The Chemical Nature of Silica in Plants.

Silica has long been known to be present in plants. Richardson (6) reported its abundance in the aerial parts of plants of the Equisetum genus and many Gramineae, constituting 50 to 70 % of the ash. He also stated that of all elements found in plants, silicon showed the greatest variation between plant parts, plants, and species of plants. Silicon usually occurs in plants in the form of its oxide, Si02, commonly called silica. Although the presence of silica has been established its chemical form in higher plants has not been reported. This study was conducted primarily to provide such information. Since opal and a quartz are produced and exist as minerals at temperatures favorable to plant growth one might expect that silica in plants would be one or the other or both. According to Her (2) the solubility of amorphous silica at ordinary temperatures is most commonly reported to be about 0.010 to 0.015%. Esau (1) stated that some plants absorb more silica than they need and inasmuch as it cannot be excreted, it is deposited in the tissues. In general, the more water absorbed by a plant, the greater the amount of silica deposited. Silica is deposited mostly in cell walls, but sometimes as bodies in the lumen of the cell. The Gramineae is the best known group depositing silica; the deposition takes place in both cell walls and cell lumina. Silica is not usually deposited in underground parts. In this work the silica was characterized by chemical analysis, p?trographie microscope studies, and x-ray diffraction analysis. The p?trographie microscope was used for the initial identification of the type of silica present. The optical characteristics of opal and quartz are quite different so that this method of study is readily used to differentiate between them. X-ray diffraction studies were conducted to supplement the information gained by the p?trographie microscope and to show the presence of minor constituents. The x-ray diffraction pattern of a quartz has been determined by Swanson and Fuyat (7) and x-ray studies of opal have been made by Swineford and Franks (8). This study has also established: (A) unreported types of silica deposition and (B) tissues other than the epidermis as sites of silica deposition. Spodograms were used for this purpose as they are the best method for studying the sites and depositional pattern of silica in situ. Spodograms are made by mounting ash material on microscope slides. The process was developed by ?ber (9) and used by Molisch (3) and Ohki (4) to get distinct, transparent skeletal deposits. Usually, transverse or longitudinal sections of plant tissues are used in the preparation of spodograms. For thin parts such as leaves, the entire tissue has been used by Ponnaiya (5). Ohki (4) studied in detail the spodograms of leaf blades of the Japanese Bambusaceae, covering 6 genera and various species. He found that the pattern of silica deposition was constant and distinct for each species. Ponnaiya (5) modified the technique for preparing spodograms. The material to be studied was placed flat between microscope slides. The material was then ashed in a muffle furnace at between 450 and 500? C. The ash was prepared for study by removing the upper slide, adding Canada balsam directly to the ash mass, and covering with a cover glass. Ponnaiya (5) working on sorghum, found silica to be present in the epidermises of the leaf and the leaf sheath, in the node and internode of stems, in the glume and awn of floral parts, and in traces on the surface of the grain. He described two types of silica deposition in sorghum leaves: (a) dumb-bell shaped structures occurring in regular rows about the veins, and (b) irregularly shaped structures occurring scattered between the regular rows of the dumb-bell shaped units.