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4. B. E. Balkovskii, A Digital Polytomous Key for Plant Identification (Naukova Dumka, Kiev, 1964) [in Russian].
5. L. P. Gvozdeva and T. Zh. Dzharmagambetov, “A Perforated-Card Key of the Flowering Plants of Kazakhstan and the Principles of Its Creation,” in Biological Sciences (Alma-Ata, 1974), Issue 1, pp. 34–43 [in Russian].