Fig. 2.

The underground stem morphological characteristics and developmental progress of Salvia castanea. (a) Annual underground stem of S. castanea, and its different cross-sections from down to up.Fig. 2c-h presented more exquisite paraffin cross-sections, revealing the gradual differentiation to form branches-like structures i.e. 3 branches, 4 branches, 5 branches, 6 branches and so on with the growth of underground stem. Fig. 2i-j were the vertical sections of underground stem, especially Fig. 2j uncovered the bifurcating process of xylem to generate multiple vessel tissues. The core of underground stem was like the results of programmed cell death, and generated the periderm-like structures in the center of underground stem to protect the new generated branches-like structures. (b) Perennial underground stem of S. castanea and cross-sections from down to up.Fig. 2k-l were the paraffin cross-sections of perennial underground stem of S. castanea, with the multiple xylem structures squeezed together. Fig. 2m was the vertical section of S. castanea, presenting multiple xylem vessels with more periderm-like structures in underground stem. Fig. 2n was the paraffin vertical section of perennial underground stem in S. castanea.