its remedy. The old tuber has been too compact for yielding to the vegetative powers of the plant.

The curl first made its appearance in this country in the year 1,764, in Lancashire, where potatoes had been first introduced into British field culture, and had been propagated without any change of seed. From Lancashire this disease spread over all the potato districts of Britain, and as the cause and cure were equally unknown, there was a general apprehension that the plant would be exterminated. Premiums were offered by different agricultural societies to those who should point out a remedy for a disease so destructive; in consequence of which many speculations and theories were raised, which, however, led to very little practical utility

The discovery of at least a temporary preventive, and therefore of the probable cause, was made, as is believed, more from accident than design, in the neighbourhood of Edinburgh. Some of the growers in that situation were in the habit of procuring seed potatoes from the cold moorland districts, and fields planted with these were free from the curl. Upon inquiry it was found that in those bleak and humid situations, the potato crop was so late that the frost came on and blackened the leaves, while they and the stems were still green, and the tubers of course not ripe: The change of climate was therefore not the sole cause of prevention, if indeed it was the cause at all, for when the full ripened potatoes were planted in the moors, the curl appeared in them, in situations where there was none in the native potatoes.

It was thus found that the curl could be prevented by using tubers that were not quite ripe.

A writer in the Gardener's Magazine for May 1827, thus ingeniously accounts for this fact:"The potato tuber is a perfect organized system, in which the circulation regularly proceeds, and if suffered

to ripen will then tend to decay; but if separated before ripe from the stem or stalk which furnishes it with blood or fruit-sap, descending from the leaves, the circulation of the blood-sap is suddenly arrested. The ripe potato, having performed all its operations, becomes more inert; but the circulation of the sap in the unripe tuber having been stopped, it starts more readily, and with greater vigour, when planted; the one appears to die, worn out with age, the other seems accidentally to have fallen asleep, and when awakened, possesses an unspent vigour and energy."—p. 317.

That over-ripeness is the principal cause of the disease, has been found by experience to be so much the case, that out of the same potato it is possible to make some sets that will, and others that will not, produce the curl. The portion of the tuber that is nearest to the cord by which it is fastened to the plant, ripens first, as any one may observe, especially in an elongated potato, where the root end is often so mealy as to fall to powder, when the top or thick end is soft and waxy. If such a potato be taken when only the small end is ripe enough to boil mealy, the eyes upon another of the same parcel that are upon the waxy part, will all produce sound plants, while curl may appear in those which are taken from the mealy end. The soil and mode of culture may have likewise some effect in producing this evil. Experience has shown that high culture and stimulating manure tend more to produce curl than poorer treatment, that this disease is less frequent in new lands than in those which have been long in culture, and that it seldom appears in cold and upland places.

The economy of this article of food, as compared with wheat, is seldom questioned, although doubts have been raised even as to its comparative cheap

ness with wheaten bread. The following statement, from Mr. Jacob's Corn Tracts, contains, as it appears to us, all the facts that can be depended upon for forming an opinion on this question:

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If an acre of land, with the same degree of labour bestowed upon it, and the same portion of manure applied to it, yields 300 bushels of potatoes, it may yield 24 bushels of wheat. The food produced by the former, at 38lbs. to the bushel, will then be 11,400lbs. in weight; the food from the latter, at 60lbs. to the bushel, will be 1,400lbs., or the weight of the wheat will be one-eighth that of the potatoes. It is difficult to ascertain the quantity of nutrition in a given quantity of either wheat or potatoes. The chemical experiments of Sir Humphrey Davy show that wheat contains about three times the quantity of mucilage or starch, and of gluten or albumen, of what is contained in a like weight of potatoes but that potatoes contain also about from three to four per cent. of their weight of saccharine matter, in which wheat is deficient, though it abounds in barley. The difficulty of estimating the nutritive power of the two substances is not wholly removed by this appeal to chemistry, because we are still ignorant of the effect which the combination of the saccharine matter with the mucilage and gluten may produce when used as aliment. A small addition of the former to the two latter may communicate to the whole mass a degree of nutritive power very far exceeding its own separate proportion of weight. Some inquiries have been made as to the actual quantity of potatoes consumed per head in families in Ireland, in Prussia, and in Saxony; but the answers varied to such a degree, as to be little satisfactory. It does not appear to me to be very far from the fact, if we estimate the proportion of the nutritive power of wheat to that of potatoes, as about seven is to two; or that 2lbs. of

wheat afford as much subsistence as 7lbs. of potatoes, though it may be doubtful if it affords so much nourishment. We have seen before, that the mean weight of the two kinds of food, from the same extent of land, is nearly as one to eight; and now assume that the consumption of an individual is yearly one quarter, or 480lbs. of wheat, or an equivalent quantity of potatoes, being 1680lbs., then one acre of wheat will produce sustenance for three persons, or one acre of potatoes will afford it to six and fivesixths*"

The productiveness and other circumstances attendant on the culture of this root certainly point it out as one of the cheapest articles of substantive food that a population can use; and therefore we may conclude that in those countries where the labouring classes are forced to subsist on this nourishment, they are in a state of indigence in which the healthy industry of a fertile land should not be placed.

Potatoes may be made to yield a spirit of a very pure quality. It is supposed that they are a cheaper material than barley from which to extract alcohol. But attempts in this country profitably to distil from potatoes have always failed. At present, however, in a distillery at Guernsey a spirit is made from them with success.

In the eastern part of Prussia, as stated by Mr. Jacob in his Reports, potatoes are applied to many useful purposes. They are cultivated to a great extent, and by converting them into starch and treacle that land is made to yield a profit which might otherwise have produced a loss. Sugar did not answer so well; "but the treacle," says Mr. Jacob, appeared to me as sweet as any from the tropics, the only perceptible difference between them was that it had less consistence."

* Jacob's Tracts on the Corn-Trade, p. 169.

CHAPTER VII.

OTHER VEGETABLE SUBSTANCES USED FOR SUBSTANTIVE FOOD.

Ir may be useful and interesting to take a rapid view of the food which the vegetable kingdom offers to large bodies of mankind for cultivation, in addition to those most important productions, the corn-plants and the potato, which we have fully noticed.

CASSAVA-Jatropha Manihot.

THIS plant is known also as the edible-rooted physicnut, and in Brazil it bears the name of Mandioc. It springs from a tough, branched, woody root, the slender collateral fibres of which swell into those farinaceous masses for which alone the plant is cultivated.

The height to which the cassava attains varies from four to six feet; it rises by a slender, woody, knotted stalk, furnished with alternate palmated leaves, which are smooth, and increase in breadth till within an inch and a half from the top, when they diminish to an acute point. The middle lobes are six inches long, and two inches broad in the broadest part; the two next are an inch shorter, and the outer lobes are only three inches long.

South America is held to be the native region of this plant, which formerly afforded the greatest part of their sustenance to the entire Indian population of that vast region. In the Mexican states, cassava is more used on the western than on the eastern coast.