Dear Sir: - have at length completed my series of observations on the insects and diseases of the Plum, Cherry and Peach-trees, a portion of which I now send for your most popular and valuable journal. Though most of the isolated facts may have come under the notice of observers, I believe no one has hitherto put them to so careful a test as I have been able to do in the last two years, trusting to nothing but ocular demonstration and tangible proof; the diseases of the trees and fruit have been carefully watched and noted down on their first appearance, and the insects all developed under bell glasses, so secured that nothing could get in or out after the branches of the trees or their fruit had been placed within them.

In the first place,'I will begin with the insect called plum curculio, hurt whoso real name is Rhynchamus nenvpltar, (No. 1,) belonging, like the curculio, to the weevil family. The individual figured in the accompanying frontispiece was developed from a grub feeding in a plum, and placed in a tumbler of earth on the first of June, in company with several other plums, all diseased; the grubs left the plums from one to four days after the fruit had fallen from the tree, and immediately entered the earth; one selected a spot on the side of the glass, enabling me to watch its progress and manner of forming its cell. It began by moistening the earth around it, until the cell was large enough to turn in comfortably; it then smooth-plastered the sides by emitting more fluid, and rubbing them with its rounded back: this was a work of two days. In these cells, apparently impervious to heat and moisture, the grubs lay during the period of their change, which in one was only four days, while the remainder appeared at various times until the 1st of August. The beetles thus early developed return to the trees, and complete the work of destruction by depositing their eggs in all the remaining fruit.

The second brood fall to the ground with the injured fruit, at their appointed season, enter the ground, form their cells as their parents had done, but remain unchanged until the following season, when they emerge from their winter home at the period when the fruit has set, and in a condition to form food for their future progeny. The crescent-shaped blemish on the plum No. 11 was formed by the snout of the beetle cutting the skin, which it carefully raised, then turning round it deposited an egg, and replaced the skin with its snout, first pushing the egg into the flesh of the plum, and then replacing the skin over it, to exclude the moisture of the atmosphere. There is never more than one egg deposited in one plum, the insect going from plum to plum, until her whole store is exhausted. Should the beetles rise from the earth before the fruit is large enough to form a nidus for their young, they deposit their eggs in the tender branches of the prune plum or morello cherry; why these peculiar trees are selected by the weevil, is a question yet to be answered.

In this neighborhood, the swellings Nos. 6 and 7, on the cherry and plum branches, began to appear about the last of May and beginning of June, and attained their full size about the middle of June; at this time the sap-vessels of the tumors were enlarged and distorted, and filled with sap, and the young grubs of the Rhynchoenua nenuphar began to appear. From this time the grubs grew rapidly, and fed voraciously. From one to six grubs were found in the tumors, according to their size. The section of a tumor, No. 8, was examined on the 10th of July, when the drawings were made; No. 9 is one of the larvae taken from the tumor; No. 10 is a magnified drawing of the head of larva No. 9. At the same time grubs were taken from tumor No. 6, on the plum branch and plum No. 11, which proved, when examined under the microscope, to be identical with those found in the cherry tumor, deciding the question, and proving that the Rhynchomus nenuphar is not only the cause of the fall of the fruit, but of the tumors in the branches.

The figures Nos. 2, 3 and 4, are magnified sections of the mouth of the perfect insect, No. 1.

Dr. William Hammond, the eminent microscopist, of the U. S. Army, to whom I am indebted for all the magnified drawings, has proved that the fungus growth on the tumors of the cherry and plum-trees is not the cause of the swelling, as Harris and others have supposed, but a congenial locality where this species of fungus finds its proper food. It is the insect which causes the tumor. The remedy is simple, but indispensable. As soon as the swelling is perceived, it must be taken off and burnt, and the plums gathered and scalded, or eaten by swine. I will here let Dr. Hammond speak for himself:

"Philadelphia, July 26th, 1859.

"Mr Dear Miss Morris: In accordance with your request, I have submitted the warts, or knots, found on the plum-tree, to microscopical examination, and, as you anticipated from your researches, have ascertained that they are not caused by a fungus. The tissue of these knots is identical in essential characters with that of the bark of the tree, except that there is not the same systematic arrangement of the elements as exists in the normal condition. Thus, on making a longitudinal section of a knot, the dotted vessels are observed both in vertical and transverse sections, and of course are much twisted and deranged. On the outside of the knots is a deposit of black matter, consisting of the Sphceria morbosa of De Schwinitz, and, growing upon that, another fungus, belonging to the genus pucchiia, probably the P. lateripes. This latter, however, is only found on certain portions of the surface, more especially in the furrows, though in a few specimens it prevailed over the whole exterior.

"There is not the least evidence of the presence of fungi in the interior of the knots. After careful examination, I am able to be positive on this point. The tissue of the knots is, as previously stated, the same as that of the bark of the tree; and from the chambers found in this, and from the character of the matters found therein, there is no doubt as to the fact of their having been inhabited by insects. As I have found the sphoeria on parts of the tree where there were no knots, it can scarcely be possible, that they owe their origin to the growth of the fungus. "The accompanying drawings will explain more definitely the appearances presented. Pig. 1 represents the internal portion of the knot; Fig. 2, the dotted vessels found more exteriorly; and Pig. 3, the external layer of sphoeria. In Fig. 4, the secondary fungus is seen.