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use of many varieties for seed, and their culture in various ways and in a great variety of soils and exposures. Men too, given to profound investi. gations in the laboratory, have not sufficiently appreciated the relative power of vitality and chemical laws.

19. Degeneration, in most of the old varieties and in many new seedlings (derived from those old varieties by but a single reproduction), and severities of weather are doubtless the two primary conditions of disease; but with them many minor conditions are associated which give enhancement to them.

In this statement of causes, I think every case of disease which I have ever witnessed admits of a satisfactory solution, notwithstanding some recent writers have ridiculed all such explanation, without, so far as appears, the patience for a candid examination.

VII. TAE TRUE THEORY OF THE POTATO DISEASE. 20. The conditions of all vegetable growth are, so far as we know them, the constitution of the plant, soil, water, air, heat, light, and usualty cultivation The proper co-operation of these seven conditions is the security of the health of the plant, while the excess, deficiency, or irregularity of the last six on the constitutional susceptibility of the plant, must cause all ordinary manifestations of disease. I cannot conceive that there exists a thoughtful, intelligent, honest mind that can gainsay a proposition so simple and so obvious. This proposition, however, seems to me to embody a whole system of vegetable pathology in the compass of a nut-shell. Without taking up these seven topics separately, I will yet contemplate the potato, in its liability to disease, in such a manner as shall include them all; taking up in order (A) the primary and (B) the secondary causes of disease.

21. A. The two primary causes of disease are, first, WANT OF VIGOR IN THE CONSTITUTION OF THE PLANT, and secondly, SEVERITIES OF WEATHER. These two causes are the basis of my article on this subject, contained in the "N. Y. Ag. Trans." for 1847. (See p. 425.)

22. First primary cause of disease—THE WANT OF VIGOR IN THE CONSTITUTION OF THE PLANT. This is properly denominated the remote cause. This want of vigor may have been original, or it may have been superinduced by time and bad culture, as I think is true of most of the old varieties of potatoes.

23. a. This want of vigor may have been original.

Whoever will originate a few seedlings from the same potato ball, much more from many balls, and more still from balls of many sorts, and will cultivate them carefully for four or five years, until their habits are fixed and character developed and stereotyped, will find that they differ almost as much in constitutional vigor as in shape and color; some will be as hardy as the Garnet Chili, the Pink-eye Rusty-Coat, and the Andes, while others will be as readily diseased as the Carter, Mercer and Ashleaf Kidney now are.

24. This difference will not be seen in ordinary healthful years, but when disease does appear, it will manifest itself in a ratio of severity conformable to the native vigor of each sort as previously ascertained by the originator, provided the circumstances of culture be strictly the same.

25. Any one such new seedling, whether of vigorous or feeble constitution, being continued in use for seed, becomes the parent of a race of corresponding vigor.

26. b. This want of vigor may be superinduced. Cultivators now find disease prevailing among sorts once kdown to be healthful; some observ. ing old men are sure that sorts which they once knew as vigorous young seedlings, have thus declined.

27. This decline may have been initiated in the character of the seed originally, or in its early development, or in some early injury to the plant, or in the influence of a bad season, which is only the continuation of a specific injury. These causes may accumulate, and their influence, or the influence of any one of them, may be enhanced, year by year, until, by and by, the occurrence of an unpropitious season will reveal the result, dimin ished vitality.

28. The analogies in the other families of the vegetable kingdom are numerous as the leaves of autumn. Thus, among annuals, who has not seen in a yard square of wheat, in a row of corn, tomatoes, cabbages, &c., diversities of vigor of which no account could be given, but that they were natural and congenital? Or among perennials, who has not observed the different health, not only in one year, but through a series of years, in a row of apple trees, grape vines, pear trees, tulip or dahlia roots, &c. ? differences that could only be considered original, or in some way superinduced.

29. Nor less striking are analogies derived from the animal kingdom. Almost every family of children, flock of sheep, or brood of chickens, pre sents diversities of constitutional energy among individual members, diversities that are not only permanent, but transmissible. Nor is it less evident that individual energy wears out by age and hardship.

30. The propriety of drawing analogies from the animal kingdom to support and elucidate positions in vegetable pathology, has been boldly denied. This is a subject mostly foreign to my themes of study, yet I will venture to suggest, subject to the correction of the learned, the following consido rations:

(1) Both animals and plants have a material organization, composed mainly of the same fifteen or sixteen simple substances, such as carbon, hydrogen, nitrogen, phosphorus, &c.

