Xylem
Xylem sap consists mainly of water and inorganic ions, although it can also contain a number of organic chemicals as well.
The transport is passive, not powered by energy spent by the tracheary elements themselves, which are dead by maturity and no longer have living contents.
When the water pressure within the xylem reaches extreme levels due to low water input from the roots (if, for example, the soil is dry), then the gases come out of solution and form a bubble – an embolism forms, which will spread quickly to other adjacent cells, unless bordered pits are present (these have a plug-like structure called a torus, that seals off the opening between adjacent cells and stops the embolism from spreading).
[17][18] Despite numerous objections,[19][20] this is the most widely accepted theory for the transport of water through a plant's vascular system based on the classical research of Dixon-Joly (1894), Eugen Askenasy (1845–1903) (1895),[21][22] and Dixon (1914,1924).
Xylem transport is driven by a combination[29] of transpirational pull from above and root pressure from below, which makes the interpretation of measurements more complicated.
[citation needed] The earliest true and recognizable xylem consists of tracheids with a helical-annular reinforcing layer added to the cell wall.
The presence of xylem vessels (also called trachea[30]) is considered to be one of the key innovations that led to the success of the angiosperms.
Specialized water transport tissues soon evolved in the form of hydroids, tracheids, then secondary xylem, followed by an endodermis and ultimately vessels.
[33] The high CO2 levels of Silurian-Devonian times, when plants were first colonizing land, meant that the need for water was relatively low.
[33] 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.
[33] This process demands a steady supply of water from one end, to maintain the chains; to avoid exhausting it, plants developed a waterproof cuticle.
[33] These wider, dead, empty cells were a million times more conductive than the inter-cell method, giving the potential for transport over longer distances, and higher CO2 diffusion rates.
Plants continued to innovate new ways of reducing the resistance to flow within their cells, thereby increasing the efficiency of their water transport.
These, the "next generation" of transport cell design, have a more rigid structure than hydroids, allowing them to cope with higher levels of water pressure.
This is an important role where water supply is not constant, and indeed stomata appear to have evolved before tracheids, being present in the non-vascular hornworts.
[33] This structure in the roots covers the water transport tissue and regulates ion exchange (and prevents unwanted pathogens etc.
While wider tracheids with robust walls make it possible to achieve higher water transport tensions, this increases the likelihood of cavitation.
For this reason, pits in tracheid walls have very small diameters, to prevent air entering and allowing bubbles to nucleate.
For instance, oaks grow a ring of wide vessels at the start of each spring, none of which survive the winter frosts.
[citation needed] Growing to height also employed another trait of tracheids – the support offered by their lignified walls.
Defunct tracheids were retained to form a strong, woody stem, produced in most instances by a secondary xylem.
However, in early plants, tracheids were too mechanically vulnerable, and retained a central position, with a layer of tough sclerenchyma on the outer rim of the stems.
[33] This allowed plants to fill more of their stems with structural fibers, and also opened a new niche to vines, which could transport water without being as thick as the tree they grew on.
[33] Despite these advantages, tracheid-based wood is a lot lighter, thus cheaper to make, as vessels need to be much more reinforced to avoid cavitation.
[citation needed] As a young vascular plant grows, one or more strands of primary xylem form in its stems and roots.
Functionally, metaxylem completes its development after elongation ceases when the cells no longer need to grow in size.
In his book De plantis libri XVI (On Plants, in 16 books) (1583), the Italian physician and botanist Andrea Cesalpino proposed that plants draw water from soil not by magnetism (ut magnes ferrum trahit, as magnetic iron attracts) nor by suction (vacuum), but by absorption, as occurs in the case of linen, sponges, or powders.
[46] The Italian biologist Marcello Malpighi was the first person to describe and illustrate xylem vessels, which he did in his book Anatome plantarum ...
[49] In 1727, English clergyman and botanist Stephen Hales showed that transpiration by a plant's leaves causes water to move through its xylem.
[50][note 2] By 1891, the Polish-German botanist Eduard Strasburger had shown that the transport of water in plants did not require the xylem cells to be alive.
