on the surface in spots which are green colored, not like the color of a leaf, however, but a very dark, deep green. A microscopic examination of the greenish knots which are found late in the

spring shows on the surface a number of threads, on which are borne small bodies which are also spores; and to distinguish them from the other spores they are called conidial spores (Fig. 13 B). As the conidial spores ripen and fall off, we find beneath the surface the beginnings of the cavities and the sacs which are to contain the winter spores. The knot grows rapidly blacker and rougher.

Having given as briefly as possible an account of the development of the black knot, it remains to state the conclusions which may be drawn from the knowledge obtained by means of the misroscope. In the first place, we may say that the disease is caused by a fungus. Why caused? you, perhaps, will ask. The word "cause" is not used in natural science to signify ultimate, but proximate cause. Ultimate causes are discussed in philosophy and theology, but they do not properly fall in the scope of nat ural science. When we say that a disease is caused by a fungus, we simply mean that the manifestation of symptoms which we collectively call the disease is invariably preceded by the presence and growth of the fungus. To illustrate: the formation of the

black knot is invariably preceded by the presence of the fungus known to botanists under the name of Sphæria Morbosa, and the onward growth of the mycelium of the fungus in the healthy stem of the plum is followed without fail by the swelling and blackened characteristic of the knot. On the one hand, we never find the fungus unless accompanied by the knot; on the other, we never find the knot unless accompanied and also preceded by the fungus. If you examine the slightly swollen branches of the Choke-cherry in the spring, before the bark has cracked open, you will find the threads of the fungus already in the stem, and later in the season you will certainly find the characteristic swell. ing and blackening. If the fungus were only found with the knot, we could not say that it was the cause of it. As the knots grow old, there is usually a number of insects and fungi found in or on them; they cannot, however, be considered the cause of the knot, as they are found in other excrescences as well. It is be cause the fungus consistently precedes, as well as accompanies the knot, that we are entitled to say that it is the cause of the knot.

to be

As to the means to be taken to prevent the spread of the disease: our knowledge of the habits of the fungus throws light upon this point. First, we have seen that the threads of the fungus extend in the stem some inches beyond the knot itself, and these threads will, the next season, be followed by a new knot. Hence, in cutting away the knots, one should cut several inches safe, we will say about ten inches - below the knots. The way the disease increases in a plant once attacked is by the onward growth of the mycelium. The next question is, how to prevent its spreading to other trees. The spreading is produced by the growth of the spores, one kind of which ripens in mid-winter, and another in early summer. The spores are all light, and easily blown about, and when they fall upon other trees, 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 trees, notwithstanding they may be exposed to a great degree of cold.

Knowing

this, we can infer that it will be safer to burn all knots which are removed.

The black knot is unknown in Europe, although the European cultivated plums and cherries are botanically the same as ours. How does it happen, then, that our trees have a disease unknown in Europe? The reason is, that the fungus which causes the disease is a native of America, and grows on our wild plums and cherries. * * * 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 be 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 aftergrowth, 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 grow 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 more 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 I can now call your attention only to two, which are common

of its development.

on grape vines, and let them serve as types, 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 yet known to occur in Europe, although both are common through 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 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 Perisporiace. 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 these threads grow up from the surface of the leaf, and at the tip divide into a number of squarish ovoid cells (Fig. 15)

FIG. 15.

which are spores corresponding to the conidial spores of the black knot. Later in the season a number of round bodies

FIG. 16.