We have adopted, for our own herbarium, the “foolscap” size. A sheet of paper receives within its fold the specimens of a single species; these are affixed to the right-hand page, when the sheet is open, and a small envelope is attached by its face to the same page at the bottom, in which loose specimens are kept for minute and special examination, or as duplicates. When the sheet is folded, the specific name is written at the right-hand lower corner, or, what is better, a strip containing that name and its number is cut from a copy of the “Index Fungorum,” kept for the purpose, and gummed in its place. The remainder of this page, which is of white cartridge paper, is occupied with memoranda referring to the species enclosed, sketches of the spores, synonyms, references to descriptions, &c. All the species-papers of each genus are placed together within a sheet of brown paper, half an inch larger in each direction, with the name of the genus written at the left-hand corner. A piece of millboard, the size of the covers when folded, separates each order. When a leaf, or other portion of a plant, is to be examined under the microscope, with the view of determining the genus and species of its parasite, it may be fastened with small pins to a piece of sheet cork, two or three inches square, and about one-eighth of an inch in thickness, such as used for lining entomological cabinets, and so placed under a lens that it may easily be brought into focus, and both hands left at liberty; or a dissecting microscope may be used for the purpose. From one of the pustules the spores may be removed on the sharp point of a penknife, and placed in a drop of water on a glass slide. A thin glass cover is placed over the drop of water, and the slide is submitted to examination. For further satisfaction it will often be found necessary to make carefully a thin section of a pustule, and place this under the microscope, a more troublesome but also much more satisfactory method. Reference to the Appendix will soon determine the name and position of the fungus, provided it belongs to the section to which this volume is devoted. If it is thought desirable to mount the spores as permanent objects, there is no obstacle to such a proceeding. The spores of the different species of _Aregma_, of _Triphragmium_, and many of the _Pucciniæ_, will be worth the trouble. We have tried several media, and only adopted Glycerine or Balsam; either of these, especially the former, if the greater difficulty of securely closing can be overcome, will answer the purpose. It should be remarked that in the examination of moulds, such as those of the genus _Peronospora_, included in this work, if any fluid be added, the acrospores are immediately disconnected from the threads, and float in the medium; so that if their mode of attachment or arrangement would be studied, that must be achieved without the addition of any fluid whatever.