IN this age of inquiry and progress, it is surprising that so little attention has been paid to the habits of parasitic fungi, and that so little is really known respecting the conditions which favor their growth and increase. To the farmer and gardener the subject is one of much importance; not a season passes without much of their labor being rendered of no avail by the influence of these almost invisible destroyers. How great, for instance, is the loss annually sustained by the attack of rust, smut, and mildew, on the wheat crop alone? and who can estimate the value of the food destroyed within the last five years, by the attack of the parasitic fungus Botrytis infestans on the potato? Almost every plant we cultivate is liable to be atacked by fungi, and he would certainly be entitled to no mean place among the benefactors of mankind, who should discover a means by which their development might with certainty, and at will, be prevented. The powers of man may not be permitted to extend thus far, nevertheless, there are grounds for concluding that by patient inquiry, and well-considered experiments carefully conducted, we may ultimately attain greater control over them than we now possess.

Different plants, as well as different animals, have their peculiar parasites, some parasitical fungi will indeed prey upon many different plants, but the attack of a species is generally confined to a certain natural order of plants, or to a genus, or to two or three species of a genus; whilst some, as in animals, seem to exist on a particular part only of one species. The parasitical fungus which is the cause of the mildew of wheat, will not live upon turneps; that which infests the turnep will seize upon the cabbage, they being nearly allied plants, but it has no power over the potato; yet the parasites of the turnep and the potato are nearly allied. For a plant to be attacked by fungi, two things, then, are required; the presence of the reproductive germs of its peculiar parasite, and the conditions required for their growth. Hence, one apparent means of securing a crop against an attack of fungi, is to obtain seeds of plants free from their parasite, and to grow them on land where no similar crop has been previously, or at least recently grown; a second mode is to destroy the germs, if seeds or plants are known to be infected, as is commonly done in the case of wheat.

It has been proved experimentally, that wheat may be inoculated with oil in germs of fungi, and thus destroy them. In old nurseries, haying a tenacious soil, and where healthy pear seedlings cannot now be raised, it may be well to try the effect of paring and burning the surface soil where the pear seeds are intended to be sown. If the burning is so managed as only to char the organic matter, the soil might be freed from the germs of fungi, and its productive powers at the same time increased.

It is doubtful, however, whether these precautions will in all cases insure the safety of a crop. The spores of fungi are probably admitted into the interior of plants, by the stomata or breathing pores of the leaves, as well as by the roots. Leaves inhale gases and absorb moisture; and how very minute must be the reproductive germs of a plant, when the plant itself, in many instances, is so small as only to be seen distinctly by the aid of the highest powers of the microscope. If they are admitted into plants by this means, then it is an important question to determine what are the conditions required for their development, and whether any of them are under our control, and by what means they can be avoided. We shall have the surest ground for hoping successfully to subdue this evil, if we can only discover the secret of its power. I think it may be assumed that fungi have not the power to destroy healthy vegetation - if they had, where would be the limit of their ravages? The Rev'd. M. J. Berkeley, who is highly distinguished for his knowledge of fungi, has observed young plants of Botrytis infestans springing from within the cells of a potato.

As the potato tuber is considered to be formed exclusively of matter which has been prepared by mature leaves, these germs could not have immediately vegetated on entering the potato plant, but were probably carried with the elaborated sap of the leaves, and deposited with it in the tissue of the tuber. Mr. Berkely has further stated that "it seemed to him most certain, from observation of those fungi which grow from the tissues of plants, that minute particles, too small to be distinguished by the highest powers of the microscope, must be carried about with the juices, and when fitting circumstances concur, proceed to act on the tissue with which they are in contact." What then are these fitting circumstances? The two tribes into which fungi may be artificially divided, have separate tasks assigned to them. The office of one is obviously to hasten the decay of matter which is already decaying; the office of the other, I believe, is to prey upon or hasten the death of that which is unhealthy. Those which flourish on dead organic matter, appear only when decay has commenced, not while it is yet fresh - a fact well known to many. "Fungi," says Mr. Solly, "are only developed in those solutions which are in that state of putrefaction favorable to their growth; moreover, they do not appear till the solution has acquired that state."There must, I believe, be unhealthy action, possibly some slight chemical change in the fluids of superior plants, before parasitic fungi can successfully attack and destroy them.

Mr. Berkeley, however, whose opinion is entitled to much deference, considers that the growth, and especially the numbers of fungi, depend upon certain atmospheric influences. There can be no doubt, whatever, that the state of the weather has considerable influence on the development and increase of fungi in ordinary cases. For instance, in moist, dull seasons, crops are observed to be always more or less affected by mildew. Any sudden check in the progress of vegetation, such as is caused when hot sunny weather is succeeded by calm dull days, or by a sudden transition from weather favorable to rapid growth, to weather cold and wet, is generally considered the precursor of blight, and favorable to the growth of fungi. Now, circumetc., predispose plants to an attack of fungi, independent of atmospheric influence; thus we read respecting the diseases of wheat - "it rarely happens that blight, rust, and mildew are felt in sunny seasons, except in confined enclosures, or marshy ground, where the evening dews stagnate, and fogs are generated." Again, "in looking over a blighted field of wheat, we may observe that the lowest and richest parts, or where it stands thick upon the ground, are more affected than those which stand higher." Wheat to which manure is directly applied, is found more subject to rust and mildew, than that which is grown upon a clean fallow.