planting is prejudicial to the health of the potato, as of every other plant. The distance apart of the rows, but especially of the hills, should vary with sorts. The naturally dwarfish kinds may be planted much more closely than the free growers. The practice of planting in drills, where the plants necessarily crowd each other, is favorable to early maturity, but is objectionable on general principles. The rule should be to plant at such distances, considering the soil and the kind used for seed, as shall allow the ground to be all covered in the final spread of each plant. To the spreading of seed in the hill there is the same objection as to planting in drills. It seems not to be well understood, that when seed for one hill is all planted in one spot, nature is never at a loss where to spread the vines and store the tubers. Where a certain amount of ground is proportionally devoted to each hill, as large a crop can be gathered where the seed is in one piece, as when it is in two or three pieces.

64. The time of digging should depend on circumstances. In a dry soil, and with a healthful crop intended for winter and spring use, defer digging until the weather is cool enough to make it safe storing them in cellars or in pits. Forks of three or four tines, and hooks similarly constructed, except that they are bent at the shank to the shape of a hoe, are safer tools for digging than hoes, not to say much cheaper. A light four-tined fork, ten inches long, where the soil is not too stony, will often throw out a whole hill at one movement. Potatoes when a little liable to disease, and when gathered hastily in bad seasons, are much more likely to be injured in the cellar, when cut with the hoe, than when dug with a fork or hook.

65. If the crop is diseased, or likely to become so, dig at once if the crop can be used, but if intended for winter stores, leave it in the soil, where decay will go on more safely, and in no larger degree than in a cellar. The soil, in this case, is the most economical place in which to purify the crop.

66. Dig in dry weather if possible. In all cases, but especially in wet weather, let the crop lie in the field, or better, under cover, until the sweating stage is passed. A potato, like an apple or squash, has a certain amount of water in the skin when freshly gathered. Dry each and all similar things before close storing, otherwise this moisture will produce decay. A well-ripened potato, thus dried, will lose very little weight dur ing the whole winter if properly stored, its natural covering, when once dried, being as impervious as India rubber. In burying potatoes in cool pits, this drying need not take place.

67. Potatoes healthfully grown, well dried, and stored in cool dry cellars, will bear to be put in large heaps; but where there is suspicion of lurking disease from the character of the soil or of the past season, or where the cellar is too warm or moist, they should be stored in open barrels, with holes bored in the sides, or in narrow bins with cracks at the sides to admit air. Cases frequently occur of crops hastened into the cellar without drying, where they are massed into large heaps, the cellar itself being too warm and ill ventilated. Potatoes so stored can not remain sound, however well they have been matured. Even in this case, injury would ordinarily be prevented by taking the precaution to have them rest on a floor raised a little and having cracks to admit the air, and small bundles

of poles, set upright over these cracks, to ventilate passages through the middle of the heap.

68. Never wash a potato before storing, especially if there is any sus picion of latent disease. Washing seems to remove from the surface of the tuber a substance analogous to the bloom on a plum or apple, and thus opens the skin to the action of the air upon the vessels of the flesh below. It probably also deranges the position of the minute scales of the cuticle, so that subsequently, in drying, the skin is never so close as in the unwashed tuber. I have tried washing single hills frequently, and in many instances have found it depreciate the health of those tubers, compared with others of the same crop. As, moreover, all potato soils are a little alkaline, such washing, by removing the soil from the surface of the tubers, makes them more liable to fermentation in cases of latent disease, a fact which illustrates the probable utility of sprinkling slacked lime over such crops in the cellar. It may also be added that the particles of earth adhering to the surface of the tubers prevent the too close contact among them, and so facilitate the circulation of the air among them.

69. Store potatoes low in the cellar, particularly when afraid of latent disease, or when the cellar is a little too warm. Of two barrels or boxes, where one stands above the other, the lower one always endures the winter storage the best. In the case of those designed for seed, germination is much slower in the lower than the upper barrel,

OBSERVATIONS ON THE FOREGOING CAUSES OF DISEASE.

70. 1st. The consideration of the two primary causes of disease (21-45) and of the four secondary causes, (46-69,) covers, I suppose, the whole ground of observations, (except of the external marks of disease soon to be noticed.) I have never yet seen a single case of disease which was not explicable when all the circumstances were ascertained, on the ground of some one or more of these causes.

