The diagram at the top left shows the structure of a root tip from an external view. At the root tip there is a protective root cap, behind which is the root meristem, an area of rapid cell division. The cells of the meristem are small and cube-shaped with only the merest hint of a vacuole. Behind the meristem is an area of cell elongation and then an area of cell differentiation where tissues are formed. We see on the diagram the piliferous layer of root hairs which lies just behind the region of differentiation. The root hairs are extensions of the epidermal cells and they grow out to invade the spaces between the soil particles. This region has no waterproofing cuticle on its outer surface, unlike other areas other the plant. The root hairs provide a large area for the uptake of water and mineral ions from the soil.

In a vertical section of the root in the piliferous region we can see that we have the following main layers, the epidermis, the cortex the endodermis and the vascular bundle at the centre. In the transverse section of the root on the previous page you will see that the vascular bundle is made up of phloem towards the outside, a layer of cambium and a central core of xylem. The cambium cells divide to produce new phloem and xylem cell as the root grows . The phloem is designed for the transport of organic molecules, and the xylem is designed for the transport of water and dissolved mineral ions.

There are three possible route by which water can travel from the soil to the central xylem vessels.

1. The vacuolar route where the water travels by osmosis through the cell walls, the cytoplasm and the vacuoles of the cells in the cortex.
2. The symplast route where the water travels only through the cytoplasm and cell walls.
3. The apoplast route where the water travels only through the porous cellulose cell walls.

There has been much debate about which of these routes the water travels by but it is likely to be by a mixture of all three to different extents. The apoplast route has evidence for it in the design of the root. The endodermis is made up of tightly packed cells which have a waterproofing layer running through the walls as a ring which connects to the ring on the adjacent cells (shown in black on the diagram). The deposits of waterproofing suberin would not be needed if water was not moving through the cell walls, and is clearly there to force water to pass out of the cell walls and through the cytoplasm of the endodermal cells. This allows the cell membranes of the endodermal cells to control the movement of dissolved ions into the xylem. The water will flow through the cell cytoplasm from cell to cell if there is an osmotic gradient in the root cortex. This is likely to occur since as water enters the epidermal cells they become diluted compared to the cortical cells beneath them. Water will move by osmosis into the cortical cells which will now be more dilute than the next layer of cortical cells and so on until we reach the endodermis.

Once the water passes through the endodermis it will move into the xylem vessels, which are dead continuous tubes made up of lines of original cells which lost their dividing walls, became secondarily thickened with an impermeable material called lignin and consequently died. Water is being pulled out of the tops of the xylem vessels by osmosis in the leaves and because water molecules are polar molecules they are electrical attracted to each other forming a chain like network. If molecules are pilled from the top of the xylem vessels this pulls the molecules all the way down the xylem vessels and will pull in the water molecules at the base of the xylem vessels. This may sound very clever but it is only what you do each time you suck on a straw. We call this the cohesion-tension theory! (I think the ‘sucking on a straw theory’ is a lot more real!)