As a good stain for showing lignin, phloroglucinol is usually supplied by schools as an alcohol solution. Treat the sections with this stain and then acidify the preparation with a drop of concentrated hydrochloric acid. The lignin on the cell wall is stained red. Alternatively, you can use pre-mixed acidified phloroglucinol. In this case, it will be a single step, but I believe that the old mixed colorant formulations may lose their hydrochloric acid content and therefore are less effective.
Phloroglucinol and hydrochloric acid are corrosive, and polyphenols are also harmful, so avoid contact with skin. Work quickly and cleanly and avoid breathing the fumes from the bottle. It is also recommended that you wear gloves when handling preparations. Once the sections are stained, I think they can be fixed in glycerin or similar fixative, which is safer than leaving them in acid dye.
If you plan to use these compounds, please discuss the necessary safety measures with your teacher, because they know better what facilities you have to handle them.
If you want to observe the three-dimensional structure of cells, you can also prepare some plant materials in the immersion solution, and then stain them in phloroglucinol. This will tell you exactly where lignin is deposited in the cell wall and let you know if lignin is deposited uniformly. You can also observe the deposition patterns of different cell types and try to associate them with functions.
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