IN EARLIER times the opponents of the "germ theory" of disease in plants were commonly ready to admit the presence of a fungus - in fact, to determine its presence was a simple matter of observation; but they contended that it merely accompanied or followed and did not cause the disease. Their position was the logical one to take until the opposite could be clearly shown to be true, and to show that the fungus causes the dis-ease, makes it necessary or at least very important, to show in what the noxious effect consists.

In some cases, as in corn smut, the fungus causes injuries of such a character that they are evident to all. In others, the character of the injury is much more difficult to determine. There are several kinds of in-juries which a fungus may produce id the plant it attacks, and often several may be caused at the same time. The one first to be mentioned is the most universal and comes necessarily on account of the essential nature of fungi. Fungi have no green coloring matter, and for this reason cannot supply themselves with food from the earth and air as green plants do ; they are compelled to take their food from some ready-prepared organic matter, i. e. from some living plant or animal or from their dead remains. If they take it from the latter, they are not true parasites, but are called saprophytes. Parasite fungi are those which take their food from their "host," that is their supporting plant (or animal) while it is yet alive. Thus an infected plant is deprived of the food which it has supplied for its own use ; the fungus thrives, and the host plant suffers more or less from starvation.

Many fungi, such as grape mildew and its allies, have special organs (suckers) (Fig. 1) for absorbing nutritive material from the cells of plants, and the cell walls are also punctured, ruptured or otherwise injured. The suckers and mycelium which enter a cell sometimes kill it and sometimes allow it to grow on with reduced vitality, a continual prey.

If the green parts are the ones attacked, as most frequently they are, the power of the plant to provide fresh food for itself is also impaired.

The light may be kept from the leaf to some extent, as when the fungus forms a layer over the surface.

Sometimes leaves are covered w i th the exudations of i n s e c t s ("honey-dew ") and a black fungus grows on this, covering the surface o f the leaf.

Also, the active power o f the leaf-green is less on account of the weaker vitality to provide food for itself.

Sometimes portions of the leaf are killed and appear as brown dry spots, as in the spot disease of the garden currant (Fig- 2) and many other plants ; or portions even die and fall out, leaving holes. When the damage is less severe, spots of various colors are seen, more or less characteristic of some special disease. Fig. 3 shows spots on mignonette leaf. Still other fungi cause the entire leaf, or a considerable portion of it, to become withered and dry, and it is usually the free end of the leaf and the leaves nearest the base of the stem that suffer most, as in rusted cereals. Certain plants, as Canadian thistle, attacked by their peculiar rust, wilt in hot sunshine, afterwards reviving ; but healthy thistles do not wilt.

Another common effect is to change the size of a plant, to make it grow either less or more rapidly than a healthy plant. We might easily expect a diseased plant to grow less rapidly, but strangely enough the reverse is very common. A spurge on which a certain common cluster-cup is growing, and a blackberry infested with orange rust, have smaller leaves but taller and more erect stems, an effect resembling that of insufficient light, as upon vines on the shaded side of a trellis, or potato sprouts in a cellar. The scab fungus may retard growth on one side of an apple or pear, while the unaffected side continues to grow. The mildew so common on plants of the mustard family often contracts the leaves of pepper-grass, . but causes considerable swellings in the stems of a nearly related plant. An exobasidium which grows on the leaves of blueberry bushes, contracts them ; other forms of the fungus growing on the allied azalea and andromeda produce inflated sacs often several inches in diameter. Many other fungi cause swellings and sometimes further distortions of the stems and leaves on which they grow.

The swelling is usually composed in part of the abnormally multiplied cells of the host plant; this may be seen in the "curl" of peach leaves, the smut of corn (Fig. 7) and the common "cedar apple" (Fig. 6) which grows on red cedar trees. Another fungus of the "cedar apple" group, growing on the trunks and branches of white cedar, causes swellings, of which some become as large as a man's head.

Fig. 2. Spot Disease of Currants.

Fig. 3. Spot Disease of Mignonette.

Injuries caused by fungi are not confined to the green parts of plants; woody stems, roots, flowers, seeds and fleshy fruits all have their parasites.

One of the slime-molds attacks the roots of cabbage, causing the disease known as "club-root." The black knot of plum trees is a familiar example of diseased and distorted stems. A certain fungus attacks the flowers of the blackberry and distorts them so that they appear double; and the disease is known as "double blossom." Various kinds of smuts are noted for attacking only certain organs or parts of their hosts ; one on the plants of the pink family fruits in the anthers ; the smut of wheat destroys the ovary ; the bunt of wheat destroys the interior of the ovary without much affecting the outward appearance ; other species growing on the troublesome grass-weeds, sand-bur and finger-grass, destroying the entire inflorescence; butone on fox-tail, while destroying the ovaries or seeds, produces almost no distortion. Corn smut is quite exceptional among the smuts, in that it grows and fruits on all parts of its host except the roots.

The young ovary is remarkably affected by the action of certain fungi.

The ergot fungus replaces the young ovary of rye by a horny- mass as shown in Fig. 4. Plum pockets are ovaries of a plum tree converted into inflated sacs by the action of a species of taphrina (Fig, 5) ; the smuts mentioned above and others attacking ovaries and seeds convert them into a mass of powdery spores. One of these found growing on a smart-weed is very beautiful under the microscope. The scab-fungus, attacking the newly formed fruit of the pead kills it and it drops off. Many leaves of various plants (e. g. of the currant. Fig. 2) fall prematurely when attacked, apparently by a process which, while abnormally induced, is yet similar to that which promotes the falling or leaves in autumn. Contrasted with premature ripening we have retarded development, diseased parts remaining green while healthy parts have become brown and dry with age.

Ripe fleshy fruits are more liable to the attacks of fungi than any other vegetable productions. The fungus may injure their quality or only their appearance and market value. This is true of the scab-fungus of apples and pears. Some fungi attack fruits only while growing, attached to the tree; others, like the potato rot, attack the growing plant and continue their ravages in the harvested product. Some of the common saprophytic molds of the household become para-sitic when of portun ity is offered, as mucor upon strawberries.

In the field and orchard, valuable plants are frequently liable to infec-t i o n from others of less value; wild plums breed black-knot for cultivated plums, wild grapes and Virginia creeper have mil-d e w s whose spores may be carried to the finest varieties.

One stage of a fungus, in some cases grows on a wild plant, another stage on a valuable crop; the cedar fungus infects the apple trees with rust and the barberry fungus carries rust to the wheat and oats. There are immense numbers of fungi of other classes which are believed to have different stages of growth, perhaps in most cases on the same host, but their histories are unknown. In almost no other field of botany would successful research yield richer results than in this. '

To sum up. we may enumerate seven ways in which fungi injure their host plants : by (1) depriving them of nourishment; (2) impairing the power to assimilate food supplies, cutting off light or weakening the vitality ; (3) abnormally accelerating or retarding growth, producing distortions ; (4) affecting not only green parts, but also roots, stems, inflorescence, flowers and fruits ; (5) causing leaves and fruit to prematurely ripen (?) and fall; (6) producing decay in matured fruits and vegetables, both before and after gathering; (7) infecting valuable plants, by means of spores grown on less valuable plants. Several of these causes are often active at the same time in a diseased plant, and it is difficult to say in many cases to which cause the greater injury is due. There can be no doubt of the vital importance of the whole subject, however, to our agriculture, and of the necessity for intelligent action looking toward the prevention of fungoid ravages in our cereal crops. Harvard University, A. B. Seymour.