The Oak (Ward)/Chapter XII
CHAPTER XII.
RELATIONSHIPS OF THE OAKS—THEIR DISTRIBUTION IN SPACE AND TIME.
The oak is a member of a very large and ancient group of dicotyledonous flowering plants, embracing the beeches, chestnuts, hazel-nuts, etc., and many other forest trees of the Northern Hemisphere.
The number of species of oaks (Quercus) is very large, probably more than 300, of which the majority belong to North America, Europe, China, Japan, and other parts of Asia. There are none in Africa south of the Mediterranean region, nor in South America orA ustralasia. Some remarkable species are found in the Himalayas, and many in the Malayan Archipelago.
The various species of the genus Quercus are arranged into groups according to differences in the form and arrangement of the scales of the cupule, the characters of the leaves, and certain peculiarities in the acorns. Many oaks, especially those of warm countries, for instance, are "evergreen," with hard, leathery leaves, quite unlike the leaves of our common British oak.
The latter is denominated botanically as Quercus Robur, but certain varietal forms of it have been distinguished, of which the commonest in this country are Q. pedunculata, a variety with the female flowers on long peduncles, and Q. sessiliflora, with the female flowers on short peduncles; but although numerous attempts have been made to define these forms, and while small differences in the petioles, lobing of the leaves, and the wood, etc., have been insisted upon at various times by observers, it appears that the two varieties graduate into one another by intermediate forms. In England, the vaxiety pedunculata is the commonest over the country generally, but in the hilly districts of North Wales and the north of England the variety sessiliflora is said to prevail. Similarly, on the Continent the latter variety is found at higher elevations than the former, though its area of occurrence is more restricted. This pronounced variability of the oak was commented upon by the late Charles Darwin, who points out, in the Origin of Species, that more than a dozen species have been made by a certain author out of what other botanists regard as mere varieties of the common oak.
De Candolle, who made a special study of this group, found the variations so enormous that, although he made something like 300 species, he decided that the majority of these were merely provisional; and he concluded, as others have done, that we have, in the numerous varieties of the species of this old genus Quercus, series of incipient species. If the connecting forms were to die out, leaving certain varieties more isolated than they are at present, systematists would elevate the latter to the rank of species.
It is interesting to observe that twenty-eight varieties of the common English oak (Q. Rohur) have been described, and that the majority of these can be grouped around the three forms pedunculata, sessiliflora, and pubescens, the latter being a somewhat hairy variety found on the Continent. No doubt we have here, again, a case where the three varieties mentioned would be accorded specific rank if the connecting forms died out, as some of them appear to be doing.
I have already stated that the oaks are a very ancient family, and their great variability is in accordance with this. It probably implies that the genus has had time during its migrations over the Northern Hemisphere to vary immensely, and that some of the varieties have become adapted to given situations, others to others. On the whole, the oak family must be regarded as a northern type which has sent extensions southward.
Now let us glance at their geological history. Something like 200 forms of fossil oaks have been described from remains, chiefly of leaves and wood, found in various parts of the world. Some of the European fossil forms remind us of species now found only in hot countries near the tropics, others are peculiar, and some are very doubtful.
The earliest remains of oaks come from the Cretaceous strata, being coeval with the first undoubted dicotyledons that have been found. Many have been found in the Tertiary also, and we have to conclude that the oaks were probably already a well-developed group of plants before the higher mammalia existed—i.e., so far as we can judge from the fragmentary records of the rocks. It seems that even the present species of oaks were already in existence in Tertiary times, and possibly some of their varieties also.
From the evidence of their fossil remains, together with the facts of their present distribution, it is at least exceedingly probable that the European oaks, including our English oak, came into existence somewhere in the East, and that, after spreading from Asia towards the West, they are now slowly retreating before competing forms—e.g., the beech. Meanwhile the English oak (Q. Robur) has been giving rise to several varieties, of which three at least (viz., pedunculata, sessiliflora, and pubescens) have become sufficiently marked to be regarded as species by those who do not consider the connecting forms.
It is not improbable that this migration of the European oaks from Asia was completed before the islands of Sicily, Sardinia, Corsica, and Britain were separated from the mainland of the Continent. Moreover, our English oak is not distantly related to certain species of Eastern Asia and of Western North America, and it has been surmised that all these related forms sprang from a common ancestor not unlike our English oak of to-day. Again, fossil leaves from Italy, found in diluvial deposits, are so like those of certain Californian oaks now existing that a common origin is also suggested, and similar leaves have been discovered in Tertiary deposits in Northwest America. If all the evidence is put together, we may conclude with Asa Gray that " the probable genealogy of Q. Robur, traceable in Europe up to the commencement of the present epoch, looks eastward and far into the past on far-distant shores."
Many of the oaks yield products which are made use of in the arts, apart from their timber, the most valuable of which comes from our European oak, the white oaks of North America, and one or two Himalayan species. In several countries oaks are grown for the sake of the bark, cups, etc., as a tanning material, and these even form important articles of export. Quer-citron, a yellow dye and tanning material, is obtained from Q. tinctoria in North America.
Cork, as used for bottling and other purposes, is obtained in Spain, the south of France, and in Algiers, from the thick periderm of Q. Suber.
Q. infectoria yields the chief galls of commerce. They are caused by the punctures of Cynips gallæ tinctoriæ, and are used for making ink and for dyeing. In these and similar galls the value depends on the presence of relatively large quantities of tannic and gallic acids which they contain.