Rust of wheat fungus grows part of its time on barberry leaves and part on wheat. In the summer, if we examine one of the rust-like patches on stem or leaf of wheat we shall see that it consists of a dense bunch of small, short stalks each one of which is terminated by an oblong red-brown spore. If we keep another patch of the fungus under observation, we shall find as the season advances, that instead of the red-brown patch it has grown darker and darker till it has become almost black. The microscope will show us that the structure of the spores has altered considerably. There is still the same bunch of stalks but they have lengthened somewhat and now each spore which terminates each stalk is divided into two parts by a wall across its narrow part. The walls surrounding the spores also appear thicker, as indeed they are. These are the winter spores, they fall to the ground eventually and there they remain, unharmed by frost or snow or rain, till the spring. In the spring they germinate and give rise, not immediately to another fungus, as might be expected, but to another kind of spore. Curiously enough these new spring-formed spores cannot grow upon wheat and unless they are carried by wind or some other agency to a barberry plant their existence is ended. Should they reach a barberry leaf, however, they will germinate, penetrate the leaf and grow for a period. Eventually the fungus appears on the lower surface of the leaf in beautiful structures called cluster cups. Under the microscope, one of these cluster cups forms a lovely object. The leaf skin is split and below the ruptured skin may be seen a flask-shaped hollow filled with chains of minute golden-yellow spores. The spores break away, one by one and favoured by fortune, are carried to a wheat plant where they germinate and give rise to the familiar rust. Any microscopist anxious for research has a life’s work before him in tracing the histories of this one class of fungi, should he feel inclined to shoulder the burden. Very many cluster cups are known and very many rusts and all that is required is an enthusiastic mycologist, as the student of fungi is called, to put the pieces of the puzzle together, so to speak. It is not so very many years ago that the connection between the cluster cups of barberry and the rust of wheat was quite unthought of.