The CÔte d’Azur whither many of my readers will be travelling—in thought, if not in reality—about Easter time, is well named the Land of Azure Blue, for it is the blueness of the sea, of the sky, and of the distant rocks and mountains, as well as much of the vegetation, which is when the sun shines, its special charm. And although one has some wet and some cloudy days, yet the sun does shine there with a strength and brilliancy not to be enjoyed in the early part of the year on the Atlantic and North Sea coast. This tract of country, more commonly known to English people as the Riviera, has very special meteorological conditions owing to its position as the narrow strip of shore-line existing between the vast mass of the Western Alps and the Mediterranean Sea. It is warmed by the sea, and lies too close under the mountains to be caught by any winds from the north, and at many points is also effectively protected from both east and west winds by rocky spurs of the great mountain chain.

The Riviera is a constant source of delight to those who love flowers and beautiful vegetation of all kinds. But few of its visitors appreciate the fact that it is really from end to end one big garden, cultivated for ages by its inhabitants, and full of plants introduced by man which at present seem at first sight to be characteristic natives of it, but are, in reality, quite distinct from its primitive vegetation. This primitive vegetation is now represented only in what is locally called the “maquis”—what we should, perhaps, term the “scrub” or “bush” in English. It comprises some pines, the juniper, the lovely rock roses, balsams, rosemary, the giant heath (bruyÈre), from which our briar-root pipes are made, the larger thyme, the myrtle, the rose of Provence, two kinds of lavender, and many aromatic plants with grey hairy leaves, and often provided with sharp thorns as additional defences against browsing goats. The delicious perfumes of these hardy inhabitants of the dry, rocky grounds, where little or no grass can flourish, are developed by them as a protection against browsing animals, who cannot tolerate much of these pungent volatile oils, although mankind extracts them and uses them in the manufacture of such scents as eau-de-Cologne and also in cookery.

Many a visitor to the Riviera never strays from the cultivated fields and roadways into this scrub-land. The olive tree, which forms so prominent and beautiful a feature in the panorama of gardens which unrolls itself as we steam or drive along the coast from Toulon to Mentone and from Mentone to Genoa and Spezzia, is not a native plant; it was introduced in prehistoric times, and has been again and again re-established by emigrants from Italy; but it was brought to Italy from the East. It is astonishing how many of the cultivated trees of the Riviera have the same kind of history—the vine came from India in prehistoric times, the fig tree more recently from Persia, the lemon from India, the orange and the peach tree from China. All of them were introduced in very ancient times to the eastern parts of the Mediterranean basin, and so gradually were carried to the shores of the Ligurian sea, and would die out here were they not to a certain extent under the care of ownership.

The so-called “mimosa,” so abundant here, with its pretty, sweet-scented, yellow blossom, is an Australian acacia, only introduced some sixty years ago; whilst the eucalyptus—a most picturesque and effective addition to the landscape—is a still later introduction from Australia. The cypress, that darkest and most shapely of conifers, long lines of which proclaim to the traveller as he passes Avignon his arrival in the true “South,” is not a native of these parts, although it flourishes in suitable situations. It was introduced in mediÆval times from the East. So, too, the palms, though some have been cultivated for centuries, have been largely imported from extra European localities in the last century. There is a native European palm. It is a kind of fan-palm, and grows here. I have gathered it in Sicily. It does not “rear its stately head” more than a foot from the ground, and is known to botanists as ChamÆrops humilis. The gigantic Mexican agave and the prickly-pear cactus were introduced in the seventeenth century from the New World, though, according to Sir Herbert Tree’s scenery, they were growing at Cape Miseno in the time of Antony and Cleopatra! Bamboos of many kinds have been introduced here from the Far East, and flourish exceedingly.

The orange tree was brought from India (whither it was carried from China) and established in Southern Europe in mediÆval times, though known to the ancient Greeks and Romans. There are as many as 120 different varieties of the orange tree now cultivated on the shores of the Mediterranean, including, besides those which are valued for their sweet juicy pulp, those which furnish bergamot oil and similar aromatic products. The “issue pea” of old apothecaries, which was bound into a cut made in a patient’s flesh for the purpose of producing inflammation and suppuration, with the notion that such treatment was beneficial, was a minute unripe orange dried, and, no doubt, to some extent, antiseptic.

