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They are also found on living vegetation, usually that which is sickly, and sometimes on that which is (apparently at least) healthful. Here come in rust in wheat, oats, &c., smut on corn, and mildew on the grape and potato, &c., &c. The mould, rust and mildew pervade the cells of the plant with fine threads or roots called mycelium. From this, small stems are thrown up through the skin of the leaf stalk, fruit, or tuber on which they are found. They are supposed to grow by imbibing the juices of the plant on which they are found, and possibly are supported in part by the air. These small threads in the plant, the stems that come through the covering of the leaf, and the seed-dust, or spores, as they are called, are usually too small to be distinctly seen by the naked eye, or even with the aid of a single microscope, and require in the latter instrument a high magnifying power. They are all found growing most rapidly in damp weather and positions, and especially in hot weather. The seeds which are very numerous, are carried about by the wind. They are supposed to grow usually not by falling on the surface, but by being imbibed into the circulation of the plant where they may lie dormant. They seem designed to be nature's scavengers for the decomposition of dead or dying matter.

a. Some varieties of them attack only dead matter. Let the limb of a tree die-let sun-scald kill the southwest side of a fruit tree-let a heap of weeds or surplus useful plants be pulled up and thrown into a heaplet hay or grain be stored imperfectly dried-let cooked food be left long, especially in damp weather-let garments hang in close rooms against damp walls, and one result follows in these and all similar cases: mould, rust and mildew, in short, fungi, attack them all, that they may the more readily be removed out of the way as offensive matter. The question whence came the mildew, that attacks a bowl of neglected milk or a neglected garment, may not be readily answered, as the study of fungal plants is by far the most difficult branch of botany. In all such cases, however, chemical changes are going on in the bodies thus attacked, which afford food congenial to these fungal plants-food that did not exist in them in their healthy state. In no one of these cases, is it true that the fungus has any original power to destroy the fabric which it attacks, but in them all the morbid state of the body invites the attack.

100. b. In other cases the fungus attacks diseased matter not yet dead. Let grapes and grain grow unhealthfully, especially under the influence of rich soil, and hot, wet weather, and rust is the consequence. So in the same weather apples and nuts will often exhibit mildew at the heart. In the cold, changeful weather of some summers, mould attacks the extremities of the vine in the watermelon and muskmelon, the leaves of the cucumber, and the fruit of the summer squash. In all these and such like cases, chemical changes in the plant, which are easily explained on pathological principles, always precede the attack.

101. I am not competent to discuss the question whether fungi of any kind are able to attack the healthful plants, especially under the directing iufluence of the experimenter. But clearly, as a great law in nature, this cannot be true, as it would strike a death blow at the most of our physical blessings. The encouragement which Divine Providence presents to man is this, that while he cultivates the soil intelligently and faithfully, it shall usually yield its full and beautiful fruits. Without the existence of such a general law, there would be an end of all security of food, and of all human enterprise.

102. The question may be asked, whether all varieties of the potato are liable to the attack of mildew. I certainly think they all are.

a. This conclusion would be probable on principle. All vegetable growth may become imperfect, if not from diminished vitality, yet from the changes and intensities of the weather, and from the character of the soil, position, bad culture, &c. Under such conditions growth is imperfect, and some degree of disease a necessary result, on the principles that govern the progress of vegetable life.

b. I think the universal liability of the potato to be at least slightly influenced by fungus is an observed fact. My strongest seedlings, such as

rpet Chili, Pink-Eye Rusty-Coat and Andes, though very rarely showing any disease of tuber, do yet frequently, under exposure to the causes detailed above, show slight disease on the foliage. Any other result than this would place the potato in a position of immunity from disease, far above any other vegetable, and a position of virtual invulnerability. (Section 95 above.) The prominent fact is, as developed in my not short or narrow observation, that while the old sorts, and even most of their seedlings of the first generation, are sure to be diseased at all times when disease is pathologically probable, my strong seedlings and the families, or rather portions of the families derived from them, are exempt, that is, exempt from any considerable degree of disease.

c. Let a great law of nature here be brought to notice. The wild buffalo, Rocky Mountain sheep, the wild horse, &c., are seldom sick. But bring these classes of animals within the influence of civilization, and all is changed. Abundance of rich food, restricted motion, and confinement to the same atmosphere in our narrow pastures, produces a certain tenderness and liability to disease. So in the vegetable world, the pear, the apple, the plum, the grape, &c., when growing wild in natural positions, where the soil is not rich, where the tree or plant starts late in the spring, and drops its leaves, and presents ripened wood early in the fall, are all healthful. The diseases to which such fruits are incidental in the experience of fruit culturists are unknown.