71. 2d. The discase of the potato is not peculiar and mysterious as it has been long and widely alleged. Considering that the whole vegetable structure is explicable by the principles of a common physiology, it were to be expected that it would be subjected also to the principles of a common pathology. Any deliberate allegation of mystery then in reference to cases of vegetable disease, would sound more like conclusions of haste and ignorance than of truth. Similar causes are likely to produce similar results in the same departments of nature, throughout all time. The mountain origin of the potato, (noticed 10-12 above,) was seen to give rise to a twofold sympathy with other plants, that is, with tropicals on the one hand and with hardy plants on the other. So, as a matter of fact, it was shown (40-42), that the potato is actually diseased with one or the other of these classes of plants as one or another type of weather prevails. There is, then, no more mystery in the disease of the potato than in that of any other plant.

72. 3d. The comparatively recent appearance of the potato disease is doubtless the most obscure point in its whole investigation. The alleged late appearance of the disease has become questionable. I can myself re

call appearances on the tubers, very like the present disease, manifested about 1809, more than fifty years ago. I saw similar appearances in the spring of 1830 or 1831, on tubers that had been kept through the winter. European writers have lately found traces of it there, seen occasionally during nearly a century past.*

73. 4th. It is the recent severity of manifestation, rather than its absolute recency, that requires explanation. I give the following solution:

a. The potato, as a semitropical plant, is not strictly congenially situated here, suffering in common with other tropicals from the unsteadiness of our climate. Time has not changed its nature in this respect any more than that of cucumbers, tomatoes or corn. A cold wet summer injures any and all these plants now just as truly as it did one hundred years ago.

b. It lies out of the ground usually from five to six months of every year, subject to many imperfections in its mode of preservation.

c. As a mountain tropical it is exposed, unlike any other tropical with which I am acquainted, to disease from two causes, that is, cold wet alternations, and hot damp intensities, (secs. 40-42, above.)

d. It is a perennial, i. e., one not grown annually from proper seed but from tubers, sprouts, &c. Hence the injuries which in annuals are overcome by reproduction by seed, are here permitted to accumulate. The influence of bad soil, bad culture, and bad weather in one year, leaves an effect on the constitutional energy of the plant which increases by the addition of succeeding years, as in the case of any other perennials and of animals. Without determining how long a young seedling potato will last, as ordinarily cultivated, it is enough to state that nearly all the sorts in use twenty years ago at the outbreak of potato disease, were old sorts. It is certain, too, that it has been seldom renewed from the sced-ball, and when so renewed, it was not by those rules of culture now found to be necessary to a somewhat permanent result.

e. Especially should it be noted that, in our anxiety to get large crops, the potato, for perhaps the last century has been overstimulated in rich soils, and by the application of fresh unfermented manures, producing upon it the same deterioration of constitutional power seen in the case of many fruit trees similarly treated. No perennial on which we depend for food can be named which has suffered in this way so much as the potato.

f. Bearing these five preceding considerations in mind and remembering their broad application to all civilized lands where the potato was largely cultivated, it is obvious that a catastrophe of severe disease could not long be delayed, and that it would be felt somewhat simultaneously over many lands, qualified more or less by the exhaustion of the varieties in use, the severities of weather, and the errors of cultivation. What gives the highest probability to this mode of explaining the sudden development of this

* In Buel's Cultivator for 1839, page 147, Abraham Halleck describes an injury to potatoes coming from a rust, so that it took thirty hills for a bushel. The rust came about the time of wheat rust. Nothing is said of rotten tubers or the appearance of the rust. This is two or three years earlier than its assigned access, and disposes of the notion that the mildew was a sudden importation. Indeed, it justifies the opinion that it is as natural as the mould cheese.

evil, is the fact that now, year by year, the exascerbations of disease are qualified by these very circumstances. With certain new strong seedlings no disease is felt. With dry, cool, or hot weather, and with even tempered weather of a mixed character, even when the seed is inferior, disease is rarely associated.

74. (5th) This disease has been felt in South America. (See Patent Office Report for 1847, pp. 141, 2.) Nor is this strange. The laws of vegetable pathology are alike applicable to all countries. Conceding to New Grenada, Peru and Chili (which seem eminently to be the native countries of the potato), the exhibition of a very benignant climate, occasions of disease might still arise. Some years might present a slight deviation from the usual uniformity. Then the effect of planting exhausted varieties, and of unwise and careless culture, would appear especially on old and feeble

sorts.

