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The wide use of hazardous formaldehyde (CH2O) in disinfections, adhesives and wood-based furniture leads to undesirable emissions to indoor environments. This is highly problematic as formaldehyde is a highly hazardous and toxic compound present in both liquid and gaseous form. The majority of gaseous and atmospheric formaldehyde derive from microbial and plant decomposition. However, plants also reversibly absorb formaldehyde released from for example indoor structural materials in such as furniture, thus offering beneficial phytoremediation properties. Here we provide the first comprehensive review of plant formaldehyde metabolism, physiology and remediation focusing on release and absorption including species-specific differences for maintaining indoor environmental air quality standards. Phytoremediation depends on rhizosphere, temperature, humidity and season and future indoor formaldehyde remediation therefore need to take these biological factors into account including the balance between emission and phytoremediation. This would pave the road for remediation of formaldehyde air pollution and improve planetary health through several of the UN Sustainable Development Goals.

Globally, the valorisation of food waste into digestate through the process of anaerobic digestion is becoming increasingly popular. As a result, a large amount of food-waste digestate will need to be properly utilised. The utilisation of anaerobic digestion for fertiliser and alternative uses is essential to obtain a circular bioeconomy. The review aims to examine the environmental management of food-waste digestate, the value of digestate as a fertiliser and soil conditioner, and the emerging uses and improvements for post-anaerobic digestion reuse of digestate. Odour emissions, contaminants in food waste, emission and leaching of nutrients into the environment, and the regulations, policies, and voluntary initiatives of anaerobic digestion are evaluated in the review. Food-waste digestate can provide essential nutrients, carbon, and bio-stimulants to soils and increase yield. Recently, promising research has shown that digestates can be used in hydroponic systems and potentially replace the use of synthetic fertilisers. The integration of anaerobic digestion with emerging uses, such as extraction of value-added products, algae cultivation, biochar and hydrochar production, can further reduce inhibitory sources of digestate and provide additional economic opportunities for businesses. Moreover, the end-product digestate from these technologies can also be more suitable for use in soil application and hydroponic use.

Particulate plastic fragments (micro and nano-plastics) in aquatic environments provide abundant solid substrates, which serve as an important habitat for a variety of microorganisms. Surfaces of microplastics (MPs) exhibit hydrophobicity that facilitate the adsorption of dissolved organic carbon (DOC) in the aquatic environment. Furthermore, MPs act as substrata, as well as a carbon source including the readily bioavailable DOC, which promote the formation of microbial biofilms. These biofilms have varied metabolic actions that govern the subsequent succession of micro- and meso-organisms habitation of MPs. The assemblage of ecosystems colonising the plastic environment is often referred to as the “plastisphere”. Polymer type, environmental conditions, including nutrient status, salinity, and season, affect the microbial composition of the biofilm. Microbial habitation accelerated by biofilm formation on particulate plastics enables the movement of microorganisms, especially in the aquatic environment, and impacts the transport and toxicity of contaminants associated with these particulate plastic fragments. This review paper describes the processes of microbial habitation and subsequent biofilm formation, the factors affecting biofilm formation, and the implications of biofilm formation on the mobility of microorganisms, degradation of MPs, and the bioavailability of contaminants associated with MPs. Refereed/Peer-reviewed

Potentially toxic elements (PTEs) adversely affect biological functions in soil including the activity of microbially mediated enzymes involved in nutrient transformation and cycling in soils. The effect of PTEs on microbial activity in soils depends on the bioavailability of PTEs. The toxicity of PTEs to soil microorganisms can be mitigated by reducing their bioavailability in soils using various organic and inorganic amendments, such as phosphate compounds, liming materials, manure, biosolids, and biochar. This chapter provides an overview of the immobilization of PTEs in soils using organic and inorganic amendments, and its impacts on the bioavailability of PTEs and the microbial response to the immobilization-induced reduction in the bioavailability of PTEs.