But when such fruits are brought under the influence of culture in a rich soil, where they start early and grow late, there is a sacrifice of hardiness. The larger and richer and surer crops of these fruits justly reconcile us to the liability, but that liability none the less exists and is inevitable. Immediately growing out of this fact is another. The less - luxurious growth, i, e., less succulent growth of the wildling, and its early fall of leaf while the weather is yet comparatively severe, admits of a simple death of its foliage. But under cultivation where the tissues of the plant are rendered succulent, and the leaves thick and juicy, and where the growth of the leaf is often driven to the extreme limits of autumn, we often see a violent and unnatural death of the leaf, and even of the points of the shoots. Fungal enemies find a natural home on such plants, and even when the wildling is free. Thus by the same law the potato, even in its strongest varieties, if grown in too rich and damp soil, with an exposure to the sun on southern and eastern slopes, amid valleys where the air is not fresh and dry, is likely to suffer from at least a little disease, if not in the tuber yet in the leaf. This is especially true where, by lateness of planting, or natural lateness of maturity, they meet the early autumn quite unripe.

IX. THE CHEMISTRY OF THE POTATO DISEASE. 103. As I am not a practical chemist, it becomes me to speak modestly on this point. A few obvious truths and facts however may be referred to with confidence.

104. The soft parts, especially of herbaceous plants, is largely composed of cellulose, which is nearly identical in chemical composition with starch, gum and sugar. These four substances readily pass into each other in the vital processes of the plant, and are all reduced to carbonic acid and water when the plant dies.

105. These substances are all elaborated in the plant, and deposited upon the tuber, the cellulose forming its cells, and the starch, gum and sugar, (the first in vastly the largest proportion) filling these cells in connection with water.

106. The tuber and all parts of the plant also contain a mucilage which is a nitrogenous compound. It is the cause of fermentation throughout the vegetable world, and may promote it in any part of the plánt. Thus the juice of the sugar cane needs to be boiled speedily after it is pressed out of the plant, otherwise fermentation will so far deteriorate the juice as to preventits granulation into sugar. The same thing is noticeable in the latest drawn sap of the maple tree, in the spring. The gradual fermentation of most vegetables, after they are crushed or cooked, or are thrown into large heaps, is to be referred to the same cause, and the final rotting of fruit also, In the spring of the year the starch deposited in trees and plants the fall before, gradually changes in the direction of sugar, and with the sugar also previously existing in many plants, is dissolved in the circulation of the tree or plant. Now let this change be interfered with by the occurrence of sudden cold weather, and we have fermentation, and death, especially in tender trees, such as peaches, and sometimes, too, plums and apples.

107. Let this reasoning be applied to the potato. Here we have a semitropical plant, liable with all other tropicals, to injury from sudden cold, wet changes, and liable, also, with most hardy plants, to injury from hot damp intensities, a liability increased by the failing of a particular sort, and by bad soil, culture, &c. Now, when either of the severities of weather just named occurs at mid-summer, or later, during the active growth of the plant, we have fermentation and disease, either in a small degree just obviously injuring the plant, or possibly in a fatal degree, ruining it. The potato, or any other plant in a similar condition of corrupted juices, now falls a prey to rust, mildew, &c.-enemies as they are sometimes deemed, but really friends provided by nature as her natural scavengers, to remove dying and dead matter out of the way. [AG. TRANS.]

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108. This chemical action in the plant, whether of the potato, grape, wheat or any other, existing in proportion to the intensity of the causes, is clearly, as has just been seen, capable of accomplishing every result seen in actual experience, from the slightest degree of rust on wheat, grapes, potatoes, &c, (a degree not appreciated at harvest time) to the largest degree which involves these plants in ruin.

109. Here, as it seems to me, is a plain, philosophical account of potato disease, and of all other ordinary diseases of vegetation. It is a plain principle in philosophy that when an adequate cause for an effect has been found, it is not wise to look for a further and additional one.

The forego ing theory fully accounts for the appearance of the fungus on the plant, without exalting that fungus to the position of the exclusive cause, and thus starting a new law in vegetable pathology, a law as applicable to any other vegetable as the potato, and a law that would set at naught all human hopes.

