## 2 Role of Macrophyytes in CWs  
The plants that grow in constructed wetlands have several properties related to the water treatment process that make them an essential component of the design. Macrophytes are the main source of oxygen in CWs through a process that occurs in the root zone, called radial oxygen loss (ROL) [17]. The ROL contributes to the removal of pollutants because it favors an aerobic micro-environment, and waste removal is therefore accelerated, whereas, in anaerobic conditions (the main environment in CWs), there is less pollutant removal. In a recent study [18] comparing the use of plants in high density (32 plants m\({}^{-2}\)) and low density (16 plants m\({}^{-2}\)) CWs, the removal of nitrogen compounds in high density CWs was twice that of CWs using a low density of plants, which is strong evidence of the importance of plants in such systems. The removal rate of total nitrogen (TN) and total phosphorous (TP) were also positively correlated with the ROL of wetland plants, according to a study involving 35 different species [19].  
The roots of plants are the site of many microorganisms because they provide a source of microbial attachment [8] and release exudates, an excretion of carbon that contributes to the denitrification process, which increases the removal of pollutants in anoxic conditions [20; 21]. Other physical effects in plant tissue in water include: reduction in the velocity of water flow, promotion of sedimentation, decreased resuspension, and uptake of nutrients. However, for roots and rhizomes in the sediment, the physical effects include: stabilizing the sediment surface, less erosion, nutrient absorption, prevention of medium clogging (in subsurface conditions) and improved hydraulic conductivity. Aerial plant tissue favors in the light attenuation (reduced growth of photosynthesis), reduced wind velocity, storage of nutrients and aesthetic pleasing appearance of the system [2; 5]. A 5-year study evaluated the influence of vegetation on sedimentation and resuspension of soil particles in small CWs [22]. The author showed that macrophytes stimulated sediment retention by mitigating the resuspension of the CW sediment (14 to 121 kg m\({}^{-2}\)). Macrophyytes increased the hydraulic efficiency by reducing short-circuit or preferential flow. Plant presence led to decreasing saturated hydraulic conductivity in horizontal subsurface flow. This study was relevant, since monitoring macrophytes is essential for understanding and controlling clogging in subsurface CWs [22].  
The removal of organic and inorganic pollutants in CWs is not only the role of microorganisms. This function is also exerted by plants that are able to tolerate high concentrations of nutrients and heavy metals, and, in some cases, plants are able to accumulate them in their tissues [23]. It has been estimated that between 15 and 32 mg g\({}^{-1}\) of TN and 2-6 mg g\({}^{-1}\) (dry mass) of TP are removed by CW plants, which was measured in the aboveground biomass [24; 25].  
Other uptakes of xenobiotic compounds (organic pollutants) are also the result of the presence of plants, involving processes such as transformation, conjugation and compartmentation [23].