Water diffuses directly from the soil into the free space of roots. Free space is that part of the root or tissue into which water reaches directly without any barrier. This space can not be measured exactly but its approximate measure is apparent free space (AFS). Its value can be estimated and expressed by the following equation:

AFS = Total solute in the tissue / solute concentration in surrounding solution in which tissue is bathed when diffusion equilibrium is reached.

The approximate measurements show that AFS of roots is 6-10% of the total volume of the tissue. Direct measurements show that water filled intercellular spaces and cell walls of root tissue constitute about 7-10% of the total volume tissue. Thus, it can be concluded that AFS of roots comprises cell walls and intercellular spaces. Cell vacuoles are not included in AFS.

The German physiologist, E. Munch, divided the root tissue into two parts, apoplast and symplast. According to him, the apoplast consists of all spaces in the root which are roughly equal to free space, i.e., cell walls and intercellular spaces. In the root tissue, the apoplast is separated by endodermis into two parts. Outer to the endodermis is the cortical part of the apoplast and inside in the stelar part, composed of conducting vessels (xylem vessels and tracheids) as shown in the diagram below.

Apoplast and Symplast

The apoplast of the outer region is not continuous with that of the inner region because of heavily suberized casparian strips of the endodermis, which are impermeable to water. Water moves freely in the apoplast of the cortex as well as of stele. But it can not move from cortex to stele or back unless it passes through the cell membranes and cytoplasm of the endodermal cells. The root may thus be regarded as an osmometer in which the endodermal cells act as the osmotically active membrane. The water and the dissolved solutes may flow unchecked through the apoplast of the cortex but to enter the stele they have to pass through the differentially permeable membrane of the endodermis.

The symplast consists of total protoplasts of the cells (contents of cell with the cell wall). Because protoplasts of cells are connected with one another through plasmodesmata (cytoplasmic strands passing through cell walls) the symplast of the whole root is a single continuous system. Vacuoles of cells are separated from the cytoplasm by membrane called tonoplast. They neither belong to apoplast nor to symplast.

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