Besides the introduced trees, we find, in ground which has been more or less under cultivation, and not, therefore, of the nature of the “maquis,” or scrub-land, some beautiful plants, such as the narcissus, iris, and various lilies. One very small and graceful tulip is, I believe, regarded as native to the soil, but a magnificent crimson tulip, as large as the varieties cultivated in English gardens, which I have found abundantly in open park-like land under olive trees at Antibes, is said to have been introduced from Persia in the Middle Ages, and to have taken kindly to the Riviera. It is the Tulipa oculus solis. In the same locality were growing many brilliantly coloured “stellate” anemones.

There is, of course, a third group or “lot” of plants on the Riviera, which consists of those brought from all parts of the world during the past century, and regularly cultivated and cared for in gardens. The climate of the Riviera enables the gardener to grow all sorts of sub-tropical plants in the open air, and a long list of them could be given. The wonderfully brilliant crimson creeper, Bougainvillia, covers walls by the roadways, and even the railway stations, with its rich colour at this season. A delightful book by the distinguished botanist, Professor Strasburger, describing and picturing in colours many of the cultivated as well as the wild plants of the Riviera, has lately been published (in English) at a small price.

The animals which come under the notice of those who go in search of spring sunshine to the Riviera are far less numerous than the plants. But there is one which is dear to all, although it makes such a noise for an hour or so about sunset that some people are irritated or even alarmed by it. This is the little green tree-frog, Fig. 1, which now comes forth from its winter sleep, and assembles in thousands—guided by the “croak” or “call” which is produced by the males. The females have a very small voice comparatively. I kept two—a male and female—through a winter in London, and when the spring came the male terrified the household one night by unexpectedly uttering his cry—loud and sharp—to which the female replied. “Wharr! biz” is the nearest expression I can give in letters to the two sounds. After a great many evenings spent in these rhythmical declamations, the little frogs collect round pools and tanks, and at last drop from the trees into the water, and there deposit their spawn. When producing his cry the male distends the skin of his throat like a balloon. The air is driven alternately from it into the lungs and back again over the vocal chords, which vibrate with no uncertain sound. These little frogs are easy to keep in an inverted bell-jar or in a fern-case, but must be fed regularly with flies and spiders, which they catch by a sudden dab of the tongue at the moment of alighting from a long leap on to the glass where the insect is crawling. They can hold on to smooth glass or leaves by means of their sucker-like toes (Fig. 1).

The colour of the upper surface of the South European tree-frog is a most vivid and smoothly laid-on grass-green. Occasionally the colour becomes altered to a brownish purple, but returns after a day or two to its usual bright green tint. A great rarity is the blue variety of this frog—the enchanted Prince of the CÔte d’Azur—blue as the sky and the sea around him—the true genius loci. I obtained one a few years ago at Mentone, and kept it alive for three years in London. Its blue was the blue of the forget-me-not or the finest turquoise. When it died (I believe of old age, and not from discomfort or disease) I examined its skin very carefully with the microscope, and compared it with that of the ordinary green tree-frog, in order to make out the cause of their difference in colour.

At Mentone there is a little shop where one may purchase green tree-frogs and ornamental cages in which to keep them. Every year the dealer has two or three specimens of the blue variety on sale—their backs and heads looking like bits of turquoise-blue kid. Visitors have sometimes wrongly supposed that the blue frogs had been artificially changed in colour, but they are real, natural varieties. A similar substitution of blue for green has been noticed as a rare variation in other kinds of frogs and toads in other countries. It really consists in a suppression of yellow pigment.

