Trichome micromorphology of the East Asiatic genus Chelonopsis (Lamiaceae) and its systematic implications

The micromorphology of trichomes of the leaves of 17 taxa (including two varieties) of the genus Chelonopsis Miq. and of six species representing four additional genera (Bostrychanthera deflexa Benth., Colquhounia coccinea Wall. var. coccinea, Co. seguinii Vaniot. var. seguinii, Gomphostemma chinense Oliv. var. chinense, G. crinitum Wall. ex Benth. and Physostegia virginiana (L.) Benth.) was surveyed by light and scanning electron microscopy. Two basic types of trichomes can be identified: non-glandular and glandular trichomes. The non-glandular trichomes can be subdivided into two subtypes: simple unbranched and branched trichomes. Based on the cell number, simple unbranched trichomes are further divided into four shapes (unicellular, two-celled, three-celled, and more than three cells), whilst branched trichomes are separated into three shapes (biramous, stellate, and dendroid trichomes). The glandular trichomes can in turn be subdivided into four subtypes: subsessile, capitate, clavate, and sunken. Non-glandular trichomes with two cells (NGTW) and subsessile glandular trichomes (GSU) are most widespread in all taxa examined. The indumentum shows considerable variation among different sections or species. Consequently, trichome micromorphology and distribution have high taxonomic value for Chelonopsis at both infrageneric and interspecific levels. The presence of capitate glandular trichomes (GCA) provides an additional morphological character to clarify the boundaries between subgenus Chelonopsis and Aequidens Wu and Li. Within subgenus Aequidens, non-glandular trichomes with more than three cells (NGMT) and clavate glandular trichomes (GCL) are important characters for sectional division between sect. Aequidens Wu and Li and sect. Microphyllum Wu and Li. Again, three forms of three-celled trichomes can be used as a distinctive taxonomic character at specific level between C. albiflora Pax et K. Hoffm. ex Limpr., C. forrestii J. Anthony, and C. souliei (Bonati) Merr. in sect. Aequidens. This study supports Wu's delimitation of subgenus and sections and the subsequent review work by Xiang et al. Additionally, distribution of trichome types is correlated with the altitudinal distribution and habitats of some species in Chelonopsis.