germinate by sending out new mycelial threads, which can enter into the stems upon which they have fallen. The object, then, should be to cut off the knots before the spores are ripe. By cutting in summer we can prevent the maturing of the winter spores by cutting early in the spring we can prevent the ripening of the conidial spores. It is not enough, however, simply to cut off the diseased branches. If the winter spores have begun to form, they go on and ripen, even if the knots are cut from the tree, notwithstanding they may be exposed to a great degree of cold. Knowing this, we can infer that it is safer to burn all knots which are removed.

as ours.

The black knot is unknown in Europe, although the European cultivated plums and cherries are botanically the same How does it happen, then, that our trees have a disease unknown in Europe? The reason is this: the fungus which causes the disease is a native of America, and grows on our wild plums and cherries. In Massachusetts it is found on the choke-cherry (Prunus Virginiana), the bird-cherry (Prunus Pennsylvanica), and the beach-plum (Prunus maritima). Farther west, it is also found on the wild plum (Prunus Americana) and on Prunus Chicasa. Being a native of America, when plums and cherries were introduced from Europe, the fungus grew upon them as well as upon our own wild species. Its injurious effects are better known on the cultivated plums and cherries, because, being cultivated for their fruit, they are more generally observed than the comparatively worthless wild species. All our wild cherries are not attacked by the fungus, as, for example, the rum-cherry (Prunus serotina); and there are a number of cultivated varieties of cherry which are not subject to the disease. In attempting to check the disease, one should not forget to remove the knots from the wild cherries growing near orchards as well as from the cultivated cherries.

Probably but few of the tumors on trees and shrubs can be said with certainty to have been caused by fungi; yet no tumor of any size is probably free from them. The number of species of fungi is enormous, and not a small proportion inhabit dead wood and bark; and the rough surface of any old tumor forms a suitable place of growth for a great many species. They are, however, not the cause of the knots, but an after-growth, and are recognized as such by those who

make a special study of fungi. Many tumors are known to be caused by insects; and, as a rule, the distortion produced arises, not so much from the attack of the insects themselves as from the effort of the plant-cells in succeeding years to perform their normal work. The injury often consists in the invasion of a leaf-bud by some very small insect; and, as a result of the irritation, the leaves constituting the bud enlarge, become hardened, and often unite into a comparatively solid mass. The next year the indurated mass itself acts as a foreign body; and there grows around it in succeeding years layers, which are all more or less distorted, until finally we have a large knot in which it is quite impossible to detect the original lesion.

In the beginning of the lecture we divided diseases caused by fungi into two general classes, tumors and blights. The latter is by far the larger and most destructive, and more generally recognized as caused by fungi. Of course, the consideration of blights on fruit-bearing plants should not be kept distinct from that of blights on vegetables; for, in a scientific point of view, they are very closely related. To describe in detail even a small portion of the blights of cultivated plants would require several lectures; and today I can only call your attention to two which are common on grape-vines in Massachusetts, and let them serve as types of two large and very destructive orders of fungi. The fungi to which I refer are found as well on wild grapes as on cultivated; and neither species is as yet known to occur in Europe, although both are common throughout the Eastern United States.

You may have noticed that the leaves of many cultivated grapes are apt to look dusty after the first of August. The dustiness, if such it really were, would, of course, disappear or diminish after a heavy rain. But such is not the case. During the damp weather the dusty look increases; and, after a while, the leaf dries and shrivels. As the leaf shrivels, the dustiness disappears; and in its place we see a number of very small black bodies scattered all over both surfaces of the leaves. In some cases, instead of looking dusty, the leaves seem to be covered with a tolerably thick white web, which extends to the leaf-stalks, and in extreme cases to the grapes themselves. The blight is often supposed

to be due to Oidium Tuckeri, the fungus which caused formerly a great deal of injury to the grape-crop in Southern Europe, and especially in the Island of Madeira. The development of that fungus is only partly known, and there is no proof that our fungus is the same. The American fungus referred to is called Uncinula spiralis, and belongs to a large group of leaf-parasites, the Perisporiacea. The dusty or webby appearance of the leaves is caused by the growth of the mycelium over the surface. The mycelial threads, although they may cover a great part of the surface of the leaves, do not enter into their interior, except that at intervals the threads are furnished with little suckers, which just penetrate into the external cells, and serve to attach the mycelium. During the summer some of the threads grow up from the surface of the leaf, and at the tip divide into a

