Petkova, Nadezhda, et al. “PHYTOCHEMICAL STUDY OF COMFREY (SYMPHYLUS OFFICINALE L.) ROOT EXTRACTS”. ENVIRONMENT. TECHNOLOGIES. RESOURCES. Proceedings of the International Scientific and Practical Conference, vol. 1, June 2024, pp. 295-9, https://doi.org/10.17770/etr2024vol1.7942.