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phytolith
Phytoliths (from Greek, "plant stone") are rigid, microscopic structures made of silica, found in some plant tissues and persisting after the decay of the plant. These plants take up silica from the soil, whereupon it is deposited within different intracellular and extracellular structures of the plant. Phytoliths come in varying shapes and sizes. Although some use "phytolith" to refer to all mineral secretions by plants, it more commonly refers to siliceous plant remains. In contrast, mineralized calcium secretions in cacti are composed of calcium oxalates.〔Piperno, Dolores R. (2006). Phytoliths: A Comprehensive Guide for Archaeologists and Paleoecologists. AltaMira Press ISBN 0759103852.〕
==Functions==
There is still debate in the scientific community as to why plants form phytoliths, and whether silica should be considered an essential nutrient for plants. Studies that have grown plants in silica-free environments have typically found that plants lacking silica in the environment do not grow as well. For example, the stems of certain plants will collapse when grown in soil lacking silica. In many cases, phytoliths appear to lend structure and support to the plant, much like the spicules in sponges and leather corals. Phytoliths may also provide plants with protection. These rigid silica structures help to make plants more difficult to consume and digest, lending the plant's tissues a grainy or prickly texture.〔Hunt JW, Dean AP, Webster RE, Johnson GN, Ennos AR (2008) (A novel mechanism by which silica defends grasses against herbivory. ) ''Ann Bot'' 102 (4):653-6. PMID 18697757〕 Phytoliths also appear to provide physiologic benefits. Experimental studies have shown that the silicon dioxide in phytoliths may help to alleviate the damaging effects of toxic heavy metals, such as aluminum.
Finally, calcium oxalates serve as a reserve of carbon dioxide. Cacti use these as a reserve for photosynthesis during the day when they close their pores to avoid water loss; baobabs use this property to make their trunks more flame-resistant.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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