Poly- and perfluoroalkyl substances (PFAS) are synthetic chemicals, which reach terrestrial and aquatic environments through anthropogenic activities. Major sources of PFAS in the environment include fire-fighting foams (aqueous film forming foam (AFFF)), wastewater sources, biosolids, and composts. Limited information is available about PFAS in wastewater, which is the focus of this review. PFAS wastewater sources include domestic effluents, industrial effluents, landfill leachates, stormwater, and agricultural effluents through their use in various applications. Land application of PFAS-contaminated wastewater can lead to the contamination of soil and groundwater, thereby reaching the food chain through plant uptake and consumption of potable water. Landfill leachates and industrial effluents contain the highest concentrations of PFAS, posing serious risks to surrounding waterways. Transformation of PFAS precursors can occur through abiotic and biotic processes within the treatment of wastewater, resulting in the formation of harmful PFAS compounds. Currently, there is limited data reported on novel short-chain and ultra-short chain PFAS and PFAS precursors. The strong thermal and chemical stability of PFAS and the complex nature of PFAS mixtures makes the remediation of PFAS in wastewater challenging. However, the review examines and compares current technologies which can treat and remove PFAS from wastewaters. In this review, the distribution, transformation, and remediation of PFAS and their substitutes in wastewater sources are covered. Refereed/Peer-reviewed

Currently, 1.3 billion tonnes of food are thrown away each year, most of which are incinerated or landfilled causing large environmental, social, and economic issues. Therefore, the utilisation of food waste as biofertilisers, such as composts and digestates, is a solution to reduce the problems created by incineration and landfilling whilst simultaneously amending soils. The improper disposal of food wastes and bulking materials can contribute to high levels of contaminants within the end-product. Moreover, the food waste and bulking materials, themselves, may contain trace amounts of contaminants. These contaminants tend to have long half-lives, are easily mobile within soil and plants, can accumulate within the food supply chain, and have moderate to high levels of toxicity. This review aims to examine the current and emerging contaminants of high concern that impact the quality of food-waste fertilisers. The paper presents the volume of current and emerging contaminants of plastics, other physical (particulate) contaminants, heavy metals, pesticides, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), per- and polyfluoroalkyl substances (PFAS), and pathogens within food-waste composts and digestates. Due to the large extent of organic chemical contaminants and the unknown level of toxicity and persistence, the risk assessment of organic chemical contaminants in the food-supply chain remains largely unknown. This study has presented available data from literature of various contaminants found in food waste, and composts and digestates derived from food waste, and evaluated the data with current regulations globally. Overall, to reduce contaminants in composts and digestates, more studies are required on the implementation of proper disposal separation, effective composting and digestion practices, increased screening of physical contaminants, development of compostable plastics, and increased regulatory policies on emerging, problematic contaminants. Moreover, examination of emerging contaminants in food-waste composts and digestates is needed to ensure food security and reduce future human-health risks.

In heavy metal-contaminated farmland, microorganisms or organic fertilizers can be used to minimize heavy metal uptake by crops to ensure food safety. However, the mechanisms by which urease-producing and metal-immobilizing bacteria combined with manure inhibit Cd uptake in wheat (Triticum aestivum L.) remain unclear. Herein, the effects of Enterobacter bugandensis TJ6, sheep manure (SM), and TJ6 combined with SM on Cd uptake by wheat and the mechanisms involved were investigated under field conditions. The results showed that strain TJ6 increased the urease activity and the proportion of strains with a high Cd adsorption capacity in SM, thereby enhancing the Cd adsorption capacity of SM in solution. Strain TJ6 combined with SM improved the rhizosphere soil urease activity, NH

Land treatment has become an essential waste management practice. Therefore, soil becomes a major source of contaminants including organic chemicals and potentially toxic elements (PTEs) which enter the food chain, primarily through leaching to potable water sources, plant uptake, and animal transfer. A range of soil amendments are used to manage the mobility of contaminants and subsequently their bioavailability. Various soil amendments, like desorbing agents, surfactants, and chelating agents, have been applied to increase contaminant mobility and bioavailability. These mobilizing agents are applied to increase the contaminant removal though phytoremediation, bioremediation, and soil washing. However, possible leaching of the mobilized pollutants during soil washing is a major limitation, particularly when there is no active plant uptake. This leads to groundwater contamination and toxicity to plants and soil biota. In this context, the present review provides an overview on various soil amendments used to enhance the bioavailability and mobility of organic and inorganic contaminants, thereby facilitating increased risk when soil is remediated in polluted areas. The unintended consequences of the mobilization methods, when used to remediate polluted sites, are discussed in relation to the leaching of mobilized contaminants when active plant growth is absent. The toxicity of targeted and non-targeted contaminants to microbial communities and higher plants is also discussed. Finally, this review work summarizes the existing research gaps in various contaminant mobilization approaches, and prospects for future research.

