METAMORPHOSIS, or entire change of form, is a familiar phenomenon in insects. The caterpillar becomes a chrysalis, the chrysalis a moth. In some birds, the plumage of the adult is very different from that of the young. In ' many marine animals, the offspring is not like the parent but like the grandparent ; one form is alternated with another, generation after generation. It is so also in ferns ; there is the inconspicuous sexual generation coming from the spore and the asexual generation, the "fern" as we ordinarily see it, bearing the spores.

Analogous or similar, or even more complicated cases are frequent among fungi. Most persons are familiar with the yellow, jelly-like balls that occur on cedar-trees in spring and are called "cedar apples" (Fig. 1). This is a fungus, and the common rust on the apple is merely another stage of the same fungus. After the first warm rain in spring, the jelly-like appendages of the cedar apples become greatly enlarged, and imbedded in them are spores such as are shown highly magnified in Fig. 2. The lower spore in Fig. 2 has germinated and has formed secondary spores. The latter can easily be obtained in large quantities by placing one of the cedar-apples on a saucer containing a little water and covering it with a tumbler. As soon as germination takes place, the secondary spores are scattered in a yellow layer over the saucer. In nature, these spores are carried to an apple or wild crab-apple tree, where they germinate and produce the rust, like that in Fig. 3. The rust appears as yellow spots with black specks on the upper surface of the leaf, and clustered cups with a fringed margin as shown magnified in Fig. 4. The cups which appear in late summer are filled with spores of a different kind from the others.

They in their turn are carried back to the cedar tree and there produce cedar-apples which live over winter and grow in the spring as before. I found last August an apple tree so yellow with rust that it was conspicuous half a mile away, and under it was a young cedar tree bearing many old cedar-apples and multitudes of new ones. The history of wheat rust is similar but more complex. Beginning with the cluster-cup stage, we find it on the barberry, not in fall, but in spring. Fig. 5 is a magnified section through a barberry leaf, showing the black specks like sunken flasks at the upper surface, and the cluster-cups on the lower. This form, when 'growing, possesses re-markable beauty; the cups, seated in clusters on red spots, are cream-white wax-like cylinders leaving a fringed margin, and are filled with spores. Long before it was understood scientifically, farmers were convinced by observation that wheat could become infected with rust from barberry bushes. This belief was so strong that a law was enacted in Massachusetts in 1755, entitled "An Act to prevent damage to English grain arising from Barberry Bushes." It was not till more than a hundred years later that the connection of the barberry fungus with wheat rust was demonstrated, but since that time the facts have been abundantly veirfied by repeated experiments.

Briefly stated, the facts are these: The barberry cluster-cup fungus (Fig. 5) appears early in spring on the young leaves. The spores of which the cups are full, are scattered by the wind, being carried with much greater ease than ordinary dust, because they are so much lighter. Many doubtless perish, but some, falling on "good ground" in the shape of wheat, oats and other cereals and grasses, germinate, penetrate the plant, and produce easily a thousand fold or more, not of cluster-cups but of rust spores. Two kinds of spores are formed sooner or later, constituting what farmers and botanists call red rust and black rust of wheat. The red rust comes first, and is characterized by oblong orange-colored spores of a single cell, on stalks which easily fall. Black rust comes later on the same mycelium or plant, and often in the same cluster, and is characterized by dark colored spores having a partition across the middle, and persistent stalks. Fig. 6 contains three spores of red rust and one of black rust. Fig. 7 is a cluster of black rust spores. The red rust spores are rapidly scattered, germinate and produce red rust again, perhaps several generations of it, before the black rust spores appear.

The latter are more permanent ; they live over winter and in spring germinate, infect the barberry bushes and produce cluster-cups again.

Wheat has three distinct species of rust; the one just described is called Puccinia graminis, another Puc-cinia Rubigo-vera, and the third, which is commoner on oats than on wheat, is Puccinia coronata. All three have similar histories, but their cluster-cups do not grow on barberry. A fourth species, which grows on Indian corn, is not known to have any cluster-cup form.

It is easily seen by a comparison of the figures, that wheat rust and apple rust belong to the same class of fungi. The black rot of the grape and the black knot of the plum trees both belong to another class, which have one of the kinds of spores in sacks (Fig. 11 and 15).

The black knot of the plum trees is one of our best known and most destructive fungi, making plum culture a failure in many places. It might easily be com batted with considerable success if cultivators would only make the effort. Its life history was worked out with great accuracy by Prof. W. G. Farlow, of Harvard University, in 1876. In the spring a new knot forms, in many cases as an extension of an old one; the branch on which it forms swells and cracks open. The microscope shows certain spores borne on the tips of threads and called conidia (Fig. 8). The knot grows on and acquires a crust-like surface which becomes marked with small pimples. These pimples indicate cavities in which spores are formed. Some of them (Fig, 10) contain spores in sacks (Fig. 11) which become mature in February. Other cavities contain spores of three different kinds, none of them in sacks. Fig. 9 shows one of these kinds (stylospores) which are borne on the slender stalks attached to the walls of the cavity. This fungus then has five different kinds of spores, two of which (Figs. 8 and 11) are of special importance, the conidia and the sack-spores.