The ancestors of modern day plants evolved in water and it is no surprise that the internal environment of a plant is 80-90% water. As land plants must remain hydrated to this level if they are to continue growing, they have evolved a number of mechanisms, such as waxy cuticles, to ensure that they do not dry out.

One of the major problem for a plants trying to grow is the water that evaporates out of the leaf  for gas excahnging. This gas exchange in the leaf does not occur all over the leaf, but rather through small adjustable parts called stomata, which usually make up only 1% of the leaf surface area.

The plant can control the loss of water from its leaves by varying the aperture of its stomata (like a tap). However, if a plant restricts the flow of water vapour out of its leaf it automatically restricts the flow of CO₂ into the leaf for photosynthesis. In short, continued hydration is essential for plant growth, but this is largely influenced by the control of transpiration.

Water flows through plants through a system of pipes called xylem at speeds of one to several meters per hour. The roots are the water absorbing organs in the soil. The ability of a plant to absorb water from the soil depends on the number, rather than the size of the roots and where they are distributed in the soil. Root densities in soils can be surprisingly high as illustrated below.

A single grass plant in 1 litre of soil:

Root hairs are more important for nutrient uptake than water because they do not contain xylem. Although water moves in saturated permeable soils at rates of between 0.01 to 0.2 meters per hour, the rate of movement in unsaturated soils (which is most of the time) is 10 to 100 times slower. Remembering that water moves in the xylem of grasses at about 1 meter per hour, we can appreciate that water moves 50 to 1000 times SLOWER in soil than roots.

The xylem becomes of necessarily larger diameter, as it collects water from smaller roots to larger roots, than the stem. The reverse occurs in the leaves where successively smaller xylem elements distribute water to the leaves, then the veins within the leaves, finally ending in small groups of cells surrounding the stomata. The end of each small xylem element is closed by a cell wall which is mainly cellulose and acts like an ultra think tissue paper. The water then seeps along the cell walls to the sub-stomata cavity, where it evaporates and moves through the stomata to the outside atmosphere. The important point about the xylem pathway is that it is a continuous column of water, not broken by bubbles, and contained in a cellulose pipe whose walls are porous in places.