X. THE REMEDY FOR THE POTATO DISEASE. 110. So long as the old enfeebled sorts continue to be used for seed, a remedy, in the strict sense, is impossible. The enfeebled vitality of those old sorts will not be invigorated, but will manifest itself as often as severe seasons occur. Taking the weather of Central New York as my guide, such sorts as the Western Red, Carter, Early Shaw, Mountain June, Early Pink-Eye, Strawberry and Ash-Leaf kidney, as the seed to be used; such a 0001, rather dry summer as 1848; a season, with the usual quantity of rain and degree of heat, but with great uniformity, as 1852; and seasons as severely dry and hot as 1854, 1856 and 1858, will all exhibit healthful crops. But in a cold wet season, such as 1857, or in hot wet ones, like 1850, 1851, 1855 and 1863, all sorts named above will prove very unreliable.

111. Much, however, may be done in overruling the action of the secondary causes of disease, (mentioned in section 46, above) such as soil, posi. tion, time of maturity and culture. In the bad years mentioned above, those who had planted on dry, poorish soils, using early sorts, and apply. ing wise culture, frequently gathered sound, or nearly sound crops of small or medium yield.

112. In a climate as unsteady and as extreme as our own, some little danger may be incurred in the use of the strongest new varieties for seed, if very heavy crops are sought. The question in regard to the potato, as in regard to wheat, cabbage, tomatoes, &c., is not what sorts are incapable of disease under the worst conditions of culture, but what sorts will proba bly suffer least under given circumstances.

113. Men of limited experience, and ignorant of simple, pathological laws, have frequently announced remedies for this disease. But, except in so far as they have sought for strong seed, and have cultivated wisely, and with a due regard to exposures, early maturities, &c., nothing has proved even a temporary remedy. Mowing off the vines when the tubers were large enough to be worth saving, has been availing; but in this case the cultivator is often held back, in the hope, which is often realised, that the disease will be light. In this case the crop should be left untouched, since after the pressure of moderate disease is passed, the crop will recuperate

and result in a fair yield of at least moderately healthful tubers. All modes of bending down and covering the vines, to prevent the mildew from dropping upon the soil immediately over the tubers, are, I think, false in theory, as they are costly and destructive in practice. Where a crop of say two bundred bushels to the acre must be sold, on the average, at thirty cents per bushel, the farmer cannot afford to practice any costly modes of cultiTation.

114. The practice of cutting and drying the seed is an old one in this country with market garderners. The reason of the practice though usually not understood, was doubtless this, that thus hybernation, i.e., the change of the starch of the tuber in the direction of sugar, is carried to a higher point than when a tuber is taken freshly from a cold cellar and put, at once into the soil. Seed so cut and dried germinates with great rapidity where the planting is carefully performed, and the soil and spring weather are not too dry. Thus the cultivator gets his crop ready for market at an earlier day than where seed is treated in the common way. I have, however, after some experience of my own in this line, yet to learn that it has any influence on the health of the crop. Indeed, all such extraordinary treatment of seed tends gradually to strain its constitutional power, and depreciate it.

115. Whatever may be true in nicely conducted experiments, the theory that the seeds of the fungus lie dormant in the tubers in the cellar during the winter, ready to break out during the succeeding summer growth, is not true in practice. Thus in my own experience, the seed grown in the very bad seasons of 1850 and 1851, when planted in the fine season of 1852, gave very healthful crops. So the very morbid weather of 1855 and 1857, was followed by the healthiest of crops in 1856 and 1858, they being both good years.

116. The first primary cause of disease discussed above (22—34), suggests the only hopeful remedy, viz.: the procuring of varieties possessed with the highest constitutional vigor. This involves two modes, importation and reproduction.

FIRST REMEDY FOR POTATO DISEASE IMPORTATION. 117. This plan of importing from some more genial climate new and vigorous sorts, was early suggested. How far that suggestion has been followed out by others, I have no means of knowing. I received in 1851, from Mr. S. P. Swain, of East Chester, N. Y., a variety called the Wild Peruvian, said to have been recently imported. In vine, tuber and habits, including health, it exceedingly resembled the old Meshanock. Of course it could not be relied upon in a sickly season.

118. In 1848, I received a variety from Bogota, South America, situated on the elevated table lands of the Andes, in five degrees north latitude, and about 8,500 feet above the ocean level. The summers at that elevated point are not as hot as ours, but they have no winters,-peppers, cabbages, and probably potatoes grow all winter. This variety was so entirely too late in its maturity as speedily to decline, though when first received it was very hardy. It bore balls freely.

119. In 1850, I received another variety from the same place, of a little

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