How long this disease has been known, is an unsolved problem. About the middle of the last century a tract was published on this subject in Italy, but this was probably not even the first intimation of its fungoid character. Before such conclusion had been arrived at, men may have struggled in the dark, through many generations, to account for a phenomenon with which they were doubtless familiar in its effects. In 1805, Sir Joseph Banks published his “Short Account,” illustrated by engravings from the inimitable drawings of Bauer, whereby many in this country learnt, for the first time, the true nature of _mildew_. [Illustration: Plate III. W. West imp. ] With a view to the clearer understanding of these parasites in the phases of their development, let us select one, and we cannot do better than adhere to that of the wheat and other graminaceous plants. A fine day in May or June dawns upon our preparations for a stroll, far enough into the country to find a wheat-field. Even now, with the area of the metropolis constantly widening, and banishing farmers and wheat fields farther and farther from the sound of Bow-bells, a corn field may be reached by a good stiff walk from Charing-Cross, or a six-penny ride at the most, in nearly any direction. Having reached the field, it may be premised that a walk into it of less than twenty yards will be sure to reward you with the fungus we are in quest of. Look down at the green leaves, especially the lower ones, and you will soon find one apparently grown rusty. The surface seems to be sprinkled with powdered red ochre, and grown sickly under the operation. Pluck it carefully, and examine it with a pocket lens. Already the structure of a healthy leaf is familiar to you, but in the present instance the cuticle is traversed with numerous longitudinal cracks or fissures, within which, and about their margins, you discern an orange powder, to which the rusty appearance of the leaf is due. Further examination reveals also portions in which the cuticle is distended into yellowish elongated pustules, not yet ruptured, and which is an earlier stage of the same disease. This is the “rust” of the agriculturist, the _Trichobasis rubigo-vera_ of botanists, the first phase of the corn mildew. To know more of this parasite, we must have recourse to the microscope; having therefore collected a few leaves for this purpose, we return homewards to follow up the investigation. We will not stay to detail the processes of manipulation, since these will not offer any deviation from the ordinary modes of preparation and examination of delicate vegetable tissues.