There are few data on the rare and endangered orchids of Renanthera Lour. Floral morphology and embryological features of anther development in R. imschootiana were investigated, and the taxonomic significance of R. imschootiana was analyzed by comparing with existing data. The results were as follows: (1) Flowers were characterized by such taxonomic features as tri-lobed labium with an unconspicous spur, cylindrical column, purple anther cap with yellow stripes, and a pair of deeply cleft pollinia connected by a well-developed stipe and a viscidium. (2) Anther primordium was differentiated into two lateral thecae containing two densely dyed microsporangia, and then, a sterile septum was formed longitudinally and unequally which divided each microsporangium into two jointly sub-microsporangium. The septum became absorbed during microsporogenesis and collapsed when anther ripened, resulting in a pair of deeply cleft pollinia. (3) The developed anther wall comprised 5-6 layers, including an epidermis, 2-3 endothecium, a middle layer, and a tapetum, which conformed to the massive type. Tapetal cell was uni-nucleated and glandular. (4) Microspore mother cells underwent a simultaneous cytokinesis during meiosis resulting in the tetrahedral or isobilateral microspore tetrads. Finally, each microspore retained in tetrads and produced 2-celled pollen via mitosis. Pollen tetrads were compactly arranged, leading to a hard cleft pollinia in each thecae. No pollen abortion was observed during anther development. The results provide new reference for taxonomy and conservation of Renanthera.
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