VIII. THE USUAL EXTERNAL MARKS OF DISEASE.

75. These marks are pretty fully exhibited in my Essays for 1847, 1848, and particularly 1851, at pp. 377-380. They have been occasionally referred to in the preceding portions of this article, but could not well be described particularly.

A. The usual external marks of disease connected with cold, wet and sudden changes of the weather.

76. When such weather follows that which has been hot and dry, the whole plant frequently assumes a yellowish color, as though starved; the young tufts of leaves wither, or at least cease growing; soon the points of the outer leaves of the plant turn steel-blue, while those in the centre are marked as with stains of iron-rust, and both rapidly dry up. More or less mildew accompanies these signs of disease. In severe cases the whole plant soon dies.

77. In the progress of disease on the herbage, but sometimes not till after its death, the tubers show brown spots of the size of a pea or larger. They are rough, and when rubbed show a loose condition of the skin, often with a little softening of the flesh beneath. Sometimes the whole tuber assumes a brown, dead look, and the flesh a brown or black color, with an unnaturally hard, bristle condition. This I think is usually the case where the disease acts slowly. In cases of the rapid action of disease on the whole plant, the tuber becomes very soft and ropy when opened, the flesh being sometimes brown, and at others white, but in both very offensive. In this disease of the tuber it is very common to find a white mould over the surface. When the disease acts with rapidity, and the flesh assumes the ropy form, the surface of the tuber is often perfectly natural, and gives no indication of its condition until it is handled, when it will, if long, often nearly break in two with its own weight, giving proof of a most active state of fermentation. These very tubers, however, if left long in the soil, become covered with white mould.

78. When the disease comes with the chills of autumn, the tuber is never soft and offensive. Indeed, there is often but the slightest discoloration of the flesh, and sometimes not even that, but just a shriveled state of

the skin. These results of late disease often do not manifest themselves until mid winter, and sometimes not till spring.

79. In cases of disease in this phase of weather, balls rarely if ever set and grow. Indeed, one of the earliest indications of disease in this connection, is the sudden falling off of the flowers, whether open or not. Flowers may fall early, from dry, hot weather, without disease, but their falling in connection with previously mentioned marks of disease, is very noticeable.

B. Disease connected with hot, damp weather.

80. In its incipient approach, especially where the soil is rich, the leaves are of the deepest green, and often a little mottled with dark spots on the largest of them. Soon these leaves, and in the end all others, become marked with fine brown dots which gradually enlarge, and run into each other, generally covering the whole leaf. In looking at the leaf (in the pretty early stage of these dots) with a hand microscope, the skin is found to be dead, and is covered with a slight mould, not discoverable to the naked eye. As the spot enlarges, however, the unassisted vision distinguishes the mould.

ease.

81. This mildew has been known from the very beginning of the potato disease. It has been by many considered as the direct cause of the disIn the autumn of 1855, Dr. H. W. Ravenel, of Aiken, South Carolina, wrote to me for specimens of this mildew, that he might compare them with European samples. They were sent to him, and he soon answered that they were identical with those of Europe. He gives it two names, "Botrytis Infestans," after Montague, and "Peronospora Devastatrix," after Caspar. The first name has usually been given to it in the United States. Professor Johnson, however, has taken half of each name, and calls it "Peronospora Infestans," although I do not know by what authority he does so. This subject is resumed in No. 83, below.

82. In severe cases, the whole plant-leaves, stalks, flower stems, and even full grown balls-all fall a prey to this disease. In light cases the leaves and flowers only, die, leaving apparently healthy naked stalks. In severe cases the whole stalk speedily turns black, becomes offensive, and soon perishes.

83. In many cases the disease, after commencing with considerable violence, is suddenly arrested by the occurrence of dry, cool, windy weather. Then frequently the spots of mildew on any one leaf cease to enlarge, and the dry portion of the leaf drops off, leaving the remainder healthful. So also some of the stalks, whose whole cuticle near the top had been diseased, until accident or light wind had broken it half off, so that it hung down, will recover, and continue to grow in that pendant position. These facts show perfectly how entirely the disease is caused by the weather. The form of this mildew may be seen in the "Patent Office Reports" for 1849, plate 7, figure 14, and the description of it on page 396 of the same volume. When seen with the naked eye it is as a brown spot, and if very mildly spreading, the spot has a visible mouldy look; seen with a microscope of moderate power, it resembles a net work of white thread. When the heart is sound, though a thick coat is brown, it is common to find potatoes