The interesting thing about the colour of the little tree-frogs is that we find, on careful examination of the skin of a dead specimen with the microscope, that there is no green nor yet any blue “pigment” present in it. I found, on examining the blue specimen which died after living three years with me, that there is only black pigment overlaid by a colourless, semi-transparent layer of skin. In this outer skin in the ordinary green specimens there is scattered a quantity of excessively minute yellow particles, which, mixed with the blue, produce the green appearance. The fact is, that the wonderful “dead” turquoise-blue of the blue frog is a colour-effect similar to that of the blue sky and the blue of the human eye. It is produced by a peculiar reflection of the light from minute colourless particles, without the assistance of any blue-coloured substance. The distinction of these two modes of producing blue colour is important.

Certain transparent bodies are so constituted that when a beam of light is directed so as to pass through them, the red, yellow, green, and purple rays which exist in colourless sunlight are stopped, and only the blue rays come through. Such a body is blue copperas, or sulphate of copper; another is methyl blue, one of the aniline dyes; another is pure water, which gives only a slight advantage to the blue rays, so that the light must pass through a thickness of 30 feet or more before the blue tint is obvious. Thus, part of the blueness of the CÔte d’Azur is accounted for—namely, the blueness of the sea when the sunlight is strong and is reflected from the white rocks and sand lying 30 feet to 100 feet below the surface of the water.

There are, of course, other self-coloured transparent bodies which allow only rays of one colour to pass. Thus, blood-red, or hÆmoglobin, the pigment of the blood, allows chiefly red rays to pass through it. Yellow rays only pass through a solution of saffron or of chromic acid; green only or chiefly through green copperas (sulphate of iron) or through leaf-green or chlorophyll. Colour is very generally due in natural objects to such transparent bodies which absorb or stop all the coloured rays of light as it passes through them, excepting those of one tint—or, to be more correct, nearly all except those of one tint.

But the blue of the blue frog and a great deal of the blue in nature is due to another cause. If you are a smoker, or the friend of a smoker, watch the fine curling lines of smoke ascending from a cigar when it is being consumed in bright sunshine. You will see that it has a blue, even an azure blue, tint as the sunlight falls upon it. But if you let the smoke get between the sun and your eyes you will notice that the little curling clouds are no longer blue, but reddish-brown, in appearance. The smoke is not a transparent blue; looked at as a transparent body, it is brown! Further, when the smoke has passed into the smoker’s mouth and is ejected after remaining there for a few seconds, the cloud no longer looks blue, even when the sunlight falls on it and is reflected from it to your eye. It is now opaque white or colourless, with, perhaps, a faint tinge of blue. This change is due—as was shown by the experiments of the late Professor Tyndall upon a variety of clouds and vapours—to the cooling of the smoke and the increased size of the floating particles which coalesce as the temperature falls. The larger particles reflect white light, and the cloud is no longer blue. A cloud formed by the finest particles gives the strongest blue to the light reflected from it, and it is to this property of the finest particles of water-cloud floating in our atmosphere that the blue colour of the sky is due.

No doubt the question arises, “Why do clouds of the finest particles reflect a predominant amount of blue light rather than yellow or green or red?” That question is answered by mathematicians in accordance with what is ascertained as to the nature and properties of light, but it would require a long treatise to put those matters even in outline before the reader. We may in the meanwhile accept the conclusions of the physicists, and interest ourselves in seeing how they apply to some of the concrete facts about colour in Nature.

There are other instances of “blueness” due to the reflection of light from a cloud of excessively minute particles besides that of the azure sky and the blue, curling smoke of a wood fire. A familiar instance is the blueness of translucent bodies, such as the “white” of a boiled plover’s egg, especially when a bit of it is placed on a dead-black ground. The bluish appearance of watered London milk is another instance. These bodies look blue owing to the fine, colourless particles suspended in them, which act on light in the same way as do the fine particles of newly-produced smoke. Another very interesting case is the blue colour of the iris of the eye of man and other animals. It is not due to any blue pigment, but to a reflection from fine particles in the translucent, but turbid, tissue of the iris overlying the dark, black chamber of the eye. White geese and white cats frequently have blue eyes, the blue being thus produced. The only pigment which occurs in the human eye is a brown one, which gives a colour varying from amber yellow to very dark brown, almost black, according to the quantity present. When a very little of it is present it gives, in combination with the blue appearance of the unpigmented iris, a green tint, so that green eyes owe their colour to the same combination of causes as does the green skin of the little tree-frogs, or “rainettes.”

