cells are, in fact, portions of the embryonic organism which persist so long as the plant lives. This continuity between the growing tip of an old oak stem and the cells of the undifferentiated embryo affords one of the most remarkable examples in nature of a link between the past and the present.

If we pass beyond the stretch of time represented by the life of a single tree, a few successive generations suffice to carry our retrospect back to the days when forests of oaks, birches, and other trees impeded the progress of the Roman invaders, and, a stage farther back, to the age of Neolithic man whose remains are occasionally found in our heaths and moors and in the submerged forests round our coast. The blocks of oak and beech, some of which are as sound as when first felled, recently discovered below the foundations of parts of Winchester Cathedral constructed at the end of the twelfth or in the opening years of the thirteenth century, are relics of Norman forests. In the course of some excavations at Brigg in Lincolnshire in 1886 a dug-out boat was found nearly 50 feet long and from 4 to 5 feet in breadth. The stem of the oak from which the canoe had been fashioned shows no sign of branching for a length of over 40 feet, a fact which points to the growth of the tree in a forest where the race for light induced the development of clean columnar stems. The Brigg 'dug-out,' now in the Hull Museum, was discovered in an old alluvial valley of the Ancholme river, formerly connected with the Humber, and it may be that it was used by Neolithic man as a ferry for river-service(7).

From the period claimed by archaeologists we pass by gradual stages into the domain of the geologist. As Huxley wrote, 'when even the dim light of Archaeology fades, there yet remains Palaeontology, which... has brought to daylight once more the exuvia of ancient populations, whose world was not our world, who have been buried in river beds immemorially dry, or carried by the rush of waters into caves, inaccessible to inundation since the dawn of tradition(8).' The length of time represented by a succession of long-lived individuals of the same species becomes enormously extended when we pass to the history of families, and disinter from the sediments of other ages the remains of extinct types. As we descend the geological series familiar types gradually disappear, and through a succession of changing floras we penetrate to the fragmentary records contained in the older rocks until the absence of documents sets a limit to our quest.

The Scots pine shares with the oak, the beech, the aspen, the yew, and several other trees the right to be included in the native flora of Britain. In the peat-beds of Scotland even up to 3000 feet above sea-level the stumps of pines occur in abundance, and in many places recent researches have revealed the occurrence of successive forests of pines, oaks, and spruces separated from one another by the accumulations of swampy vegetation(9). The spruce fir has long ceased to be a member of the British flora, but in a few localities in the Scottish Highlands patches of primeval pine forests remain. The accompanying photograph (Fig. 1), taken by my friend Mr A. G. Tansley, in the Black Wood of Rannoch in north-west Perthshire, shows a few trees of Pinus sylvestris growing in their native soil: the form of the older tree (A) suggests comparison with that of a well-grown beech such as we are familiar with in English plantations. This spreading dome-shaped habit seems to be a peculiarity of the Highland tree, and is one of the characters which have led some botanists to regard it as a variety (Pinus sylvestris var. scotica) of the ordinary Scots pine. Though it is doubtful if any relics of primeval pine woods are left in England, abundant evidence of the former existence of the Scots pine is afforded by the submerged forests exposed at low-tide on many parts of the English and Welsh coasts and at the base of some of the English peat moors. During the construction of the Barry docks on the north coast of the Bristol Channel a few years ago, the exposed sections of peat and forest beds were investigated by Dr Strahan and by Mr Clement Reid. There is evidence of a subsidence of the land to an extent of 55 feet since the formation of the lower peat-beds containing oak, hazel, willow, and other trees. The pine, unknown in Wales during the historic period, was recognised in the Barry cutting. The occurrence of a polished flint implement assigns a date to the uppermost portion of this old land-surface(10).

It is impossible within the limits of a small volume to discuss in detail the evidence furnished by the records of the rocks as to the relative antiquity of the different constituents of the present vegetation of Britain. In later chapters a few selected plants are described which are pre-eminently ancient types. Before passing to the consideration of the data on which the geological history of plants is based, brief reference may be made to one of the most interesting and difficult problems of botanical research, namely the history of the British flora.