H.  Tsukaya, Nakajima, M., and Okada, H., ?Didymoplexiella cinnabarina (Orchidaceae): a new species from Muller Range, Central Kalimantan, Indonesia?, Acta Phytotax. Geobot., vol. 56(3), pp. 207 - 212, 2005.

K.  Suetsugu, Tsukaya, H., Nurainas, N., and Okada, H., ?Thismia sumatrana (Thismiaceae), a new species from West Sumatra, Indonesia, with discussions on the taxonomic identity of Thismia clavigera?, PhytoKeys, vol. 113, pp. 59 - 67, 2018.

H.  Tsukaya, Yokota, M., and Okada, H., ?Chromosomal characteristicsof 0xygyne shinzatoi (Burmanniaceae), and its phylogenetic significance?, Acta Phytotax.Geobot., vol. 58(2/3), pp. 100 - 106, 2007.

H.  Tsukaya and Okada, H., ?A new species of Lecanorchis Blume (Orchidaceae, Vanilloideae) from Kalimantan, Borneo?, Syst. Bot., vol. 38(1), pp. 69 - 74, 2013.

H.  Tsukaya and Okada, H., ?A new variety of Didymoplexis cornuta (Orchidaceae) from West Kalimantan, Borneo?, Acta Phytotax. Geobot., vol. 62 (2/3), pp. 89 - 93, 2012.

H.  Tsukaya and Okada, H., ?A color variaton of Epirixanthes species (Polygalaceae) found in West Kalimantan, Borneo, Indonesia?, Acta Phytotax. Geobot., vol. 62 (2/3), pp. 95 - 97, 2012.

H.  Tsukaya, Suleiman, M., and Okada, H., ?Didymoplexiella trichechus (J.J.Sm.) Garay and a new variety of Didymoplexis cornuta J.J.Sm. (Orchidaceae) in Borneo?, Acta Phytotax, vol. 62 (2), pp. 105 - 110, 2014.

H.  Tsukaya, Suleiman, M., and Okada, H., ?A new species of Epirixanthes (Polygalaceae) from Imbak Canyon, Sabah, Borneo?, Phytotaxa, vol. 266 (2), pp. 146 - 150, 2016.

H.  Tsukaya, Suleiman, M., and Okada, H., ?A new variety of Thismia hexagona Dan?ák, Hrone?, Koblová et Sochor (Thismiaceae) from Sabah, Borneo, Malaysia?, Acta Phytotax. Geobot., vol. 65, no. 3, pp. 141-145, 2014.

H.  Tsukaya and Okada, H., ?Two new species of Sciaphila Blume (Triuridaceae) from Kalimantan, Borneo, with a new record of S. thaidanica from Borneo?, Syst. Bot., vol. 38, no. 3, pp. 600-605, 2013.

H.  Tsukaya and Okada, H., ?A new species of Thismia (Thismiaceae) from West Kalimantan, Borneo?, Syst. Bot., vol. 37(1), pp. 53 - 57, 2012.

H.  Tsukaya, Nakajima, M., and Okada, H., ?Kalimantanorchis: a new genus of mycotrophic Orchid from West Kalimantan, Borneo?, Sys. Bot., vol. 36(1), pp. 49 - 52, 2011.

H.  Tsukaya and Okada, H., ?Thismia mullerensis (Burmanniaceae), a new species  from Muller Range, central Kalimantan?, Acta Phytotax. Geobot., vol. 56(2), pp. 129 - 133, 2005.