The expedients which at various times have been resorted to with a view to prevent the disease, such as by obtaining the seed-tubers from late situations, or by raising them before the haulm had naturally decayed, or by planting late in the season, so that they could not have time to ripen, all indicate that under-ripe watery tubers afford the most healthy and vigorous plants, and some security against the disease.

The influence of the dry state of the tuber in producing curl, has also been proved experimentally. Sets taken from the waxy, or least ripened end of a long kidney potato subject to curl, were found to produce healthy plants; whereas, sets from the opposite dry end of the same tubers, did not vegetate at all, or produced curled plants. Mr. Knight conceived that curl originated in the preter-naturally inspissated state of the sap, and he, from a number of tubers, the produce of wholly diseased plants, carefully detached the shoots when about three or four inches long, and planted them; as they had now little to subsist upon, except water, not a single curled leaf was produced, though more than nine-tenths of the plants which the same identical tubers subsequently produced, were much diseased.

There can hardly be any question, then, that curl is in some way induced by the perfectly ripe or dry state of the seed-tubers. But then it seems equally certain that the potato was formerly free from this disease, and that varieties do not become subject to it till they have been some time in cultivation. Sir John Sinclair, in his work on the potato, said, " if continued too long, they are liable to disease, as the curl." Sheriff, an eminent Scoth farmer, observed, " time or old age, never fails ultimately to bring on the curled or shriveled disorder." How are these seeming inconsistencies to be reconciled? Either the potato formerly, or varieties in the earlier years of their existence, never ripened their tubers, or perfect ripening alone is not sufficient to account for this disease. There must be some other undiscovered agent at work, which has power over those plants only that are the produce of ripe tubers of aged varieties; or else, in the progress of time, a change takes place in the tubers of a given variety; the texture must become more solid, the fluids thicker and less abundant, and therefore incapable of supporting healthy vegetation.

A given species of plant requires a certain range of temperature, and a certain amount of light, to enable it to grow in a healthy and profitable condition. The Palms of the tropics will not grow to any useful purpose in the United States; nor will our apple trees thrive in the tropics. An excess, or a deficiency, of heat and light, are alike injurious; both lead to functional derangement, ending in general debility. A gardener, on receiving an exotic plant new to him, would desire to know its native country, and what soils and situations it preferred in its wild state, in order that he might determine what mode of culture would be most likely to be suitable to it. But different species differ widely in their power of adapting themselves to different climates, and this the gardener can only learn by experience, aided by his knowledge of the geographical range of the plant in question. Does our knowledge, then, of the native country of the potato, and of the various climates in which it is cultivated, warrant the conclusion that the curl disease is caused simply by the over-ripening of the seed tubers? that is, (if I understand correctly what is implied,) is the amount of heat and light of European summers, greater than the plant naturally requires for its healthy growth? At the first glance, it seems hardly probable that a plant which is a native of the tropical regions of South America, should be over-ripened in the climate of England. Of all cultivated plants, the potato is most accommodating.

It is cultivated in every latitude from the torrid to the frigid zone, and if it is liable to be so over-ripened in England, as to cause it to be diseased, then what should be the condition of the plant when grown in the West India Islands; in the burning sands of the Gape, or under the hot and brilliant summers of the United States? The quantity and quality of the secretions of a given plant, and the solidity of its tissue, depends partly on the amount of light and heat to which it is exposed, and if the concrete state of the sap, or the dry condition of the tuber of the potato, which gives rise to curl, was simply or solely the effect of over-ripening, in England, it should follow that the plant would be useless in the United States of America, and the West Indies, or at the Cape.

I am not aware that any satisfactory evidence exists, to show that the ripe state of the tuber of an aged variety induces a certain condition of the plant, which is favorable to the attack of .some animal or vegetable parasite; but there are facts on record which seem to justify the conclusion that a change does take place in the tubers of a given variety, when in the course of time its vigor declines, the tissue becomes more solid and drier, and the fluid thicker and less plentiful; or in other words, the tubers of a given variety become more dry and farinaceous.

In animals, it is well known that certain changes do take place as age creeps on. The bones gradually become more solid and brittle; the muscles more rigid, and the fluids thicker and less abundant; and various important organs, on the due exercise of which health depends, become impaired, and incapable of performing efficiently their respective functions. The opponents of Mr. Knight's theory have dwelt much upon this fact, when doubting the accuracy of the conclusions at which he had arrived respecting the limited duration of individual plants. An animal, say they, becomes worn out, or dies, when old, in consequence of a structural change in many of its most important organs; but in plants or trees there is nothing analogous to this. I suspect, however, that there is a closer analogy between plants and animals in this respect, than has hitherto been supposed to exist.