596 POTATO In cultivation l the potato varies very greatly not only as to the season of its growth but also as to productive ness, the vigour and luxuriance of its foliage, the presence or relative absence of hairs, the form of the leaves, the size and colour of the flowers, &c. It is probable that a more careful investigation of these peculiarities, and especially of those connected with the microscopical ana tomy of the leaves, would give serviceable indications of the varieties most or least susceptible to the disease, a point at present hardly if at all attended to. As to the tubers, they vary greatly in size, form, and colour ; gardeners divide them into rounded forms and long forms or "kidneys"; "lapstones" are more or less flattened; and " pebble " varieties are long potatoes broader at one end than at the other. The colour of the rind, yellowish, brown, or purple, furnishes distinctions, as does the yellow or white colour of the flesh. The colour of the eyes and their prominence or depression are relatively very constant characteristics. These variations have originated chiefly by cross-breeding, but not invariably so, as some varieties rarely, if ever, produce flowers in Britain, and yet " sports " have been observed in their tubers and have become the parents of new varieties. Various methods have been pro posed for the prevention or arrest of the ravages of the fungus which causes the " potato disease " (see below). In addition to different modes of cultivation, attempts have been made to secure varieties less liable than others to disease, and, although no great measure of success has been attained, still the matter is not without promise, seeing how the early varieties, as before stated, escape the full virulence of the malady. Other attempts have been made to infuse a hardier constitution by hybridizing the potato with hardy species such as S. Dulcamara and S. nigrum. Hybrids were accordingly raised by Mr Maule, but they all suffered from the disease as much as the parents, and it is to be feared that the hybrids raised between the common potatoes and some of the six species mentioned by Mr Baker may suffer a like fate. This, however, remains to be proved. Mr Maule, disappointed with his hybridization experiments, then tried the effect of grafting. With this view he grafted S. nigrum on to a shoot of the potato. New tubers were formed, the foliage being wholly that of S. nigrum. In another ex periment he grafted the potato on to S. Dulcamara. In one case tubers were produced on the graft (the potato), but none on the Dulcamara stock, either above or below ground, while in another case tubers were actually pro duced on the underground portions of S. Dulcamara. Mr Maule s experiments were most ingenious, but the theory he gave in his The Potato, what is it? (Bristol, 1876) will not commend itself to physiologists, and there is no evidence to show whether the grafts he obtained were attacked by the parasite or even whether they had a chance of being so. Mr Maule s experiments, especi ally the one last mentioned, afford confirmation of the possibility of graft hybridization being effected. Various experimenters, especially Mr Fenn, have asserted that by engrafting an eye of one variety into the tuber of another, not only will adhesion take place but the new tubers will present great variety of character; and this indeed seems to be the case from the numerous specimens shown by Mr Fenn at the Royal Horticultural Society, but it can hardly be considered as established that the variations in question were the result of any commingling of the essences of the two varieties. The wound may simply have set up that variation in the buds the occasional existence of which has been already noted. The last-cited experiment of Mr Maule s, however, is much more conclusive. Mr A. Dean also (Gardeners 1 Chronicle, 2d September 1876, p. 304) 1 See HORTICULTURE, vol. xii. p. 286. succeeded in grafting the potato on to the tomato, with the result that, although no tubers were found on the root of the tomato, numerous tubers were produced on the sides of the branches of the potato. Another experiment may be here mentioned as throwing light on the formation of tubers, one wherein Mr Burbidge observed the production of tubers at the portion of an ordinary cutting of S. Com- mersoni inserted in the soil. In this case no tubers were formed above ground. (M. T. M ) POTATO DISEASE. There are few agricultural subjects of greater importance than the culture of the potato and the losses entailed by potato disease. The number of acres in Great Britain alone under cultivation for potatoes is generally more than half a million (543,455 in 1883, 562,344 in 1884); the average weight of the produce per acre may be taken at five tons, the average price about 5 per ton, so that the commercial value of each year s crop commonly ranges between 13,000,000 and 15,000,000. It is not unusual in bad seasons for a single grower to lose from 1000 to 1500 through disease; for the market grower some times not only loses the entire produce, or nearly so, but loses also the value of the seed, the guano, the farm-yard manure, the rent, and the labour. Growers sometimes lose 30 per acre in one season, for, exclusive of the dis eased produce, 10 may be put down to guano and dung, 4, 10s. to rent, tithes, and taxes, 6, 10s. for seed, and 2 for digging ; added to this there are ploughing, har rowing, overlooking, earthing up, sacks, carriage to and fro, and many minor expenses. The losses range in amount according to the virulence and general extent of the dis ease. In extreme cases every tuber is lost, as the produce will not even pay the cost of lifting. The year of the great potato famine in Britain was 1845, but the Rev. M. J. Berkeley, in his famous essay on the potato murrain published by the Royal Horticultural Society of England in 1846, stated that a very serious disease of the potato named the " curl " had at that time been known in Britain for more than half a century. We now know that the " curl " is a condition of the true potato murrain. As a rule, although there are a few exceptions, the disease occurs wherever the potato is grown. It is known in South America, in the home of the potato plant. The disease of potatoes is caused by the growth of a fungus named Peronospora infestans, Mont., within the tissues of the host plant, and this fungus has the peculiar property of piercing and breaking up the cellular tissues, and setting up putrescence in the course of its growth. The parasite, which has a somewhat restricted range of host plants, chiefly invades the potato, Solanum tuberosum, L. ; the bittersweet, S. Dulcamara, L. ; S. demissum, Lind. ; and S. cardiophyllum, Lind. It is also very destructive to the tomato, Lycopersicum esculentum, Mill., and to all or nearly all the other species of Lycopersicum. At times it attacks petunias and even Scrophulariaceous plants, as Anthocersis and Schizanthus. A second species of Perono spora is known on Solanaceous plants, viz., P. Hyoscyami, D.By., a parasite of the common henbane. In England the disease is generally first seen during the last ten days of July ; its extension is greatly favoured by the warm and showery weather peculiar to that period of the year, and according as the warm and humid weather of autumn is late or early the murrain varies a little in its time of appearance. To the unaided eye the disease is seen as purplish brown or blackish blotches of various sizes, at first on the tips and edges of the leaves, and ultimately upon the leaf -stalks and the larger stems. On gathering the foliage for examination, especially in
humid weather, these dark blotches are seen to be putrid,