Due to the increasing concerns on global ecosystems and human health, the environmental risks posed by microplastics (MPs) and nanoplastics (NPs) have become an important topic of research. Their ecological impacts on various faunal species have been extensively researched and reviewed. However, the majority of those studies perceive these micro(nano)-plastics (MNPs) as a single entity rather than a collective term for a group of chemically distinct polymeric particulates. Each of the plastic polymers can possess unique physical and chemical behavior, which, in turn, can determine the possible environmental impacts. Furthermore, many studies explore the adsorption, absorption, and release of other environmental pollutants by MNPs. But only a handful of them explore the leaching of additives possessed by these polymers. Data on the environmental behavior and toxicity of individual additives associated with different polymer particulates are scarce. Knowledge about the leachability and ecotoxicity of the additives associated with environmental MNPs (unlike large plastic particles) remains limited. The ecological impacts of different MNPs together with their additives and the basis of their toxicity have not been explored yet. The present review systematically explores the potential implications of environmentally predominant polymers and their associated additives and discusses their physicochemical characteristics. The review ultimately aims to provide novel insights on what components precisely make MNPs hazardous to the fauna. The paper also discusses the major challenges proposed in the available literature along with recommendations for future research to throw light on possible solutions to overcome the hazards of MNPs.

Advances in genomic science make it desirable to include genomic controls in soil-plant-atmosphere-continuum (SPAC) models by methods proposed in this paper. Molecular genetic concepts suggest that a differential equation similar to ones used in neural networks can be used to model single-gene elements of larger systems. Natural modifications to the equation incorporate temperature dependency. Multi-gene components based on this element function as Boolean logic gates, linear arithmetic units, delays, differentiators, integrators, oscillators, coincidence detectors, and bi-stable devices. Related genetic circuitry from real organisms is shown. Genomic integration with SPAC models entails whole-plant modeling with realistic morphology. Plants are networks of parts, iterated in time and space under genetic control, that induce and modulate conservative SPAC mass/energy flows. Network developmental rules can be stated as Lindenmayer grammars whose symbols represent plant parts programmed as software objects. A structure is presented for simulators based on these concepts. The discussion argues that prior object-oriented plant modeling approaches (i) do not reflect how plants actually develop morphologically and (ii) may represent processes in tactically unwise ways at a time when genomics is advancing knowledge of process interactions. Finally, genomics and expanding computing power redefine concepts of model “simplicity” and “complexity” to favor increased realism.

This chapter defines seven different units: force, weight, work, energy, power, pressure, and heat. A force is a push or pull exerted on a body. If an unbalanced force acts on a body, the body accelerates in the direction of the force. Conversely, if a body is accelerating, there must be an unbalanced force acting on it in the direction of the acceleration. The unbalanced force acting on a body is proportional to the product of the mass and of the acceleration produced by the unbalanced force. The mass m of a body refers to its inertia, and the weight w of a body is the pull or force due to gravity acting on the body which varies with location. (Inertia is the tendency of matter to remain at rest if at rest, or, if moving, to keep moving in the same direction, unless affected by some outside force.) Weight w is a force with a direction approximately toward the center of the earth. A force does work on a body when it acts against a resisting force to produce motion in the body. The energy of a body is its ability to do work. As the energy of a body is measured in terms of the work it can do, it has the same units as work. Power is the time rate of doing work. Average power = (work done)/(time taken to do this work) = force applied × velocity of body to which force is applied, where the velocity of the body is in the direction of the applied force. Pressure or p is a force per unit area.

This chapter describes the dual-probe heat-pulse technique, also called the dual thermal probe technique, to determine soil water content. The theory of the method is given, which depends upon measuring the thermal properties of soils. In development of the theory, information in the classic book, Conduction of Heat in Solids by Carslaw and Jaeger, is presented. Advantages of the method are cited. It measures accurately soil water content. It is the only instrument that can measure soil water content near the soil surface. It offers a method to monitor not only soil water content but also other hydrological properties of the soil, including soil water flux density. From a plant physiological point of view, the method provides unique opportunities to measure water uptake by plant roots on a small scale. A biography of Jaeger is given in the appendix.

