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The color of glomalean fungal spores has not been easy to communicate in the literature for several basic reasons: (i) each person has a different perception of a particular color and its name, (ii) equipment and light conditions used to record color are not standardized between (or even within) laboratories, (iii) most color charts in general use are designed for other purposes (e.g. classification of soils, etc.) thus do not bracket the range of colors typically found in spores of most glomalean fungal species, and (iv) charts often differ in color specification methods and thus are not comparable.
Color variation is much greater amongst spores within some species compared to others. For example, range of color in many Acaulospora and Entrophospora species is narrow (shades of the same color), but can range from white to dark yellow in some species of Glomus. This variation must be carefully documented to determine if color is sufficiently conserved to represent stable discontinuities between species rather than contingent population-level variation. Color measurements apply not only to intact spores, but also to subcellular structures after spores have been placed in a mountant (including Melzer's reagent).
We have chosen to create one chart which represents most of the range in color of intact spores and their structures after being mounted in PVLG or PVLG + Melzer's reagent (or variations thereof). It can be viewed here as an optimized html document (with resolution too low for adequate printing) or as a printable PDF file. We still have many printed copies of this chart, and 1-2 can be requested at no charge by email to: jbmorton@mail.wvu.edu. The funds to cover the cost of printing and mailing were taken from culture fees.
The chart was created in the graphics software program CorelDRAW for PC-compatible machines. Any graphics package using Process or Spot Color Palettes would have served equally well. We used the Process Color Palette because almost any combination of colors could be blended and printed accurately. Process Color encompasses two models: CMYK (Cyan, Magenta, Yellow, blacK) and RGB (Red, Green, Blue). Each model produces color differently. The RGB model was avoided because colors are subtractive and thus do not parallel biological processes. For example, 100% of all three colors produces white; 0% produces black. The CMYK model was chosen because colors are additive. A percentage of each color component is combined to determine the final color product. From a practical standpoint, all printing (e.g. paper, transparencies) employs inks in the CYMK model.
Computer users with graphics packages can duplicate the chart or any intermediate color combinations not shown, especially with today’s high-memory graphics cards and high resolution monitors. Computer users can compare the printed chart with the image on their monitor and then make adjustments to obtain comparable output. Then, other intermediate color combinations may be generated as needed.
Yellow is given the most weight in the chart, with each column assigned a different increment alone or in combination with black. The reason for this emphasis is that yellow is the dominant underlying color in spores of many species. With yellow as the base, cyan and magenta are added in percentage increments in rows. High percentages of cyan and magenta represent colors found in reactions of the spore wall (Glomus, Gigaspora, Scutellospora) and in inner flexible walls (Acaulospora, Entrophospora, Scutellospora) to Melzer's reagent. Colors not shown include black and black with a red tinge, both of which encompass the darkest color extremes of Glomus constrictum, Scutellospora nigra, S. coralloidea, and S. gregaria spores. Spores of most other species will be black if their contents are heavily parasitized by saprophytic fungi, bacteria, actinomycetes, or combinations of these.
The process of comparing spore color with chart colors rarely is the same for different workers. To minimize the disparity, readings should be taken of spores extracted only from fresh, mature cultures. Measurements from field-collected spores have little diagnostic value, because color is modified easily by biotic and abiotic factors. The light source on spores and the chart should be matched as much as possible. This can be achieved most easily under a stereomicroscope using a light source with two fiber optic tubes. One tube is aimed at a cluster of spores and the other is aimed at the appropriate region of the color chart. Angle of lights, distance from lights to paper and spores, also should be as similar as possible. Despite normal limitations, the color chart can be an important tool in assessing color variation among spores and reporting these values so they are interpretable by others.
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