We commend the following interesting and valuable article, to the notice of our readers. Ed.

A. J. Downing, Esq. - Dear Sir: When considering various explanations which had been advanced to account for the disease of the potato, known as the " curl," I have been led to inquire whether varieties of plants, as they become old, do not afford other evidence, beside that stated in my previous paper, of a progressive diminution of vital power, leading to functional derangement, debility, and death.

I am doubtful whether I shall not be trespassing too much on the patience of your readers, by recurring again to this subject; but the question I conceive has a great practical bearing, and if the view I now send you, of the effects of age, should prove on further investigation, to be well founded, the most sceptical can hardly be otherwise than convinced, because of the exact and conclusive evidence it will afford of the truth of Mr. Knight's conclusions, respecting the limited duration of varieties of plants propagated by extension.

It would seem that the potato, in the earlier years of its culture in Europe, was either entirely or comparatively free from disease. The first notice I have met with of the " curl," (a disease so called from the leaves contracting or curling, instead of expanding,) is in a paper in the Transactions of the London Society Of Arts; wherein it is said, that the disease was probably first noticed in Lancashire, about 1764; about that time a man observed a few plants in his crop which decayed, or seemed to ripen sooner than the rest, and he straightway concluded that somehow he had luckily obtained a new and early kind; be accordingly marked the plants with a view to cultivate them, but was much disappointed and perplexed by the result of his experiment. Baron Hepburn, in a communication to the Board of Agriculture, said the curl was unknown in Scotland before the years 1778 or 1779. Dr. Anderson, in an essay on the potato, in the Bath Papers, remarked, that the only thing which seemed to be positively certain with regard to curl, is, that it was not known in the northern parts of the country, till a very few years ago, and at that time it was much less prevalent in the north than in the south.

Towards the close of the 18th century, the curl prevailed in the potato crops to such an extent, as to give rise to much discussion, and many experiments, with a view to discover the cause of the malady, and by what means it could be prevented.

Many observations might be cited to prove that this first mild form of disease of the potato, could not be attributed solely to any peculiarity of soil, season, or mode of culture, but that it was peculiar to, and therefore inherent in, certain varieties for the time being. I learn from two prize essays in the Transactions of the Society Of Arts, for 1790, that it was known at that time, that certain varieties only were subject to the disease; that it was hereditary, and that the only effectual mode of getting rid of the evil, was by discarding the affected varieties. One of the writers, Mr. Pitt, said, " the curl in potatoes is doubtless owing to degeneracy - to the particular variety being worn out. I have known three to fail by curling in this county, (Staffordshire.) The national remedy, therefore, is, the raising and reproducing fresh varieties, a practice which has never been interrupted by any difficulty." Mr. Holt, who wrote from the neighborhood of Liverpool, observed, " the cause of the disease, so far as lean learn, appears to be nothing more than a degeneracy of the plant.

This district, for some years, suffered great injury from curled potatoes, but few crops of late years have failed of being much infected with this disorder, for whenever the curl has appeared, in ever so small a degree, that stock has been rejected by the attentive cultivator, and new seed obtained." Hence the conclusion based on these facts, as we read in Martyn's edition of Miller, " the circumstances of the old sorts being now almost entirely cut off by curl, renders it probable that the disease is incident to declining varieties of potatoes, as canker is to declining varieties of fruit".

About the time the curl was so prevalent in England, it seems to have prevailed to a considerable extent on the continent of Europe, also. A reward of 1,200 francs was offered in 1775, by the Royal Academy of Brussels, for the best treatise on the cause of the disease. The prize was awarded to a writer who concluded that it was the result of a degeneracy of the plan trowing, as he supposed, to its being an exotic. He advised that Dew varieties should be obtained from Virginia, the potato being supposed at that time to be indigenous to that country. His advice was followed, and the remedy proved efficient. So the Belgian, like the English cultivators, found that the most effectual, or only certain means of restoring their crops to health, was by substituting healthy varieties for those which were subjected to the disease, Many well-informed men have concluded that curl is caused by the over-ripening of the seed tubers, and the facts are certainly too numerous, and too well authenticated, to admit of doubt, proving that the state of ripeness, or rather the dry condition of the tuber, does exercise a considerable influence on, if it is not the immediate cause of the curl.

The authors of several of the earlier papers for instance, observed, that when the curl was rife among the crops in rich, low-laying, early soils, it had never been experienced in neighboring hilly districts, having a northern aspect, where vegetation was more backward, and where the crops had not the same chance of becoming perfectly ripe. It was also frequently observed, that curled plants proceeded from large, hard tubers, which did not decay in the ground, as usually happens. Others had noticed that small potatoes, which had been thrown aside for pigs, but which were planted for the want of a sufficient number of sets of larger potatoes, droduced entirely healthy, smooth-leaved plants.