tion in the advance of the disease in the tree, corresponding with changes in the meteorological conditions. Doubtless there are some such variations, but not such as are appreciable on account of passing storms. Even winter weather does not offer complete immunity to the trees. During two winters I have watched the slow progress of the disease in the branches of the same tree and am positive there was no mistake as to the nature of the phenomena. In the first case the gradual destruction was continued during the spring; in the second, progress immediately ceased upon the expansion of the buds.

In many parts of the country, patches of bark upon the trunks of apple and pear trees, often near the ground, are found diseased or dead. Far too often these extend around the tree and put an end to its existence. This is ordinarily supposed to occur in winter. Is this another form of the blight of which we treat?

MICROSCOPICAL.

The organism to which we attribute the death of our fruit trees is so minute that a magnifying power of two hundred diameters is necessary to make out its outline at all, and one thousand diameters are required for careful study. During the course of its development, it assumes various shapes, the different forms usually appearing together in the field of the microscope. What seems to be the most characteristic form consists of two oblong joints with rounded ends, which have a transverse diameter of about Too of a millimetre. The two articles usually seen together have a length of about 5 of a millimetre. They are comparatively thicker than Bacterium termo and their motions are less rapid. They slide forward with a slightly undulating motion, they turn over and on end, but never glide across the field. It is impossible specifically to identify these creatures by form alone. So far as we now know, this may be the same as a common omnivorous little agent 13 which converts sugar, amylaceous matter, lactic, tartaric, citric, malic and music acids and albuminous substances into carbon dioxide, butyric acid and hydrogen, whenever and wherever the conditions permit. Should this prove to be the case, it would not necessarily, if presumably, invalidate its agency in pro

13 Schutzenberger, on Fermentation, p. 209.

A. A. A. S., VOL. XXIX.

38

ducing this disease of the pear; but it might render less hopeful the discovery of remedial treatment.

The most conspicuous change that can be observed with the aid of the microscope, in the tissues affected with blight, is the disappearance of the stored starch. The cell walls are not dissolved, or in any way altered except the staining by oxygenized material in later stages of the disease. The protoplasm of young cells remains, until death takes place. In older cells the thin protoplasmic lining of the walls can be made out after the starch has all disappeared.

Unless the Bacterium does dissolve for itself a passage way through the cell wall, it cannot possibly gain entrance in the adult state; yet they are found swarming in cells absolutely closed so far as a power of one thousand diameters can reveal. In studying this I have used a Spencer's one-tenth of recent construction and have been unable to detect the least corroding of the cellulose wall. It is scarcely necessary to add that the young tissues affected by the blight have in their cell walls no open pores, such as occur in the cells of older wood; neither are there ducts or other channels for their passage.

Accepting Nägeli's 14 theory of the molecular construction of the cell wall, we can only understand how these organisms pass from eell to cell in their deadly work by supposing their germs are less than the molecular openings, hence ultra-microscopical, and that in this condition they pass in water the cellulose barrier, developing into visibility within; or, that in the germ condition, they are of such plastic consistency that, amoeba-like, they are able to creep through the narrow spaces between the molecules. It is well known that the spores of many parasitic fungi, e. g., Peronospora infestans, causing the potato rot, send through the epidermis of the leaf an exceedingly fine tube through which the contents of the spore pass and accumulate in the swelling end of the penetrating tube. From the latter the growing mycelium itself makes its way through the walls of the inner tissues. The minute opening in the epidermis becomes closed, and all indications of the entrance are obliterated. If anything of this kind occurs in the penetrating of our Bacterium it is upon too small a scale to be made out.

14 Sachs' Text Book of Botany, Eng. trans., p. 588.

After a cell has been invaded by the moving particles, the first thing noticeable is the agitation of the starch granules. They swing to and fro like the Brownian movement in small particles. By passing under the microscope a thin fresh section from the healthy to the diseased parts, one can witness the gradual diminution of the granules of starch until they disappear entirely from the cells.

There is absolutely no trace of other fungous growth in the tissues examined by me until after death has taken place; neither have any been found for several days after this in sections of twigs or diseased parts of trees left in the meantime out of doors. In culture baths, ordinary molds soon appear on the affected parts. If the death of the trees were due to the larger fungi like that of the potato rot and wheat rust, surely some one would have found it. None are too small for the modern microscope and its masters, nor too inscrutable to be past finding out!