Publisher Summary Soil not only permits entry of water and nutrients and their storage there, but it also allows wasteful passage past roots. An older method to analyze movement of solutes through the soil is based on measuring pore volumes. This method does not divide the soil water into mobile, θm, and immobile, θim, regions, but considers the water in the soil to be the sum of both, the total volumetric water content, θ. By measuring pore volume, the chapter considers the physical process of the movement of solutes in fluids. It presents information concerning what takes place in a porous medium as one fluid displaces another. To analyze concentration changes of solute in the effluent from a soil column, the chapter presents the data in a standard form called a breakthrough curve. A breakthrough curve is a plot of the ratio C/Co versus the number of pore volumes of effluent collected. Here C is the concentration of the solute found in the effluent (or leachate; the wastewater effluent is not referred to here, but the liquid that comes out of the bottom of a column of soil when it is leached). Co is the initial concentration of the solute in the displacing fluid, and the pore volume Vo is the volume of the porous medium occupied by fluid.

This chapter defines water content and water potential. The water potential and its components are then described. All water in soil and plants is subjected to force fields originating from four main factors: the presence of the solid phase (the matrix); the gravitational field; dissolved salts; and the action of water pressure. These force fields can be defined in units of potential energy or in head (length) units. The reason why a length unit can represent a potential energy unit is given. Head units are used to show the force fields on water in a column of soil. The direction of movement of water between two tensiometers is determined by calculating the sum of the matric potential and the gravitational potential. A biography of William L. Powers, who is coauthor of a classical book on soil physics from which information in this chapter was taken, is given in the appendix.

This chapter tells how to measure plant water potential using thermocouple psychrometers. The theory of their use is given. It includes knowing the relation between water potential and relative humidity (called the Kelvin equation); the Peltier thermoelectric effect; Joule heating; thermoelectric power; and the relationship between vapor pressure and temperature (called the psychrometric equation). Each of these topics is discussed. A sample problem is given showing how to get partial pressure of water vapor in air, relative humidity, and dewpoint temperature from wet and dry bulb readings obtained from a psychrometer. Calibration of thermocouple psychrometers and how one takes measurements with them are described. The importance of isothermal conditions during measurements is emphasized, and reasons for them are enumerated. Four types of thermocouple psychrometers are distinguished: the isopiestic thermocouple psychrometer; the Spanner thermocouple psychrometer; the Richards-and-Ogata thermocouple psychrometer; and the dewpoint hygrometer. Appendices give biographies of Peltier, Joule, and Kelvin.

Publisher Summary The term potential evapotranspiration must be defined when we talk of evapotranspiration. The concept of potential evapotranspiration was put forth by Thornthwaite (1948) and Penman (1948). The chapter uses the definition of Rosenberg, noting that his definition is similar to that of Penman. Rosenberg says that potential evapotranspiration (abbreviated as ETP by him, but as PET by most others) is “the evaporation from an extended surface of a short green crop which fully shades the ground, exerts little or negligible resistance to the flow of water, and is always well supplied with water. Potential evapotranspiration cannot exceed free water evaporation under the same weather conditions.” In fact, real (actual) evapotranspiration differs from the potential under most circumstances. The reasons for these differences are best explained by reference to the conditions imposed by the definition of potential evapotranspiration and by an analysis of the reality of these conditions. The chapter first looks at the influence of extended surfaces (or what is called the influence of fetch) on potential evapotranspiration. Second, it looks at the influence of crop height. Many of the important crops grown worldwide are not short: corn, sorghum, winter and spring grains, cotton, and trees. The taller the crop, the more effective is its exchange of energy with the ambient air. Third, it looks at the influence of crop cover. Row crops do not normally shade the ground fully except in some cases at advanced stages in their development.

This chapter tells how to measure plant water potential using pressure chambers. Most pressure chambers are based on a design developed by Scholander at the Scripps Institute of Oceanography in the 1960s. His pressure chamber is described. Measurements made with it are compared to measurements made with thermocouple psychrometers, because measurements made with thermocouple psychrometers are the standard ones. They are based on sound physics using the Kelvin equation. One of the first scientists to compare measurements made with the Scholander pressure chamber and thermocouple psychrometers was John Boyer, and his comparisons are presented. Advantages and disadvantages of the Scholander pressure chamber are given. Other methods, aside from the Scholander pressure chamber, used to determine water potential with pressure chambers are described. These include the pump-up pressure chamber in which pressure is created by pumping the instrument as one would a bicycle pump. Appendices give biographies of Scholander and Boyer.

