Let the first bright day in May witness the student beside a cluster of plants of _Mercurialis perennis_, which it will not be difficult to find in many localities, and, on turning up the lower leaves, he will meet with our first illustration of a genuine _Uredo_, in the form of yellow confluent patches, with a powdery surface (Plate VII. fig. 133). This will be _Uredo confluens_. By the way, the generic name is in itself suggestive, which it always should be in all instances, but unfortunately is not; it is derived from the Latin word _uro_, “I burn,” and is peculiarly applicable in instances where the leaves acquire a blistered, burnt, or scorched appearance, occasioned by the presence of the fungus. The microscopical features of the spores of this genus are, briefly, a more or less spherical form, without any pedicel or footstalk Plate VII. fig. 134). These spores are at first produced each in a separate cell, but when ripe become free, and are at times with difficulty distinguished from such forms as _Trichobasis_, unless the pedicels in the early stage of the latter genus are regarded. A rare species in Britain is the oak-leaf rust (_Uredo Quercus_), in which the sori or pustules are minute, and at first yellow, but afterwards orange. It occurs on the under surface of the leaves, and was, we believe, first found in this country by Mr. D. Stock, in the neighbourhood of Bungay, nearly five-and-twenty years since. That gentleman informs us that it was not at all uncommon, but always appeared on the young shoots which had sprung up from the trunks or roots of trees that had been cut down. We are uncertain whether it has been found by any one since that time in this country, although it is not uncommon in France. The leaves of the common sorrel are often sparely sprinkled with the pustules of a rust (_Uredo bifrons_, Grev.), which derives its specific name from the fact that the pustules, which appear on both surfaces of the leaves, are often opposite to each other Plate VII. fig. 137). These pustules are generally seated on a discoloured spot (Plate VII. fig. 138 enlarged), and are surrounded by the remains of the ruptured epidermis. The spores are globose and brown Plate VII. fig. 139). We have not met with any other _Uredine_ on the sorrel leaves, though one having a similar appearance to the naked eye is not uncommon on several species of dock. The fern rust (_Uredo Filicum_), which occurs on the under surface of the fronds of two or three species of ferns, we have never met with, and do not think that it can be considered common. It has been found in the West of England, on _Cystopteris_, and Sowerby collected it, probably not very far from London.