We are sorry to be obliged to say, nothing satisfactory has yet been settled, regarding the nature of either of these disease or the remedy.

If our correspondent wishes us to add our speculation, to the pile of speculations already before the public, it is at his service.

We believe both the potato disease and the Sycamore malady to be not the same disease, but diseases owing their origin to causes quite similar. We think them both the result of an attack of the growing parts by peculiar fungi, the seeds of which are invisible to common eyesight, floating about in the air. Wherever these seeds of fungi light upon vegetation to which they have a natural affinity, they take root in the young vegetable tissue - propagate themselves and gradually destroy the healthy functions of the plant. In the potato, the fungus attacks the tops, but its decomposing influence is not confined there. - like mould - which is a species of fungus - its influence, so destructive to the life-tissues of the plant, penetrate to the root and appear there in the form of the rot. In the Sycamore, the smallest and tenderest young shoots are first attacked - the poison of the fungus thence gradually extends in a blackened filth-line down the branches, directly in connection with the young shoots, until at last the whole tree is poisoned - healthy vital action ceases, and the trunk dies.

The fungus ripens its invisible seeds in these decaying plants and trees; these seeds floating in the air seize upon other healthy trees, and thus, little by little, the disease extends all over the country.

Some fifteen years Ago, Buttonwood disease appeared at the South. Ten years ago it began to be fatal in Philadelphia. At that time it had not reached New-York, where the trees were still green and flourishing. It gradually spread northward, it has since reached Canada and will extend all over the continent. The only mitigation of it seems to be in severely heading back the whole top of such trees as arc affected, boring a hole in the trunk, filling it with sulphur and plugging it up tightly. We have known trees so affected put out a new head and recover a healthy appearance again. But there is now little doubt that the disease will exterminate the present generation of Plane or Buttonwood trees from the United States altogether. It is a little curious that the plane tree of Europe (Platanus orientalist though so closely resembling our native button wood, is not liable to the disease though standing near affected trees. We have had an opportunity of observing this in our own grounds, and were told in England last year, that a long time ago this very plane tree disease appeared in England and swept off most of the American species (P. occidentalis,) while the European plane tree remained untouched.

Such being the case and the growth of the oriental plane being the more ornamental of the two, no one will plant our native species for the present - but select the oriental and especially the pyramidal plane tree, now to be had in some of the nurseries.

The potato disease has extended gradually but rapidly in the same manner all over the world. At the present time its effects have raised the price of potatoes, as an article of food for winter use, nearly four-fold in many parts of the Union.

We notice that recommendations have lately been made of the use of powdered sulphur in the hills when planting. If it could be sufficiently pulverised and divided by mixing it with ashes or some such substance to render its use feasible in an economical point of view, we should think it more likely to answer the purpose than any other substance - simply because we know that sulphur is the only remedy for certain kinds of mildew and blight - the result of the attack of fungi - yet successfully applied. The chief point, therefore, the remedy being known, is to discover how to apply it with practical benefit.

And how is it that these fungi suddenly make their appearance all at once and spread all over the earth - readers will naturally ask? It is not easily answered, the most probable solution seems to be that they are the result of some electrive agency, and are deposited on the earth by its agency. The phenomena of colored rain, which has been observed and carefully examined by naturalists in various parts of the World, seems to us to bare some relation to those mysterious and sudden eruptions of vegetable disease. In many cases recorded in the Journal of Science, this rain has fallen when the sky was unclouded - showing its direct dependence upon electric phenomena. In 1845 a shower of inky black rain fell in a district in England, and was believed there to be the origin of the potato disease. Prof. Bailey, of West Point, one of our closest microscopic naturalists, has recorded two instances of colored rain which have fallen in this country, in Silliman's Journal. His analysis of this rain showed it to be composed mainly of the pollen of Pine trees - but an analysis of some colored rain which fell in England, in 1849, showed distinctly the spores of fungi.

The whole subject is still far from being understood, but it is one which is taking a shape so serious to the cultivation of the soil, that men of science should bestow more attention upon it

Remarks #1

This is a very interesting question - the duration of varieties - and one which we shall be glad to see discussed by practical men in our columns.

It is not a little curious that a variety that is considered to be worn-out by cultivators in an old and long settled part of a country, if taken to a new country, or new soil well adapted to it, will immediately resume all its original vigor. This has been lately illustrated by potatoes of old and favorite sorts, that could with difficulty be preserved from the rot in this part of the United States, but which, taken to California and planted, produced immense crops of potatoes, of very large size and unrivalled excellence, entirely free from rot, and showing all the habit of the most healthy new variety. It would appear from such facts as these, (which have given rise to the practice so well known among farmers and gardeners of "changing seed,") that the variety wears out the soil where it is grown before it becomes deorepid in itself.

Mr. Loudon, in the Suburban Horticulturist, the work on gardening which he published just before his death, gave an excellent expose of the different views on this subject in the following paragraph.

All the plants of a variety which have been procured by division, for example all the plants of any particular variety of grape, apple, or potato, being in fact, only parts of one individual, it has been argued by Mr. Knight, that when the parent plant dies, all the others must die also; or to put the doctrine in a more general form, that all varieties are of but limited duration. This opinion, though it has been adopted by many persons, has not met with the approbation of Professor De Candolle, who says that the permanence of the duration of varieties, so long as roan wishes to take care of them, is evident from the continued existence of varieties the most ancient of those which have been described in books. By negligence, or by a series of bad seasons, they may become diseased, like some of our varieties of apple or potato; but by careful culture they may be restored, and retained, to all appearance, for ever. We are not sure that De Candolle's theory will hold good with the finest fruits and florist's flowers. The species might be recovered, but we question whether in many instances that will be the case with the variety. Perhaps a hypothesis might be devised which would coincide with both authorities.

It would coincide with that of De Candolle, if Mr. Knight had spoken with reference to actually wild varieties only; but with regard to improved varieties, as they are understood in a horticultural point of view, they are doubtless prone to decay, in proportion to their degree of departure from the physiological perfection which enables the wild variety to maintain itself continually on the surface of the globe, independent of the care of man. A wild variety will produce seed under favorable circumstances, but many highly improved varieties, in a horticultural sense, do not perfectly mature their seeds under any circumstances whatever; and, therefore, must be physiologically imperfect, and being so, a priori, if it be admitted that imperfection is a principle of decay, it will not be denied, that no plant imperfectly constituted can carry on its functions but for a more or less limited time, even under the most favorable circumstances,