Page:EB1911 - Volume 22.djvu/216

Rh little known but very distinct tuberous species from central Mexico.

A review of the localities in which the presence of S. tuberosum and its tuber-bearing allies has been ascertained shows that, broadly, these varieties may be divided into mountainous and littoral. In either case they would not be subjected, at least in their growing season, to the same extremes of heat, cold and drought as plants growing on inland plains. Again, those forms growing at a high elevation would probably start into growth later in the season than those near the coast. The significance of these facts from a cultural point of view is twofold: for, While a late variety is desirable for culture in Great Britain, as ensuring more or less immunity from spring frost, it is, on the other hand, undesirable, because late varieties are more liable to be attacked by the potato disease (Phytophthora infestans) which as a rule appears about the time when the earliest varieties are ready for lifting, but before the late varieties are matured.

In cultivation the potato varies very greatly not only as to the season of its growth but also as to productiveness, 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. The tubers vary greatly in size, form and colour; gardeners divide them into rounded forms and long forms or “kidneys,” and there are of course varieties intermediate in form. 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 arisen chiefly through cross-breeding, though not entirely so, there being a few cases upon record of the production of “sports” from tubers that have become the parents of new varieties, but authentic cases of the sporting of tubers are few and far between. If, on the other hand, the true seeds of any of our cultivated varieties are sown, the seedlings show very wide variations from one another and from the parents. In this Connexion it is very interesting to observe that Messrs Sutton of Reading find that the seedlings of many of the varieties of potato that occur spontaneously in different parts of America come quite true to type from seed.

The potato thrives best in a rather light friable loam; and in thin sandy soils the produce, if not heavy, is generally of very good quality. Soils which are naturally wet and heavy, as well as those which are heavily manured, are not suitable. Indeed it is best, except when there is ample space, to grow only the earlier kinds in gardens. If the soil is of fair quality the less manure used upon it the better, unless it be soot or lime. Gypsum, bone-dust, super phosphate of lime and nitrate of soda may also be used, and wood ashes are advantageous if the soil contains much vegetable matter; but the best results are usually obtained when farmyard manure is supplemented by artificial, not by using artificial alone. Potatoes are commonly propagated by planting whole tubers or by dividing the tubers, leaving to each segment or “set” one or two eyes or buds. The “sets” are then planted in rows at a distance varying from 15 in. to 3 ft., the distance being regulated by the height of the stems, and that between the sets varying from 6 to 12 in., 8 in. being a good average space for garden crops, with 2 ft. between the rows. The sets may be put in 6 in. deep. The planting of whole tubers instead of the cut sets usually gives a better return. The full-sized tubers are, however, preferable to smaller ones, as their larger buds tend to produce stronger shoots, and where cut sets are used the best returns are obtained from sets taken from the points of the tubers-not from their base. Thomas Dickson of Edinburgh long ago observed that the most healthy and productive crop was to be obtained by planting unripe tubers, and proposed this as a preventive of the disease called the “curl,” which sometimes attacks the young stems, causing them and also the leaves to become crumpled, and few or no tubers to be produced; in this Connexion it is interesting to note that Scottish and Irish seed potatoes give a larger yield than English, probably on account of their being less matured. It has also been noted that the sprouting of the eyes of the potato may be accelerated if, while still unripe, it is taken up and exposed for some weeks to the influence of a scorching sun. The best sets are those obtained from plants grown in elevated and open situations, and it is also beneficial to use sets grown on a different soil.

The earliest crops should, if possible, be planted in a light soil and in a warm situation, towards the end of February, or as early as possible in March. In some cases the tubers for early crops are sprouted on a hotbed, the plants being put out as soon as the leaves can bear exposure.

The main crop should be planted by the middle of March, sprouted sets being used; late planting is very undesirable. Those intended for storing should be dug up as soon as they are fairly ripe, unless they are attacked by the disease, in which case they must be taken up as soon as the murrain is observed; or if they are then sufficiently developed to be worth preserving, but not fully ripe, the haulms or shaws should be pulled out, to prevent the fungus passing down them into the tubers; this may be done without disturbing the tubers, which can be dug afterwards.

Forcing:—The earliest crop may be planted in December, and successional ones in January and February; the varieties specially suited for forcing being chosen. The mode of cultivation adopted by the London market gardeners is thus in substance explained by Cuthill: A long trench, 5 ft. wide and 2 ft. deep, is filled with hot dung, on which soil to the depth of 6 in. is put. The sets employed are middle-sized whole potatoes, which are placed close together over the bed, covered with 2 in. of mould, and then hooped and protected with mats and straw, under which conditions they will sprout in about a month. A bed of the requisite length (sometimes 100 yds.) is then prepared of about 2 ft. thickness of hot dung, soil is put on to the depth of 8 in., and the frames set over all. The potatoes are then carefully taken up from the striking bed, all the shoots being removed except the main one, and they are planted 4 in. deep, radishes being sown thinly over them and covered lightly with mould. When the haulm of the potato has grown to about 6 in. in height the points are nipped off, in order to give the radishes fair play; and, although this may stop growth for a few days, still the potato crop is always excellent. After planting nothing more is required but to keep up the temperature to about 70°, admitting air when practicable, and giving water as required. The crop is not dug up until it has come to maturity.

Potatoes are also grown largely in hooped beds on a warm border in the open ground. The sets after having been sprouted, as above, are planted out in January in trenches 2 ft. deep filled with hot dung, the sets being planted 5 in. deep, and over all radishes are sown. The ridges are then hooped over, allowing about 2 ft. of space in the middle, between the mould and the hoop, and are covered with mats and straw, but as soon as the radishes come up they are uncovered daily, and covered again every night as a protection against possible frosts. This is continued till the potatoes are ready for digging in May.

Potatoes are sometimes grown in pots in heat, sprouted sets being planted in 11-in. pots about two-thirds full of soil, and placed near the glass in any of the forcing-houses, where a temperature of from 65° to 70° is to be maintained. The plants are duly watered and earthed up as they advance in growth.

There are few agricultural subjects of greater importance than the culture of the potato and the losses entailed by potato disease. It is not unusual in bad seasons for a single grower to lose £30 per acre in one season. In extreme cases every tuber is lost, as the produce will not even pay the cost of lifting.

The best-known disease of potatoes is caused by the growth of a fungus named Phytophthora infestans, 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; the bittersweet, S. Dulcamara, and other species of Solanum. It is also very destructive to the tomato, Lycopersicum esculentum, and to all or nearly all