The plant cell wall serves a variety of functions. Along with protecting the intracellular contents, the structure bestows rigidity to the plant, provides a porous medium for the circulation and distribution of water, minerals, and other nutrients, and houses specialized molecules that regulate growth and protect the plant from disease. Growing pant cells are surrounded by a polysaccharide-rich primary wall. This wall is part of the apoplast, which is the free diffusional space outside the plasma membrane. Apoplast itself is largely self-contiguous and contains everything that is located between the plasma membrane and the cuticle. The primary wall and middle lamella account for most of the apoplast in growing tissue. The symplast, the inner side of the plasma membrane in which water and low-molecular-weight solutes can freely diffuse, is another unique feature of plant tissues.