Structure and Function of Silica Bodies in the Epidermal System of Grass Shoots

Abstract

Silica (SiO₂.nH₂O) is deposited in large quantities in the shoot systems of grasses. In the leaf epidermal system, it is incorporated into the cell wall matrix, primarily of outer epidermal walls, and within the lumena of some types of epidermal cells. This biogenic silica can be stained specifically with methyl red, crystal violet lactone, and silver amine chromate. At the ultrastructural level, the silica in lumens of silica cells, bulliform cells and long epidermal cells is made up of rods about 2.5 μm in length and 0.4μm in width. Ultimate particles in the rods range from 1 to 2 nm in diameter. In contrast, silica in the cell wall matrix of trichomes and outer walls of long epidermal cells is not rod-shaped, but rather, forms roughly spherical masses. Detailed analyses are presented on the frequencies of occurrence of the different types of epidermal cells that contain silica in the leaves of representative C₃ and C₄ grasses. The C₄ grasses have higher frequencies of bulliform cell clusters, silica cells, and long epidermal cells, whereas the C₃ grasses have higher frequencies of trichomes. No correlation was found in the frequency of occurrence of silica bodies in bulliform cells for C₃ grasses as compared with C₄ grasses. Of all the grasses examined, Coix, Oryza, and Eleusine had the highest densities of such bodies, and some taxa had no silica bodies apparent in their bulliform cells. The idea that silica bodies in bulliform cells and silica cells might act as “windows’ and trichomes might function as ‘light pipes’ to facilitate light transmission through the epidermal system to photosynthetic mesophyll tissue below was tested. The experimental data presented do not support either of these hypotheses.