STRUCTURE OF GRAVITY‐SENSITIVE SHEATH AND INTERNODAL PULVINI IN GRASS SHOOTS

Abstract

The location and some morphological, anatomical, and functional aspects of the gravity‐sensitive pulvini of a selected number of grass shoots are examined. There are two distinct gravity‐sensitive regions near the nodal regions of Gramineae. One, the leaf sheath pulvinus, is located at the base of the sheathing leaf bases, and is characteristic of the subfamily Festucoideae. The other, the internodal pulvinus, is located at the base of the internode, a little above the nodal joint. Most members of the Panicoideae possess internodal pulvini, in addition to more or less developed leaf sheath pulvini. Three members of the Oryzoideae examined possess leaf sheath pulvini only, while Phragmites australis (Arundinoideae) possesses both leaf sheath and internodal pulvini. Leaf sheath pulvini of some grasses develop hairs, cork‐silica cell pairs or stomatal apparatuses over the epidermis while many others are devoid of any such idioblasts. Both the leaf sheath and internodal pulvini of all grasses examined preferentially exclude, or accumulate very little silica, whereas the regions of the shoot immediately above and below the pulvini in these same grasses accumulate large quantities of silica. Pulvini remain unsilicified or poorly silicified throughout their life and even after several days following geotropic bending. Pulvini are also distinguished from the regions above and below them by the lack of lignin in the bundle cap cells. Lignin is found only in the xylem vascular tissue, and this consists of annular and helical vessel elements only. The bundle cap cells are rich in pectin and are described as collenchymatous. All pulvini possess specialized zones of cells containing starch statoliths. In response to horizontal displacement of the shoots, the lower side of the pulvini grows by cell elongation only. The collenchymatous cells stretch in a manner that results in alternately thin and thick regions of cell wall.