The problem of the rise of water in tall plants is as old as the science of plant physiology. This chapter considers the cohesion theory, which is the best formulation to explain how water can get to the top of tall trees and vines. The cohesion theory assumes that diffusion of water from the noncollapsible xylem elements in contact with the leaf cells creates a state of tension within the water columns in the xylem vessels. This tension is possible because of the cohesion of water molecules and their adhesion to the hydrophilic walls of the xylem elements. Tension in the water columns is assumed to lift water from the roots to the leaves, in addition to reducing the potential energy of the water in the root xylem tissue until water diffuses from the soil into the root during absorption of the water. The cohesion theory assumes continuity of water columns, laterally and vertically, in the conducting elements of the xylem tissue. These water columns ultimately are placed under tensile strain. But widespread rupture is believed not to occur in the water columns under tensile strain owing to the purported cohesive properties of water when entrapped in small capillaries.

This chapter describes the use of infrared thermometers to measure canopy temperature. They are used to monitor drought stress, because plant temperature and water use are related. If plants are water stressed, transpiration is reduced and the canopy temperature increases, and the increase can be measured with an infrared thermometer. The temperature measured with an infrared thermometer gives the black-body temperature of the canopy; thus, principles of black bodies are reviewed. The instruments are called infrared thermometers, because they measure radiation in the infrared region. Calibration of an infrared thermometer is illustrated. Portable, commercially available, hand-held infrared thermometers were developed in the 1970s. The pioneering work using them, done by Ray Jackson and colleagues at the U.S. Water Conservation Laboratory in Phoenix, Arizona, is described. The advantages of infrared thermometers are listed, including their ease of use and ability to take rapid measurements. An appendix gives the biography of Jackson.

This chapter describes the theory and use of tensiometers. They are used to measure, when the soil is not too dry, the energy status of the soil. They measure the matric potential, formerly called the soil moisture tension. A tensiometer consists of a porous, permeable ceramic cup connected through a water-filled tube to a manometer, vacuum gauge, or pressure transducer. The tension that a tensiometer measures cannot exceed about three-quarters bar or 750 cm of water column. This is due to air coming out of solution at the reduced pressure to break the continuity of the water column. At tensions >750 cm, pressure apparatus is used, which is explained. Different types of tensiometers are illustrated. Temperature effects on tensiometers are described. The applications of tensiometers in agricultural research are listed. A biography of L.A. Richards, one of the early developers of the tensiometer, is given in the appendix.

This chapter describes how leaves intercept the Sun's rays. To understand the process, basic principles of astronomy are presented. Sidereal time, solar time, apparent solar time, mean solar time, and standard time are defined. The procedure for determining standard time from apparent solar time is given. Interception of direct-beam solar radiation then is discussed. It is based on the direct-beam solar radiation equation that has been in the literature for over 200 years. It can be used on any sloping surface, but in this chapter we apply it to the sloping surface of a leaf. Four angles need to be known to solve the equation: azimuth and altitude angle of the Sun and azimuth and altitude angle of the leaf. An example is given showing how to use the equation to determine light interception on a leaf. An appendix gives the biography of Kepler, whose work laid the foundation for astronomy.

This chapter reviews the anatomy of stems and then shows how one places a leafy stem in a pressure chamber to determine graphically, via a pressure–volume curve, the two main components of the plant water potential: the osmotic potential and the turgor potential. Pressure is applied to the cut-off shoot, and the volume of sap exuded from the cut end is measured. One then graphs pressure versus volume. The theory of pressure–volume curves is based on the work by van't Hoff, who used data of the botanist Wilhelm Pfeffer to develop his law on osmotic pressure. The chapter also tells how to measure plant water content and relative water content. Osmotic potential can be determined using an osmometer, and its theory is presented, which depends upon knowledge of the Clausius–Clapeyron equation. Direct measurement of turgor potential with a pressure probe is reviewed. Appendices give biographies of Pfeffer, van't Hoff, and Clausius.

This chapter describes how to measure water content in the soil using time domain reflectometry (TDR). The TDR technique measures the velocity of propagation of a high-frequency signal down waveguides in the soil. The velocity is related to the dielectric constant of the soil, which is then related to the water content. Basic physical definitions for period and frequency are given. It is noted that frequency is measured in a unit called the hertz, named after H.R. Hertz. Measurements taken in the time domain and the frequency domain are distinguished. Coaxial cables and waveguides, needed for TDR measurements, are described. It is shown that water extraction by roots at different depths can be determined by using TDR. A biography of Hertz is given in the first appendix, and a biography of S.A. Schelkunoff, a developer of the coaxial cable, is given in a second appendix.

