In more than one periodical the botanical name of this plant has been given as Mentha arvensis, var. purpurascens. It will be well, therefore, to point out that this is an error before the statement is further copied and the mistake perpetuated. The plant has green foliage, with not a trace of purple, and less deserves the name purpurascens than the true peppermint (Mentha piperita), of which a purplish leaved form is well known. The mistake probably arose in the first place in a printer's error. The history is as follows:
For some years past a large quantity of a substance called menthol has been imported into this country, and extensively used as a topical application for the relief of neuralgia, and in some instances as an antiseptic. This substance in appearance closely resembles Epsom salts, and consists of crystals deposited in the oil of peppermint distilled from the Japanese peppermint plant. This oil, when separated from the crystals, is now largely used to flavor cheap peppermint lozenges, being less expensive than the English oil. The crystals deposit naturally in the oil upon keeping, but the Japanese extract the whole of it by submitting the oil several times in succession to a low temperature, when all the menthol crystallizes out from the oil and falls to the bottom of the vessel. The source of the Japanese peppermint oil has been stated to be Mentha arvensis, var. javanica. On examining several specimens of this plant in our national herbaria I found that the leaves tasted like those of the common garden mint (Mentha viridis), and not at all like peppermint, and that therefore the oil and menthol could not possibly be derived from this plant.
I then asked my friend, Mr. T. Christy, who takes great interest in medicinal plants, to endeavor to get specimens from Japan of the plant yielding the oil. After many vain attempts, he at last succeeded in obtaining live plants. These were cultivated in his garden at Malvern House, Sydenham, and when they flowered I examined the plant and found that it differed from other forms of M. arvensis in the taste, in the acuminate segments of the calyx of the flower, and in the longer leaf stalks; the leaves also taper more toward the base. Dr. Franchet, the greatest living authority on Japanese plants, to whom I sent specimens, confirmed my opinion as to the variety deserving a special name, and M. Malinvaud, a well known authority on mints, suggested the name piperascens, which I adopted, calling the plant Mentha arvensis, var. piperascens. Specimens of the plant kindly lent by Mr. Christy for the purpose were exhibited by me at an evening meeting of the Linnaean Society, and by a printer's error in the report of the remarks then made, the name of the plant appeared in print as Mentha arvensis, var. purpurascens.
I trust that the present note, through the medium of The Garden, will prevent the perpetuation of this error. This is the more important, as I hope that the plant will come into cultivation in this country. It is a robust plant of rapid growth, as easily cultivated as the English peppermint, and seems to require less moisture, and is therefore capable of cultivation in a great variety of localities. The increasing demand for menthol, which can only be procured in small quantities from the English peppermint, and the high price of English peppermint oil, lead to the hope that instead of importing menthol from Japan, it will be prepared in this country from the Japanese plant.
With the appliances of more advanced civilization, it ought to be possible for the oil and menthol to be made in this country at less price than the Japanese products now cost.
At the present time large quantities of cheap peppermint oil are imported into this country from the United States, and Chinese oil is imported into Bombay for use in the Government medical stores. There is no reason why this should be the case if the Japanese plant were cultivated in this country. In Ireland, where labor is cheap and the climate moist, this crop might afford a valuable source of income to enterprising cultivators. It may be interesting to note here that the plant used in China closely resembles the Japanese one, differing chiefly in the narrower and more glabrous leaves. I have therefore named it Mentha arvensis f. glabrata, from specimens sent to me from Hong Kong, by Mr. C. Ford, the director of the Botanic Gardens there.