No solvent will extract any pigment from the skin of the blue frog—nor by the finest trituration can one obtain any coloured particles from it; only fine black granules can be separated. Alcohol removes the yellow pigment from the skin of a green tree-frog (killed, of course, for the experiment), and for a minute or two the skin becomes blue when its yellow pigment is thus removed by immersion in spirit; but it rapidly becomes a dull greyish-brown in colour, and so remains; the green cannot be preserved in spirit-specimens. It is not fully explained how such a uniform “dead” blue is produced by the reflection of light from fine particles, as that observed in the blue frog’s skin.

It appears that the blue and the green colour in the feathers of birds is in most, if not all, cases produced in the same way as the blue and green of the tree-frog’s skin. It would be interesting were it found possible to produce a full dead-blue colour by experimentally placing a coat of a translucent but turbid colourless medium on a dead-black plate. This, however, has not been done as a deliberate experiment. It is, however, recorded that Goethe was delighted to find what he considered to be a confirmation of his theory of colour when a friend showed him an oil-painting of a gentleman in a black coat which when wetted with a sponge turned bright blue. The picture had been recently “restored,” and the varnish on the black coat was not “dry.” It was precipitated by the water from the sponge, mixing with the spirit which held it in solution. A fine colourless cloud was thus produced overlying the black paint of the coat, and, as in the case of the cerulean frog, a dead-blue colour, due to reflection of the light by the fine particles, was the result. Some friendly physicist might repeat this experiment and study the matter in detail. The red, orange, and yellow colours of birds’ feathers are produced by pigments which are either insoluble or only soluble with great difficulty in fluids of the nature of ether. There is, however, an exception in the case of the African birds called Turacous, or Plantain-eaters. These birds have some large quill-feathers in the wing of a rich crimson colour. This splendid red pigment can be washed out of the feathers by water which is slightly alkaline, and a fine blood-red solution is obtained. Why this curious exception exists we do not know. The extracted colour is found to contain the element copper as one of its chemical components. Plantain-eaters kept in cages have sometimes washed all the colour out of their feathers owing to the water supplied to them for bathing and drinking having become foul and ammoniacal, and thus capable of dissolving the red pigment.

The cultivation on the Riviera of flowers for sale as “cut flowers” in Paris, London, and Berlin, in the colder months of the year, is now an enormous business, bringing many thousands of pounds yearly to the small gardeners around HyÈres, St. Raphael, Nice, and Mentone. Roses, violets, carnations, “mimosa” of various kinds, anemones, lilies, and narcissus are sent literally in tons by quick trains several times a week from these realms of sunshine to the dreary North. The commencement of this trade was due to the suggestion made some fifty years ago by Alphonse Karr, the French poet and journalist, who had a beautiful garden of his own at St. Raphael, and found that he could produce flowers in profusion through the winter. Two years ago I visited this garden (which now belongs to a French painter) at the beginning of April, and found it full of interesting flowers and shrubs, enormous bamboos, palm trees, some twenty different “mimosas,” eucalyptus of several species, camellia trees, and rose-bushes in quantity.

The influence of man on the vegetation of a favoured locality like the Riviera is more striking than in the North. But it is worth remembering that the most familiar tree in England—the common elm—is not a native, but introduced from South Europe. Our native elm is the wych-elm, or mountain elm—a much handsomer tree, in the opinion of many, than the so-called “common elm.” There are doubts as to whether both the spruce and the larch were not introduced by man at a very remote time, so that the Scotch fir would be our only aboriginal pine. The oak, beech, birch, ash, hawthorn, poplar, and alder are undoubted native English trees. The holly-oak or evergreen oak, the sycamore, plane-tree, sweet chestnut, horse chestnut, walnut, and probably the lime or linden tree have been introduced by migrating men at various periods into our islands. With the exception of rye and oats none of the plants which we cultivate for food are derived from our own wild plants, and none of our domesticated animals have been produced from native wild kinds.