This chapter gives the history of the soil–plant–atmosphere continuum (SPAC) and tells of early researchers who pioneered the concept. The analogy for water movement through the SPAC and the flow of electricity is then presented. Ohm's law governs the movement of electricity through a circuit, and it is detailed. Electrical analogs depend upon a knowledge of the law of resistance, which is illustrated. A Wheatstone bridge is used to measure the resistance in electrical analogs, and its principle of operation is described. The unit used for electrical resistance, the ohm, and the unit used for electrical conductance, the Siemen, are presented and their relation shown. The catenary equation used for the movement of water through the SPAC, which links water movement from the soil to the plant and from the plant to the atmosphere, is presented and its proof is derived. Appendices give biographies of Ohm, Wheatstone, and members of the Siemens family.

The water molecule is composed of two hydrogen atoms and one oxygen atom. The water molecule is positively charged on one side and negatively charged on the other and is, thus, a dipole. The two hydrogen atoms share a pair of electrons with a single oxygen atom. The two hydrogen atoms of the water molecule are separated at an angle of 103 to 106 degrees, measured with the oxygen atom as the apex of the angle and with the two hydrogen protons as points on the angle sides. The electron pairs shared between the oxygen nucleus and the two hydrogen protons only partially screen (neutralize) the positive charge of the protons. The result is that the proton side of the molecule becomes the positive side of the water molecule. The chapter lists 15 properties of water. For example, water has the highest specific heat of any known substance except liquid ammonia, which is about 13 percent higher. The heat of vaporization of water is the highest known. The heat of vaporization is defined as the amount of heat needed to turn one gram of a liquid into vapor, without a rise in the temperature of the liquid. The heat of fusion of water is unusually high. The heat of fusion is the quantity of heat necessary to change one gram of a solid to a liquid with no temperature change.

This chapter describes the two main methods used to determine the flow of sap in plants. They are the heat-pulse method used on woody plants and the heat-balance method usually used on herbaceous plants. In the heat-pulse method, flow is obtained from the time required for a heat input (the pulse) to travel from its source to a sensor up or down the stem. Sap flow is measured in units of length/time. In the heat-balance method, a heater is wrapped around a portion of a stem, and flow is obtained by balancing heat fluxes into and out of the stem. Sap flow is measured in units of mass/time. The heat-pulse method was developed in 1932. The heat-balance method was developed about 30 years later and was promulgated by Cornelius van Bavel. Each method has advantages and disadvantages, and they are discussed. A biography of van Bavel is given in the appendix.

This chapter defines field capacity, permanent wilting point, and available water to plants, which is the difference between field capacity and the permanent wilting point. The permanent wilting point was described in 1912 by Briggs, a physicist, and Shantz, a botanist, who called it the wilting coefficient. The nonlimiting water range (NLWR), first defined by John Letey at the University of California in 1985, acknowledges that water may not be equally available to plants between field capacity and the permanent wilting point. The NLWR includes the effects of aeration and mechanical resistance on water availability. In follow-up Letey's work, researchers have defined the least limiting water range, which sets limits for water uptake based on the wetness and mechanical resistance of the soil. Biographies of Briggs and Shantz are given in the appendix along with documentation showing they were two of the most important scientists of their time.

This chapter gives attention to water movement in plant roots. It applies Poiseuille's law for the flow through the xylem. But first it reviews root anatomy and the cell types that make up the xylem. Elongating roots usually possess four regions: the root cap, the meristematic region, the region of cell elongation, and the region of differentiation and maturation. But these regions are not always clearly delimited. The root cap is composed of loosely arranged cells and is usually well defined. Because it has no direct connection with the vascular system, it probably has no role in absorption. It is said to be the site of perception of the gravitation stimulus, but this is debatable. The meristematic region typically consists of numerous small, compactly arranged, thin-walled cells almost completely filled with cytoplasm. Relatively little water or salt is absorbed through this region, largely because of the high resistance to movement through the cytoplasm and the lack of a conducting system. Usually there is a zone of rapid cell elongation and expansion a few tenths of a millimeter behind the root apex. It is difficult to indicate a definite zone of differentiation because various types of cells and tissues are differentiated at different distances behind the root apex. Typically, sieve tubes of the phloem differentiate before the xylem elements. As the newly enlarged, thin-walled cells at the base of the zone of enlargement cease to elongate, they become differentiated into the epidermis, cortex, and stele, which constitute the primary structures of roots.