TITLE:
Cambial Variants Combine Successive Cambia and Intraxylary Phloem in Convolvulaceae in Taiwan
AUTHORS:
Sheng-Zehn Yang, Po-Hao Chen, Chien-Fan Chen
KEYWORDS:
Arcs/Segments, Convolvulaceae, Dissected Xylem, Lobe Stem
JOURNAL NAME:
American Journal of Plant Sciences,
Vol.11 No.3,
March
26,
2020
ABSTRACT: Secondary stem growth develops different cambial
variants in Convolvulaceae. Here, the cambial variant features of 17 species in
seven genera (Convolvulaceae) in Taiwan are presented. The xylem rings produce
various successive cambial types and the primary xylem and intraxylary phloem are
produced in the outer and inner pith, respectively. The two Argyreia species have round or
elliptical stems with continuous secondary xylem and phloem rings. In the eight Ipomoea species, secondary growth has
two to four layers of successive cambia and xylem, dispersed with parenchyma
tissue, except for I. nil and I. violacea with one layer of successive
cambia. The first secondary xylem segments are separated by few or many xylem
rays: I. cairica and I. obscura have three to five xylem rays and I. triloba has numerous xylem rays. The first secondary xylem of Erycibe
henryi is divided by numerous
rays in small stems, but develops more layers, as adjacent segments separated by xylem rays, in larger stems. Owing
to continuous vascular bundle division, it has a dissected xylem. The secondary growth of Distimate tuberosa and Operculina turpethum produces two
layers of successive cambia; the secondary successive cambium is divided evenly
by rays into many arcs/segments. The stem
thickness of I. pes-caprae subsp. brasiliensis increases via concentric successive
cambia. Owing to shallowly lobed stems with directional successive cambia, I. obscura and I. triloba stems are irregularly shaped. I. littoralis and Stictocardia
tilifolia stems are triangular without directional successive cambia. I. hederifolia and Merremia gemella stems have two to three deep lobes. Parenchyma
proliferation results in thicker cortex with mucilaginous canals. The xylem
vessels are dispersed and diffuse-porous; the vasicentric paratracheal
parenchyma around them is common to all species. A bracketed key was developed
comparing the secondary xylem structures of the 17 species, providing a basis
for further morphological studies.