number of squarish ovoid cells (Fig. 4), which are spores corresponding to the conidial spores of the black knot. Later in the season a number of round bodies (Fig. 5) are formed on the threads. They are at first yellowish, and afterwards black. These black bodies are hollow, and contain a number of sacs (Fig. 6), in which are spores which may be said to correspond to the winter spores of the black knot. The black bodies have attached to them a number of peculiar threads or appendages, which are rolled up at the end, from which the name Uncinula is derived.

The second form of blight which occurs on grapes begins to appear about the same time as the first, and may be mixed with it on the leaf; or more frequently it occurs alone. The first form of blight may be found on either side of the

leaf. The second occurs only on the under surface, and appears when fresh like frost-work. The leaf-stalks are sometimes covered by the fungus, but it does not attack the grapes themselves. An examination of the frost-like spots on the under surface shows that they consist of branching threads, on whose tips are borne oval bodies (Fig. 7), which are the conidial spores. The threads do not, however, grow all over the surface of the leaf, but make their way from the in

Fig. 7.

terior into the air, through the holes. which abound on the under surface of most leaves, and are known by the name of "breathing-pores." If we follow the threads still farther, we shall find that they penetrate through all parts of the leaf and stems, making their way between the leafcells. The threads are also furnished with small suckers, which push their way directly through the walls of the plant-cells into their interior. The bodies which we may call the winter spores of this blight are not found on the surface, but embedded in the leaf. They are round, and have thick walls. The name of the second form of grapeblight is Peronospora viticola.

A comparative study of the two blights which we have just described is instructive. They both first appear on the leaves about the first of August, and both cause them to shrivel and drop off. One sometimes attacks the grapes, while the other does not. The two blights may be distinguished with the naked eye by an ordinary observer, as one forms a sort of dusty-looking web on any part; the other, frostlike spots on the under surface of the leaves. Both have conidial spores, which grow on stalks in the air. Both have what we may for convenience call winter spores, which ripen late in the autumn.

Those of the Uncinula are in the

round black bodies on the surface of the leaf: those of the Peronospora are in the interior of the leaf.

A microscopic examination, then, shows us that it is not correct to speak of grape mildew, or blight, as a distinct disease. We have just seen that there are at least two different fungi which produce a blight; and the two differ decidedly in their habit and growth, so much so, that the means taken to prevent the growth of one will not apply to the other. Let us consider this practical point more at length. We will suppose that the grape-raiser recognizes that his plants are attacked by the first form of blight described, — Uncinula spiralis. As a microscopic examination shows that the fungus is on the surface, and not in the interior, of the leaves, it is plain that the object should be to check the growth of the mycelium on the leaf. The injury that the Uncinula does to the grape is, that it covers the leaves, which in a certain sense may be said to be the respiratory organs of a plant, so that the necessary supply of light and air is shut off. The growth of the fungus may be checked by the use of sulphur strewn over the plants. We must also consider how the disease is propagated from one plant to another. The conidial spores already described are light, and easily blown from one plant to another. Wherever they fall, if the weather is only moist enough, they begin to send out threads, which form the mycelium of a new Uncinula. The threads only grow to any length, as far as we yet know, when the spores have fallen on or near grape-vines. From this we can infer that the Uncinula does not live entirely upon material found in the air, or accidentally on the surface of the grape-leaves, but that it also requires some peculiar substance produced only by the grape-plant. A great many of the species of fungi, however, which are botanically closely related to the Uncinula, are not limited to the plants of a single genus, but grow indifferently on plants which are not nearly related botanically.

Another question also arises, -How does the fungus survive the cold of winter? The conidial spores which can spread the disease rapidly are killed by the cold. The round black bodies which contain the winter spores are much tougher. The winter spores are not ripe until late in the autumn, and fall to the ground with the leaves on which