Sambong plants were collected in Luodian (Guizhou, China), authenticated by Prof. Yu-Xin Pang. Voucher specimens (TCGRI 10011) have been deposited at the Traditional Medicinal Plant Germplasm Nursery of South China, Tropical Crops Genetic Resources Institute, Chinese Academy of Tropical Agricultural Sciences, Hainan, China.

Fresh and healthy stems tips and leaves were collected. They were directly cut into 5 mm long or 4 × 4 mm² in area without fixed. Each pieces of stems tips or leaves were embedding on a sample table with Jung tissue freezing medium (Leica, German) under −18˚C for 10 min, and then cut in Leica CM1900 under −18˚C. The thickness range is 10 - 30 μm. The sections were observed with a computer-equipped microscope.

Fresh and healthy leaves were collected and cut into 1 × 1 mm pieces. Fixed in 2.5% paraformaldehyde―3.0% glutaraldehyde (dissolved in MPBS, pH 7.2) and kept under 4˚C for 12 h. Then wash the leaf pieces with phosphate buffer for three times. Post-fix the pieces with osmic acid under room temperature for 2 h, then wash them with phosphate buffer for three times. Dehydration the pieces with 50%, 70%, 85%, 95%, 100%, 100% ethanol, then transition into epoxy propane. Embed the pieces with Epon812 epoxy resin. Kept them under 40˚C for 24 h, then 60˚C for 24 h. Cut the pieces with Leica EM UC6 microtome with a thickness of 900 nm. Stain the sections with 0.5% toluidine blue. Observe the sections with a computer-equipped microscope.

The sambong leaf was firstly originated from leaf primordium nearby the stem tip. The first leaf grows fast, that the mitosis there happens frequently, and the cytoplasm manufacture rapidly (Figure 1(a) and Figure 1(b)). Midrib develops earlier than the blade (Figure 1(c)). The mesophyll cells in the first few leaves have not yet differentiated into the spongy parenchyma and palisade parenchyma (Figures 2(a)-(d)). Then the vascular tissue formed (Figure 2(e)). In the mature leaf, the spongy parenchyma and palisade parenchyma were fully developed (Figure 2(f)).

The stomas were firstly developed from the epidermis cells of the sambong leaf. There are two kinds of epidermal

hairs in sambong leaf epidermis. The non-glandular hairs do not secrete. These hairs are usually long, consists of multiple cells with thin cytoplasm. The glandular hairs secrete. They are much shorter than the non- glandular hairs, usually 40 - 60 μm by length, consists of two columns of cells with dense cytoplasm (Figure 2(d)). Most epidermal hairs generate from the epidermis cells on the first leaf. Less epidermis hairs were found generated on mature leaf. As the leaf grow, the density of the epidermal hairs decrease (Figures 2(d)-(f)).

The mature sambong leaf is bifacial leaf. Leaf epidermis is consisted of one layer of cells. The upper epidermis is thicker than the lower one. There were stomata on the upper and the lower epidermis while the stomata frequency in lower epidermis is higher. The mesophyll tissue in leaf was developed into palisade parenchyma and spongy parenchyma. The former is nearby the upper epidermis, and is consisted by a layer of columnar cells. The latter, loose irregularly shaped cells, packed with large air spaces, allow rapid diffusion of gases to.