(seen in Fig. 14 at 4, 5, 6 and Fig. 16) are formed on the threads. They are at first yellowish and afterwards black. These black bodies are hollow, and contain a number of sacs (seen in the cen. ter of figure 14 and Fig. 17), in which are spores which may be

FIG. 17..

said to correspond to the winter spores of the black knot. These 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 tops are borne oval bodies (seen in the branches in figure eighteen), which are the conidial spores. The threads do not, however, grow all over the surface of the leaf, but make their way from the interior into the air, through the holes which abound on the under surface of most leaves, and are known by the name of "breathing spores." If we follow the threads still farther, we shall find that they penetrate through all parts of the leaves and stems, making their way between the leaf cells. 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 are imbedded in the leaf. They are round and have thick walls. The name of the second form of grape blight is Peronospora viticola.

A comparative study of the two blights 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, frost-like 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 shows us that it is not correct to speak of grape milldew, 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 habits 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 grower 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 Uncinulu, 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 till late in the autumn, and fall to the ground with the leaves on which they are growing. They remain dormant during the winter, and when spring comes, germinate and make their way into the nearest grape vines. It would not be unwise in the autumn to collect and burn all the grape leaves, that is, as far as practicable, in districts which have in the summer suffered from the Uncinula. In this place we should bear in mind what has been said about the black knot extending from wild species to the cultivated. This blight can extend in a similar way, as it is found on wild vines, but as far as we yet know the fungus does not grow on any wild plants except grape vines.

If we turn now to the Peronospora, which grows on grape vines, we see that the preventive measures, which in the case of the

21- HORT.

Uncinula would be of advantage, would here be of little avail; because the fungus is not confined to the surface, but pervades the whole plant, and in fact does not grow through the breathing pores into the air until it has already traversed a considerable part of the interior of the leaves and stems. The first warning of the presence of the fungus, viz., the white spots on the under surface of the leaves, is not to be interpreted as showing that the disease is beginning, but that it is already far advanced. To sprinkle sulphur on the leaves is quite useless in the case of the Peronospora, for it will not affect the miss of the fungus which is in the interior. The only thing which will check the disease is to dimin ish the moisture in the air, but that, unfortunately is beyond human control. If the season happens to be dry, all very well; if very wet, then the Peronospora, once started, will grow in spite of everything.

The disease spreads from plant to plant durtng the summer by means of the conidial spores. They may be carried about by the wind and rain, just as the conidial spores of the Uncinula; but when they fall on a place sufficiently moist they germinate, but in a different way from the conidial spores of the Uncinula. The contents of the spores separate into a number of distinct bodies, which break through one end of the spore and escape, leaving the empty spore-wall behind. The bodies which escape, to which the name of "Zoöspores" is applied, swim about by means of two hair like threads called "cilia," which are in constant motion. Being very small, they are able to move about in the moisture which is found on the ground and on plants when it is not dry weather. They swim about for only a short time, and then the cilia drop off, and the Zoöspores come to rest. They then give off threads like the conidial spores of the Uncinula, and the threads penetrate into the interior of the grape plants on which they may be. Once inside, the threads constitute a mycelium, which extends through the plant at a rate corresponding with the external moisture, and finally the threads make their way through the breathing pores into the air, and produce new spores. It wil be seen that the conidial spores of the Peronospora have an advantage over those of the Uncinula, because they produce a num

ber of Zoospores, generally from five to fifteen, each of which is capable, under favorable conditions of producing a new mycelium and spores. Like the corresponding bodies in Uncinula, the conidial spores are destroyed by cold.

The winter spores of the Peronospora are thick and tough, and are produced in the interior of the grape leaves by a rather complicated process, which, although interesting from a botanical point of view, need not be described here. They fall to the ground with the leaves in the autumn, and are set free by the rotting away, during the winter and spring, of the leaves in which they are contained, and as the season advances they germinate, and enter the nearest vines, but the details of the germination have not yet been made out. The remarks already made about burning the grape leaves apply also in this connection. The mycelium of the Peronospora in the interior of the vines affected during the summer may remain dormant during the winter, and start up again when the warm weather returns. Just how much harm the Peronospora does to the grape vines is not easy to decide. I have, on a previous occasion, expressed the view that it is not unlikely that the harm done has been exaggerated, because the fungus never attacks the berries, and it does not cause the leaves to shrivel and dry up until comparatively late in the summer, when, as some say, their room is better than their company, for what is especially needed is, that there shall be plenty of sunlight to ripen the grapes, which is not the case when the foliage is luxuriant, and covers up the branches. Whether the shrivelling of the leaves in the latter part of August permanently injures the vines, and injures the crop is a point to be settled, not by the botanist, but empirically by the grape grower, and as far as can be learned, on this point opinions differ.

My object in describing the two principal blights on the American grapes has been to show that an accurate and scientific knowledge of the causes of diseases in plants requires a careful microscopic study, and that such study is not without definite and practical results. The time has passed when the labors of botanists should be considered of interest only to special students of science. From them the farmer may learn certain facts of which