## 1 Introduction  
Silicon (Si) is a non-essential yet helpful element in most higher plants [1]. Si enhances the resistance of plants to various biological and non-biological stresses [2]. A critical role of Si in plants is to reduce the translocation of As from roots to the shoot and to stimulate the antioxidant system [3]. Silicon deposits within cell walls of the plant endodermis and epidermis, forming amorphous opals called phytoliths [4]. Phytoliths consist mainly of Si and contain many elements and organic matters that were part of the plant, have strong resistance against weathering, and are considered to act as carbon sinks [5]. The encapsulation of metal(loid)s within phytoliths represents a new field of research, and there are two suggested roles of metal(loid)s encapsulation in phytoliths. The first is the sequestration of toxic materials from plant metabolism and the detoxification of the materials [6]. The deposited silica in the plant body also helps the dependent mechanism of the cell wall to block toxic material from the plant cell [7]. The second is the encapsulation of toxic material in soil after plant organic matter degrades. There are two opposing theories regarding phytolith-encapsulated metal(loid)s. The first one states that phytoliths can be considered stabilized metal(loid) sinks because the phytolith decomposes more slowly than plant organic matter [8]. The second considers metals stored within phytoliths as a possible leaching source, thus necessitating management for reducing the rate of runoff [9, 10, 11]. Another hypothesis suggests that phytoliths resistant to degradation would sequester metal(loid)s inside plants, thereby reducing the metal(loid)s-associated toxicity [6, 12]. The previous studies mentioned above have been inconclusive on the matter; it is agreed that phytolith-encapsulated metal((iod)s) could affect the fate of metal((iod)s) and, consequently, the environment at contaminated sites. Most previous studies on metal((iod) accumulation in phytoliths were conducted with Gramineae species [6; 8; 10; 12]. These are well-known phytolith accumulators and are appropriate models for studying phytoliths. However, the ability of different plant species to accumulate metal(iod)s also needs to be considered, particularly for hyperaccumulators of specific metal(iod)s, to improve our understanding of phytolith-encapsulated metal((iod)s. _Pteris multifida_ is a well-known As hyperaccumulator [13] that is grown and researched in Korea [14; 15]. _P. multifida_ phytolith accumulation has also been previously studied [16]. Further, _P. multifida_ is expected to have the potential of storing large amounts of arsenic inside phytoliths. Higher soil water content and plant water uptake can increase Si uptake into plants, as well as its accumulation in phytoliths [17], potentially affecting metal((iod) accumulation in phytoliths. The differences in the phytolith amount and shape were considered in ancient agricultural irrigation [18]. _P. multifida_ is a well-known As hyperaccumulator, with the relationship between soil water and As accumulation being well established [19]. Understanding the differences in As accumulation within _P. multifida_ phytoliths with regard to different soil water contents is of interest for understanding how phytolith-encapsulated As may affect the environment. Comparison of Gramineae and _Pteris_ species can be a guide for formerly studied species and newly suggested species. Two species that are also comparable in As contaminated the metal mine area as widespread abundant plant species and, in most of the studies, _Pteris_ species accumulate more As than Gramineae species [20; 21; 22; 23]. In this study, As accumulations in _Pltragmites australis_ and _P. multifida_ were compared for different soil water contents. Comparing _P. multifida_ with _P. australis_ can be a helpful case study to understand possible amount of phytolith encapsulated As and characteristics of As encapsulation in phytolith of As hyper-accumulator.