This section is from the book "Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc.", by George Francis Atkinson. Also available from Amazon: Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc..
In many cases it is desirable to obtain spores in a mass on paper in order to know the exact tint of color produced by the species. Often the color of the spores can be satisfactorily determined by an examination of them under the microscope. One cannot always depend on the color of the lamellae since a number of the species possess colored cystidia or spines in the hymenium which disguise the color of the spores. The best way to determine the color of the spores in mass is to catch them as they fall from the fruiting surface on paper. For the ordinary purpose of study and reference in the herbarium the spores caught on unprepared paper, which later may be placed in the packet with the specimen, will answer. This method has the advantage of saving time, and also the danger of injury to the spores from some of the fixatives on prepared paper is avoided. If for purposes of illustration one wishes pretty spore prints, perfect caps must be cut from the stem and placed fruiting surface downward on paper prepared with some gum arabic or similar preparation spread over it, while the paper is still moist with the fixative, and then the specimen must be covered with a bell-bar or other receiver to prevent even the slightest draft of air, otherwise the spores will float around more or less. The spores may be caught on a thin, absorbent paper, and the paper then be floated on the fixative in a shallow vessel until it soaks through and comes in contact with the spores. I have sometimes used white of egg as a fixative. These pieces of paper can then be cut out and either glued to cardboards, or onto the herbarium sheet.
This should be done as soon as possible after collection. A large table in the sorting room is convenient, upon which the specimens may be spread, or grouped rather, by species, the individuals of a species together, on sheets of paper. Surplus dirt, or wood, leaves, etc., can be removed. A few of the specimens can be turned so that spores can be caught on the papers. If only one or a few specimens of a given species have been found, and it is desirable not to cut off the cap from the stem, the plant can be supported in an upright position, a small piece of paper slit at one side can be slipped around the stem underneath the cap, on which the spores will fall. Sometimes it will be necessary to cover the plant with a bell-jar in order to prevent it from drying before the spores are shed. Experience with different species will suggest the treatment necessary.
Very few probably realize the desirability of making notes of certain characters while the plants are fresh, for future reference, or for use by those to whom the plants may be sent for determination. It is some trouble to do this, and when the different kinds are plentiful the temptation is strong to neglect it. When one has available books for determination of the species, as many as possible should be studied and determined while fresh. But it is not always possible to satisfactorily determine all. Some may be too difficult for ready recognition, others may not be described in the books at hand, or poorly so, and further the number of kinds may be too great for determination before they will spoil. On these as well as on some of the interesting ones recognized, it is important to make a record of certain characters. These notes should be kept either with the specimen, or a number should be given the specimen and the notes kept separately with the corresponding number.