(2) The proximate principles in both are somewhat alike. Thus plants have woody fibre, starch, gum, sugar, and various acids. Animals have fibrin, albumen, gelatin, ozmazome, and numerous acids. .

(3) Each has an anatomical structure, of which the minerals have largely the support, and which exhibits many points of comparison in the vascular system of each.

(4) The mode of nutrition presents many interesting points of comparison.

(5) Both depend on the stimulation of light and heat.

(6) The sexual organization and mode of fecundation are much the same in both.

(7) Both hold important relations to climate.

(8) The causes of disease, arising from original feebleness, over-stimulation, bad location, and irregular weather, are much the same.

fibrino woodyroximate hosphoru

(9) Many plants and many animals are capable of and subject to hyber. nation.

(10) Both plants and animals sleep.
(11) Culture affects both, in regard to variety, size, health, &c.
(12) Both have important relations to motion.

(13) Both are capable of affecting others, and being affected themselves by parasitic condition.

(14) In both, life is a mystery, in which, for a time, vital energy resists chemical law.

(15) In both, age has its limitations.

(16) The results of death are similar; each becomes food to the other, and each, also, food to the other species in its own class. Now, I ask whether it should be considered strange and improbable that there should be striking pathological analogies between two systems exhibiting so many palpabe general analogies?

31. Experience and theory combine to convince the intelligent cultivator of plants that races become enfeebled and finally perish. My appeal here is not to the mere theorist, but to the actual cultivator, acting through a long period of time, and with a diversified experience.

32. An important fact in regard to the potato (the same is true of other plants), is, that in years of marked disease the old varieties may be classified in the order of their liability, so that where the conditions of culture are strictly the same, the intelligent cultivator knows with considerable certainty which varieties will be most severely affected. The fact of such & ratio of liability to disease governs the choice of the cultivator in his selections for seed. He may plant the Mercers and the Carter variety, not with any hope in severe seasons of escaping disease, but because, not withstanding that liability, they are sorts having important qualities for the table.

33. Some, who deny the possibility of a gradual enfeeblement of the constitution of some varieties of potato, have attempted to explain the confessed exemption of some new sorts from disease, on the ground of a thicker caticle on both leaf and tuber. This admission is all I could desire, for what, I ask, constitutes vigor of constitution in any plant? is it not its physical conformation as related to the earth and air in which it lives and grows ? A plant with a strong cuticular covering on its leaves and tubers, will probably exhibit, for the time being, an equally strong bark on its stem and root, and a strong vascular system in the interior of the plant, filled with well elaborated secretions. In one word, it will be a vigorous plant, and of course healthful.

34. Appeal has been made against this asserted tendency to degeneration of vital energy, to certain varieties of the grape, and the pear, which are supposed to be as old as the time of the Cæsars, and which are claimed as still vigorous. Admitting this claim to great age to be indubitably established, I still answer, in reference to such varieties of plants as I suppose are in a declining state: first, it is hardly fair to compare a woody tree, having its permanent home in the soil, with an esculent root, that needs to be dug and stored, usually, seven out of twelve months; secondly, it is not fair to compare the same woody tree, a native of a temperate or cool climate, with a mountain tropical, having a double liability to disease. (See sections 10-12 above).

Thirdly, the natural age of the pear, grape, &c., may be vastly greater than that of the potato. Most plants, both perennial and annual, have their natural limitations of age, as measured by months and years; the same is true of insects and quadrupeds. The question of the limits of natural age is one of fact, and not of theory—a question, therefore, which, in many cases was long since settled. The natural age of the potato at Bogota or Valparaiso (where, after maturity, it hybernates and grows again in perhaps from four to six weeks, if left in the soil), may be long, but in a climate as variable and intense as our own, it may be much shorter. The nasturtion, scarlet runner and Lima bean, are all perennial in tropical climates, but not in our own. The Germans are said to have a rule that no seedling potato should be continued in use longer than fourteen years. But certainly, on pathological grounds, no such rule can be wisely established, since all have not the same original hardiness, nor are all treated with the same care. My Pale-Blush-Pink-Eye is a seedling of 1850, and although not of the highest vigor, and a little liable to dwarf, is now as reliable as ever. The Western Red, Carter, Ashleaf Kidney, Early Pink-Eye, Early Mountain June, Early Shaw and Winter Pink-Eye, have all been known to me twenty, and some of them thirty years, and while most of them do moderately well in healthful years, no one of them escaped severe disease in 1863, so far as I know of their culture. Fourthly, Allowing the asserted great age of some varieties of pears, grapes, &c., and their present good health, it is yet equally true that other varieties of pears and grapes are hopelessly declining under the wisest culture.

