Figure 1. Appearance and surface structures of the cactus.

(a) Optical image of a plant of O. microdasys stem covered with well-distributed clusters of spines and trichomes. (b,c) Magnified optical images of a single cluster with spines growing from the trichomes in the top (b) and side (c) view. (d) SEM image of a single spine divided into three regions, the tip (e) with an apex angle (2α) and oriented barbs, the middle (f,g) with gradient grooves, and the base with belt-structured trichomes. (f,g) Magnified images of regions near the base and tip of the cactus spine, respectively. The microgrooves near the base are wider and sparser than those near the tip. (h) Magnified image of a single barb with an apex angle (2β) covering the tip of the spine (e). Scale bars, 5 cm (a), 500 μm (b,c), 100 μm (d), 20 μm (e–g) and 2 μm (h).