An interesting instance of the predisposing influence of an excess of manure, is mentioned in the Agricultural Gazette. Wheat which had been top-dressed with guano, " was a good crop, and free from symptoms of blight, excepting in the spots where the sacks were put down; here the straw was blighted, evidently from receiving an overdose." Now, all these different circumstances would tend to make wheat plants unhealthy, hence the attack of fungi - it cannot be attributed to atmospheric influences in these cases; since wheat growing in large open fields, or on the higher parts of fields, or on clean fallows, instead of on land recently manured, or if plants growing at proper distances apart, escaped. The atmospheric conditions under which the healthy and blighted plants were growing, must have been the same in most of these instances, but other conditions which affected the health of the plants, were not the same; the inference therefore, is, that the latter ore most likely to be the conditions which led to the attack of

Andrew Knight considered that one of the principal causes of mildew was the want of sufficient moisture in the soil, more especially if excessive humidity in the air, and low temperature succeeded warm bright weather. The pea when cultivated late in the fall is very liable to be attacked by mildew, and Mr. Knight found that by deepening the soil and by copious watering he could prevent its appearance. In a forcing house he found it equally easy of appropiate management to introduce or prevent the appearance of mildew. " When be had kept the mould very dry, and the air in the bouse damp and unchanged, the plants soon became mildewed, but when the mould had been regularly and rather abundantly watered, not a vestige of the disease has appeared." The development of fungi in these cases also was obviously not dependent solely on certain atmospheric conroots or by the leaves, it is probable that both the early and late varieties were inoculated with the germs at the same time, but in one the conditions were favorable for their development while in the other they were not favorable, and the difference must obviously have been owing to some internal cause as both were subject to the same external influences.

It has been commonly observed that varieties of the potato are usually attacked as they approach maturity, as if a certain cessation or decline of the growth of the plant favored the growth of the parasite; varieties of the potato are not however invariably mildewed at a certain stage, or when the plants have nearly perfected their tubers, neither is wheat and other grain crops always, though generally, blighted by rust or mildew when the plants are in ear. The vitality of an annual plant like wheat, or of the potato, whose stems endure but for a season, is probably more energetic at the commencement of their growth than when they have nearly performed their functions, and they may in consequence be better able to resist the action of ordinary external influences at that time than afterwards. If this be so, then it is not unlikely that different varieties of our cultivated plants growing under the same circumstances may offer greater resistance to attacks of fungi than others seeing that they possess different degrees of constitutional vigor.

This is a point in the inquiry of considerable interest, whether according to the vigor or controlling power of the vital principle of a plant, so will be its power of contracting the action of ordinary external influences and its comparative freedom from disease arising from attacks of fungi.

Many of our garden plants are propagated by extension, that is by buds, cuttings, layers or roots instead of by seeds. - and notwithstanding the "tale of woe" revealed to your correspondent, Mr. Marshall, by the gallant old apple tree, which in spite of its age and infirmities persisted in believing that it retained the vigor of youth, I may be permitted to say that the evidence of apple trees and other plants seems to me to afford substantial grounds for coinciding with the views advanced by Andrew Knight, that each plant propagated by extension has a limited duration, that it cannot by any known means be continued equally healthy and vigorous forever; but that sooner or later the progeny will gradually decline in vigor become unhealthy and unproductive, not suited to the purposes of the cultivator and consequently extinct. This also suggests an interesting branch of the inquiry, whether as a variety declines in vigor it becomes more subject generally to attacks of fungi, or if an attack is more virulent and damaging than on younger and more vigorous varieties, growing under the same circumstances.

The hop is generally propagated by cuttings, and a writer who has raised plants from seed, in consequence of his old plants dying off, and the hops becoming small, and of a bad color, said, "We write from experience; having raised very many hop plants from seed, we hare found them to be much more energetic and vigorous than those which have been raised from cuttings, their luxuriant growth enabling them to withstand the effect of blight."It is well known that some varieties of the potato have suffered much less from the blight than others, while some varieties recently raised from seeds, in the United States and Prussia, are said to have continued free from the disease, though growing near to others which were infected. I believe the potato plant, considered in the mass or as a species, to be in an unhealthy and degenerate condition; that it is, in fact, heredeterarily diseased, the accumulated result of maltreatment and neglect in the culture of the plant, reach of controversy, though possibly not of cavil. The unprecedented attack on the potato, so much more extensive and virulent than the usual attacks of fungi on other cultivated plants, I consider to be the result of its previously unhealthy condition.

If these views are well founded, then the question, What are the conditions required for the growth and increase of parasitic fungi? has a great practical bearing, if considered with reference to this plant only, because if their growth and increase does not depend solely on atmospheric influences, but is favored by the unhealthy state of the larger plant, then we may reasonably hope, that by restoring the plant to its pristine vigor, with judicious selection, through several successive generations of seedlings, with improved culture, we shall obtain varieties which, when propagated by divisions of the tuber, will, for a time at least, resist the attacks of their parasite, just as wheat, turneps and other crops annually raised from seeds, now do; further than this we are not justified in anticipating. What we know of the attacks of fungi on other crops, forbids the hope now that the parasite of the potato is established in this country, (for several facts seem to indicate that it is a recent introduction to North America and Europe,) that our crops will ever be entirely free from mildew when the potato is restored to the highest rate of health which it is capable of attaining.