35. Second primary cause of disease-SEVERITY OF WEATHER AND CLIMATE. This is properly denominated the immediate cause. Weather, signifies the meteorological condition of a country for a short space of time, as for a day, month or season, while climate indicates its condition permanently.

36. Air, light, heat and moisture in their coöperative influence, make weather. Air and moisture are sources of food to plants, while light and heat are stimulants. A certain happy combination of these elements and influences constitutes what is called good weather, and certain other combinations are called bad weather; since experience shows that the first has a favorable influence on all organized beings, whether animal or vegetable, while the last has an equally unfavorable influence.

37. Vegetable growth is the development and progress of a plant under the influences of the soil and the weather, modified very much, however, in all ordinary cases, by cultivation. The resulting condition at harvest, in regard to health, is usually modified as much by weather as by all other considerations combined.

38. Experience must determine the quality of the weather and climate appropriate to each species of plant. The decision of this question is much facilitated by observing the operations of nature in locating one plant in a tropical region, and another in a cooler one. There is no contravention of this arrangement in the fact that in this climate we are able to cultivate from sixteen to twenty tropical, herbaceous plants. Our summers, though short, are in the degree of their heat nearly tropical. Hence, by selecting varieties of any tropical species that have a short period of growth, and by taking advantage of sheltered positions, and the aid of glass to forward plants in the early spring, we are successful in their cul

ture.

39. It is inferable from what has been said above (sections 7-12) that, in a climate as changeful as ours, and as liable to sudden and severe intensities, the potato will not always make a healthful growth.

40. Accordingly it is a matter of well settled experience, that in seasons when the progress of ordinary warm weather is suddenly interrupted by that which is cold and wet, and continues three or four days with more or less severity, the potato always suffers, especially when nearly full grown. At that time its extent of foliage and the particular stage of its secretions, render it more liable to injury than when only a few inches high, and receiving nutriment from the parent tuber. When sudden changes from dry hot to dry cold weather occur in summer, the potato and other tropical plants suffer usually very little. The reasons seem to be that the warm dry state of the soil about the roots maintains in a good degree a warm circulation through the whole plant, nor is the berbage of the plant chilled half so rapidly as when wet. Another important consi. deration is, that the circulation of the plant is less watery, than when a cold chill follows a rain. The circulation is also less overcharged in quantity, and for both these reasons vital energy will be less liable to yield to the force of chemical law, disposing the plant to a state of fermentation. The fact of this difference of effect between cold wet changes, and those which are dry, is very marked and very instructive.

41. Mean while all other tropicals usually cultivated by us suffer similarly, and generally in a higher degree, because really much less hardy than the potato, although usually receiving more careful culture. I have not room here to detail the indications under which they suffer. They are familiar to every market gardener, and are pretty fully described in my essay in the "Transactions” of 1847, pages 442-3, and in that of 1848, pages 411-14, under the head of the collateral argument. The influence of such weather ou tropicals at midsummer, when it continues only three or four days, is a little like a similar influence late in September, under which, without frost, tropicals frequently perish after eight or ten days of suffering.

42. Again, when the ordinary weather of summer, especially from the middle of July to the middle of August, is marked by a succession of warm rains, which keep the earth saturated with moisture, and the foliage of the plant almost constantly wet, the showers being intermitted sometimes by burning sunshine, and at others by close cloudy days, the whole period being unrelieved by any dry and bracing weather, the potato is always affected unfavorably. At the same time many hardy plants suffer; wheat and other grains rust, and cherries rot when such weather comes somewhat early, and at a later period apples and plums rot also, walnuts become black at the heart, and cabbage decays in ripening; grapes are also liable to suffer both in fruit and foliage, through a pretty long period, showing like

the potato, evidence of a double sympathy as noticed above (section 10). . 43. Severely hot dry weather never diseases the potato, although less

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