In very young tissues such as the tips of apple tree shoots, all parts except the epidermis seem to be equally affected; but in older limbs the chlorophyll-bearing parenchyma of the bark is the first and usually the chief seat of the disease. The bast is not affected and not unfrequently a concentric layer of bast cells divides the healthy from the diseased parts. Sometimes invasion through such a close layer is gained by way of the medullary rays. Contrary to the usual opinion, the cambium is by no means the seat of the disease. This layer of thin walled cells often retains its vitality when all outside of it perishes, in which case a fresh layer of new bark is sometimes formed and the limb or tree survives. When the disease proceeds upward, as it often does, the xylem or wood inside the cambium is usually stained by the ascending water colored in its passage through the dying and brown parts. The staining is especially noticeable in the fibrovascular bundles which turn out into the leaves, just where the water rises to supply the transpiration from the leaf surfaces. This stained wood may be still healthy in every particular; there is no evidence of the progression of the disease in the wood itself. The leaves however are invaded from the bark and doubtless may be on the other hand the starting point of the infection. have no direct experiments upon this point, but have found infected and dying leaves on healthy stems. During dewy nights

infected leaves become smeared with the exuding virus which upon drying looks like varnish. This dried material retains its vitality for some time, but how long is not known. When moistened with water, the bacteria move as usual and otherwise exhibit every characteristic of life.

CHEMICAL.

My colleague, Professor H. A. Weber, kindly undertook to dedetermine for me the kind of fermentation which takes place in the diseased bark. He found, first, that carbon dioxide is abundantly given off, and that in this respect the affected bark differs wholly from that in the healthy condition. Secondly, butyric acid was determined as a considerable product of the fermentation. This turned my attention to the published accounts of the butyric fermentation of organic products. Among the earliest and best known are those of Pasteur 15 of France, upon sugar and calcium lactate. He traced the changes observed to the action of a minute organism, which received the name of vibrion butyrique.

In 1850 Mitscherlich 16 of Germany communicated to the Academy at Berlin his studies upon the fermentation of cellulose and expressed the opinion that it was due to a particular living atom which he found in the fermenting material. Fifteen years afterward, Trecul of France found in macerated vegetable substances a living organism which he believed arose spontaneously by a direct transformation of the protoplasm of the closed cells. To this he gave the name of Amylobacter, after assuring himself that in a certain stage of its existence it stored starch within itself. In 1877, Van Tieghem 17 of France transferred this living thing to the genus Bacillus, naming it Bacillus amylobacter. From observations upon it he satisfied himself that it could and did penetrate the cell wall in the adult condition, thus overthrowing as he thought the theory of spontaneous origin. Here the battle wages. Trecul 18 still holds his first opinion and criticises Van Tieghem's proofs.

In June, 1879, Prazmowski 19 of Leipzig announced that this Bacillus amylobacter produced, in the fermentation of carbonaceous substances, butyric acid, and forthwith concluded that it was iden

15 Comptes Rendus, t. 52, p. 344 (1861) and Schützenberger, on Fermentation, p. 213. 16 Comptes Rendus, t. 88, p. 205, t. 61, pp. 156 and 436, t. 65, p. 513.

1 Bulletin de la Societé botanique, March 23, 1877. Comptes Rend., t. 88, p. 205. 18 Comptes Rendus, t. 88, p. 401, 1879.

19 Comptes Rendus, t. 89, p. 5, 1879.

tical with the Vibrio described by Pasteur. Van Tieghem confirms this, and it now appears probable that the butyric fermentation of at least all dead carbonaceous compounds, soluble or insoluble, is directly or indirectly produced by the same wonder-working little creature. Does it now extend its dominion, and conquering the life forces of our trees seize upon their treasured products and ruthlessly appropriate them to uses of its own? Whether this microscopic organism of the pear blight proves to be the same as that described or not, the evidence now given of disease in plants produced by bacteria contributes something to the germ theory of disease in animals and may lead to very important scientific and practical results.

In closing, I wish gratefully to acknowledge valuable suggestions from and assistance by Dr. H. J. Detmers, commissioned by the department of agriculture at Washington to investigate the diseases of domestic animals, and who is the author of a most important report upon Swine Plague or Hog Cholera.

FIELD WORK BY AMATEURS. By ELLEN HARDIN WALWORTH, of Saratoga Springs, N. Y.

Ir is announced, I believe, that one of the aims of this Association is to make natural science popular, to encourage its pursuit among all classes of people. It is because I have such an understanding of its aims, that I presume to speak a word in behalf of the class who love science, yet can give to it but a limited portion of their time and thoughts.

Such a class of persons are important factors in the development of every department of knowledge and art. The professor, the artist, the specialist may have higher aims, they certainly do more thorough work, yet they would scarcely be understood, appreciated and encouraged if there did not exist the intermediate class who admire, applaud and exhibit the work they cannot themselves perform.

I therefore deprecate the scorn with which the professional too