are the cause of mildewed corn, and recent scientific investigations have shown that there is truth in the

belief, which is not always the case with popular adages. Dr. M. C. Cooke, in his "Fungi," gives the following:

"There is a village in Norfolk, not far from Great Yarmouth, called 'Mildew Rollesby,' because of its unenviable notoriety in days past for mildewed corn, produced, it was said, by the Berberry bushes, which were cut down, and then mildew disappeared from the cornfields, so that Rollesby no longer merited its sobriquet. It has already been shown that the cornmildew (Puccinia graminis) is dimorphous, having a one-celled fruit (Trichobasis), as well as a two-celled fruit (Puccinia). The fungus which attacks the Berberry is a species of Cluster-cup (Æcidium berberidis) in which little cup-like peridia, containing bright orange pseudospores, are produced in tufts or clusters on the green leaves, together with their spermogonia. De Bary's observations on this association of forms were published in 1865. In view of the popular belief, he determined to sow the spores of Puccinia graminis on the leaves of the Berberry. For this purpose he selected the septate resting spores from Poa pratensis and Triticum repens. Having caused the spores to germinate in a moist atmosphere, he

B

placed fragments of the leaves on which they had developed their secondary spores on young, but fullgrown, Berberry leaves, under the same atmospheric conditions. In from twenty-four to forty-eight hours a quantity of the germinating threads had bored through the walls and penetrated amongst the subjacent cells. This took place both on the upper and under surface of the leaves. Since, in former experiments, it appeared that the spores would penetrate only in those cases where the plant was adapted to develop the parasite, the connection between P. graminis and Acid. berberidis seemed more than ever probable. In about ten days the spermogonia appeared. After a time the cut leaves began to decay, so that the fungus never got beyond the spermogonial stage. Some three-year-old seedlings were then taken, and the germinating resting spores applied as before. The plants were kept under a bell glass from twenty-four to forty-eight hours, and then exposed to the air, like other plants. From the sixth to the tenth day, yellow spots appeared, with single spermogonia; from the ninth to the twelfth, spermogonia appeared in numbers on either surface; and a few days later, on the under surface of the leaves, the cylindrical sporangia of the Æcidium made their appearance, exactly as in the normally developed parasites, except that they were longer from being protected from external agents. . . . It seems, then, indubitable so far that Ecidium berberidis does spring from the spores of Puccinia graminis.”

Other experiments have been made in which the spores of Ecidium have been placed on healthy

plants of rye, and in five or six days after, these plants were affected with rust, whilst the remainder of the crop was unaffected. Dr. Cooke, in his valuable book, gives detailed instances of other species in which this polymorphism occurs.

Fig. 20 is a representation of a small portion of the Potato fungus (Peronospora infestans), which at times destroys the entire crop in large

FIG. 20.

districts. It is produced by spores which, falling on the Potato plant, develop, and their hyphæ enter the stomata or breathing-pores of the leaf, and penetrating its cellular tissue, absorb all nutriment from it. From the leaf the hyphæ or mycelium traverse the substance of the stem, and finally reach the cells of the tuber the Potato itself. Mr. Worthington G. Smith has recently made some valuable researches into the life-history of this fungus, with the result of adding considerably to our knowledge of it. Fig. 21 is a modified reproduction, on a small scale, of one of that gentleman's illustrations, representing the section of a small portion of potato leaf. a, a are two of the hairs with which the leaf is furnished; b, b, and c, c, the cellular tissue of the leaf. d is a branch of the fungus emerging from a breathing-pore or stomate, and "is no other than a continuation of a thread of spawn or mycelium which lives inside, and at the expense of the assimilated material of the leaf. When this thread emerges into the air, as here shown,

it speedily ramifies in different directions, and bears fruit at the tips of the branches, as at e, e; these fruits

are commonly arrested in growth while still small, and they germinate in an exactly similar manner with the zoospores themselves, and may be considered somewhat analogous with seeds. The Potato fungus has another method of reproducing itself in the 'swarm spores,' as shown at f, g. These are so called because, on the application of moisture (as supplied by rain or dew, or when applied artificially), the vesicles set free a swarm of from six to fifteen or sixteen other bodies known as 'zoospores,' so named because they are furnished with two lash-like tails, and are capable of moving rapidly about like animalcules. This rapid movement usually lasts for about half an hour, and (like the dust-like conidia, or 'simple spores,' before mentioned) the swarm spores generally enter the breathing-pores of the leaf, and there germinate. So potent, however, are the contents of these bodies when set free, that

they are capable of at once corroding, boring, and entering the epidermis of the leaf, or even the stem or tuber itself. These zoospores are best seen when within the vesicle ƒ, where they arise from a differentiation of the contents; but when once set free (), they are, from the extreme rapidity of their movements, very difficult to make out. In almost half an hour they cease to move, their lash-like tails (cilia) disappear, and having burst at one end, a transparent tube is protruded, which is a similar mycelium in every respect with that produced by the simple spore, and which grows, branches, and fruits in a precisely similar manner." ""*

But besides these zoospores and simple spores, the Potato fungus produces a third kind called "oospores," which were unknown until Mr. Smith's careful study of the fungus by night and day through its various changes and developments revealed its existence. Referring to our fig. 21 again, we notice several large, round cells marked i, and some smaller k. The larger is the oogonium, and the smaller the antheridium. The oogonium is analogous to the ovary, and the antheridium to the anther, in flowering plants. These two bodies come in contact, and the antheridium pushes out a small tube which enters the cell-wall of the oogonium, and through it a portion. of the antheridium contents is emptied into the oogonium. This fertilises the oospores which are contained within, and when the latter are mature the mycelium vanishes, and the resting spores are set free. It sinks into the earth, and remains quiet during the winter,

* Worthington G. Smith, "Monthly Microscopical Journal," 1875.