Page "Xylem" Paragraph 32

During the Silurian, was readily available, so little water needed expending to acquire it.

By the end of the Carboniferous, when levels had lowered to something approaching today's, around 17 times more water was lost per unit of uptake.

However, even in these " easy " early days, water was at a premium, and had to be transported to parts of the plant from the wet soil to avoid desiccation.

This early water transport took advantage of the cohesion-tension mechanism inherent in water.

Water has a tendency to diffuse to areas that are drier, and this process is accelerated when water can be wicked along a fabric with small spaces.

In small passages, such as that between the plant cell walls ( or in tracheids ), a column of water behaves like rubber – when molecules evaporate from one end, they literally pull the molecules behind them along the channels.

Therefore transpiration alone provided the driving force for water transport in early plants.

However, without dedicated transport vessels, the cohesion-tension mechanism cannot transport water more than about 2 cm, severely limiting the size of the earliest plants.

This process demands a steady supply of water from one end, to maintain the chains ; to avoid exhausting it, plants developed a waterproof cuticle.

Early cuticle may not have had pores but did not cover the entire plant surface, so that gas exchange could continue.

However, dehydration at times was inevitable ; early plants cope with this by having a lot of water stored between their cell walls, and when it comes to it sticking out the tough times by putting life " on hold " until more water is supplied.