Inside a plant, the apoplast can mean the space outside of cell membranes, where material can diffuse freely; that is, the extracellular spaces. Apoplast can also refer especially to the continuum of cell walls of adjacent cells; fluid and material flows occurring there or in any extacellular space are called apoplastic flow or apoplastic transport. The apoplastic route is one way by which water and solutes are transported and distributed to different places through tissues and organs; another way is symplastic flow. To prevent uncontrolled leakage to unwanted places, in certain areas there are barriers to the apoplastic flow: in roots the Casparian strip has this function[clarification needed]; On the outside of the skin of certain plant parts is a protective waxy film called plant cuticle to achieve this (protection against e.g. drying out, but also waterproofing against soaking). Air bubbles occupying extracellular spaces can also hinder apoplastic transport. The apoplast is important for all the plant's interaction with its environment: The main carbon source (carbon dioxide) needs to be solubilized, which happens in the apoplast, before it diffuses through the cell wall (also called plasma membrane) into the cell's inner content (cytoplasm) (thereby then entering the symplastic possibility of flow) to be used (by the chloroplasts) for photosynthesis. In the roots, ions diffuse into the apoplast of the epidermis before diffusing into the symplast, or in some cases being taken up by specific ion channels, and being pulled by the plant's transpiration stream, which also occurs completely within the boundaries of the apoplast. Similarly, all gaseous molecules emitted and received by plants such as plant hormones and other pheromones must pass the apoplast. In nitrate poor soils, acidification of the apoplast increases cell wall extensibility and root growth rate. This is believed to be caused by a decrease in nitrate uptake (due to deficit in the soil medium) and supplanted with an increase in chloride uptake. H+ATPase increases the efflux of H+, thus acidifying the apoplast.[clarification needed - maybe better an extra chapter for this?] The apoplast is a site for cell-to-cell communication. During local oxidative stress, hydrogen peroxide and superoxide anions can diffuse through the apoplast and transport a warning signal to neighbouring cells. In addition, a local alkalinization of the apoplast due to such stress can travel within minutes to the rest of the plant body via the xylem and trigger systemic acquired resistance. The apoplast also