<P> In most plants, pitted tracheids function as the primary transport cells . The other type of tracheary element, besides the tracheid, is the vessel element . Vessel elements are joined by perforations into vessels . In vessels, water travels by bulk flow, as in a pipe, rather than by diffusion through cell membranes . The presence of vessels in xylem has been considered to be one of the key innovations that led to the success of the angiosperms . However, the occurrence of vessel elements is not restricted to angiosperms, and they are absent in some archaic or "basal" lineages of the angiosperms: (e.g., Amborellaceae, Tetracentraceae, Trochodendraceae, and Winteraceae), and their secondary xylem is described by Arthur Cronquist as "primitively vesselless". Cronquist considered the vessels of Gnetum to be convergent with those of angiosperms . Whether the absence of vessels in basal angiosperms is a primitive condition is contested, the alternative hypothesis states that vessel elements originated in a precursor to the angiosperms and were subsequently lost . </P> <P> To photosynthesize, plants must absorb CO from the atmosphere . However, this comes at a price: while stomata are open to allow CO to enter, water can evaporate . Water is lost much faster than CO is absorbed, so plants need to replace it, and have developed systems to transport water from the moist soil to the site of photosynthesis . Early plants sucked water between the walls of their cells, then evolved the ability to control water loss (and CO acquisition) through the use of stomata . Specialized water transport tissues soon evolved in the form of hydroids, tracheids, then secondary xylem, followed by an endodermis and ultimately vessels . </P> <P> The high CO levels of Silurian - Devonian times, when plants were first colonizing land, meant that the need for water was relatively low . As CO was withdrawn from the atmosphere by plants, more water was lost in its capture, and more elegant transport mechanisms evolved . As water transport mechanisms, and waterproof cuticles, evolved, plants could survive without being continually covered by a film of water . This transition from poikilohydry to homoiohydry opened up new potential for colonization . Plants then needed a robust internal structure that held long narrow channels for transporting water from the soil to all the different parts of the above - soil plant, especially to the parts where photosynthesis occurred . </P> <P> During the Silurian, CO was readily available, so little water needed expending to acquire it . By the end of the Carboniferous, when CO levels had lowered to something approaching today's, around 17 times more water was lost per unit of CO 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 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 . </P>

What is the function of xylem and phloem in plants