Complete tylosis formation in a latest Permian conifer stem

Abstract

Background and aims: Our knowledge of tylosis formation is mainly based on observations of extant plants; however, its developmental and functional significance are less well understood in fossil plants. This study, for the first time, describes a complete tylosis formation in a fossil woody conifer and discusses its ecophysiological implications.

Methods: The permineralized stem of Shenoxylon mirabile was collected from the upper Permian (Changhsingian) Sunjiagou Formation of Shitanjing coalfield, northern China. Samples from different portions of the stem were prepared by using the standard thin-sectioning technique and studied in transmitted light.

Key results: The outgrowth of ray parenchyma cells protruded into adjacent tracheids through pits initially forming small pyriform or balloon-shaped structures, which became globular or slightly elongated when they reached their maximum size. The tracheid luminae were gradually occluded by densely spaced tyloses. The host tracheids are arranged in distinct concentric zones representing different growth phases of tylosis formation within a single growth ring.

Conclusions: The extensive development of tyloses from the innermost heartwood (metaxylem) tracheids to the outermost sapwood tracheids suggests that the plant was highly vulnerable and reacted strongly to environmental stress. Based on the evidence available, the tyloses were probably not produced in response to wound reaction or pathogenic infection, since evidence of wood traumatic events or fungal invasion are not recognizable. Rather, they may represent an ecophysiological response to the constant environmental stimuli.

Keywords: China; Shenoxylon mirabile; conifer wood; ecophysiological response; fossil plant; late Permian; tylose.

Fig. 1.

Shenoxylon mirabile from the upper Permian Sunjiagou Formation, Shitanjing coalfield, northern China. (A–D, F) Transverse sections: (A) tylose-filled tracheids in a concentric zonate distribution (arrows indicate the tylose-free tracheids in the middle of the image which appear brighter in transmitted light); (B) tylose-free tracheids irregularly distributed within the zones of tylose-filled tracheids; (C) close-up of densely spaced tyloses in tracheids; (D) isolated tyloses showing spheroidal or ellipsoidal shape; (F) a tylose sometimes shows an irregular outline. (E) Tangential longitudinal section showing a small pyriform tylose growing out from the parenchymatous ray cell through a pit. Scale bars are as indicated on the images. Specimen and thin sections illustrated in this paper are housed in the Palaeobotanical Collections of the Yunnan Key Laboratory for Palaeobiology, Yunnan University, with catalogue number YKLP20005.

Fig. 2.

Shenoxylon mirabile from the upper Permian Sunjiagou Formation, Shitanjing coalfield, northern China. (A–G) Radial longitudinal sections: (A, B) successive fields showing tracheids in the tylose-filled wood zones; (C) densely spaced tyloses in the tracheids; (D) vertically aligned tyloses in tracheids; (E) densely spaced and vertically aligned tyloses in isolated tracheids; (F) isolated tyloses showing spheroidal or ellipsoidal shapes; (G) vertically aligned tyloses near the end wall of tracheids which have a smaller diameter. (H) Tangential longitudinal section showing tyloses with a smaller diameter toward the tapering ends of the tracheids. Scale bars are as indicated on the images.

Fig. 3.

Shenoxylon mirabile from the upper Permian Sunjiagou Formation, Shitanjing coalfield, northern China. (A, B) Radial longitudinal sections of secondary xylem in the same area in different focal planes, showing bordered pits on the radial surface of tracheids (A) and the tyloses inside the tracheids (B). (C–F) Tangential longitudinal sections of the secondary xylem: (C) single spheroidal tylose shows a narrow base; (D) single ray cell protrudes into different tracheids (arrows); (E) multi-tyloses emerging from the ray cells through the pits into single tracheid; (F) ray cells and tylosis initials commonly contain black substances. (G, H) Radial longitudinal sections of pith and primary and secondary xylem: (G) densely spaced tyloses (arrow) occurring in the innermost portion of the heartwood; (H) densely spaced tyloses (arrow) in the tracheids of the primary xylem. Abbreviations: PX, primary xylem; SX, secondary xylem; PTX, protoxylem; MX, metaxylem. Scale bars are as indicated on the images.