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Vascular Tissue in Plants - Xylem March 2004 Vascular tissue in plants - Xylem National Biology Support Service Plant tissue types ascular tissue - there are two different types of vascular tissue in the plants from Xylem Tissue the Kingdom Plantae - xylem and phloem. xylem VXylem is composed of two basic types of elongated fibres water-conducting cells: tracheids and vessels which together with fibres and parenchyma cells make up this complex tissue. Xylem functions in the movement xylem of water and salts throughout the plant. tracheids xylem Structure: vessel During the development of the tracheids and the vessels their cell walls become impregnated with lignin, making them woody and impermeable. The walls of these water-conducting cells are perforated by pits, which allow water and salts to pass sideways between the cells. Mature tracheids and vessels are Phloem In contrast to xylem, this consists of living cells dead cells due to the lignin in their walls. Xylem is the called sieve tube elements. An element has two components - wood in a woody plant and so also functions in the a sieve tube cell and a companion cell. These, together with support of the plant. other cell types - fibres, sclereids and parenchyma make up this complex tissue. End to end sieve tube cells form Shape; continuous pathways. The end wall of each cell is perforated Tracheids are long, tapered cells, which have angled by pores through which cytoplasmic strands stream. These end-plates with pits that allow water to pass from cell perforated end walls are called sieve plates (because they to cell. Xylem vessels are wider and usually shorter, do resemble a sieve). not taper and lack end-plates. The end walls of the vessels break down so that the cells join end to end to When sieve cells are mature their cytoplasm is pushed to the form a continuous hollow tube. sides of the cell and they have no nucleus. They are kept alive and supported in their function by companion cells, Location: without which they cannot work. Companion cells have Tracheids are found both in Gymnosperms and in exceptionally large numbers of mitochondria, indicating that Angiosperms whereas vessels are only found in the they are very active metabolically. more evolved Angiosperms (flowering plants ). Sieve tubes and companion cells form the functional unit for The possession of vessels provides Angiosperms with a translocation - the transport of organic solutes in the major advantage over Gymnosperms and ferns as they phloem. allow for very rapid water uptake and transport. Besides vascular tissue there are two other fundamental categories of plant tissue: ermal tissue - these tissues usually occupy the outer layer of all plant organs so they cover and protect the plant e.g. the epidermis of the leaves secretes a cuticle that helps the plant retain water. Dermal tissue is also D responsible for environmental interaction such as light passage, protection from pathogens e.g. the cork cells in the bark of a tree, gas exchange etc. round tissue - this tissue occupies the space between the dermal tissues and the vascular tissues. It is, however, much more than a filler. In leaves, for example, the ground tissue is the layer (mesophyll) where G photosynthesis occurs. In roots, the ground tissue may store sugars or starches. This newsletter contains both prescriptive and non-prescriptive material page 1 Did you know? Water movement through the xylem • H. Dixon and J. Joly were Irish. The water flows (usually upwards) According to their theory the through the xylem tracheids and the continuous columns of water • The mechanism of water movement is xylem vessels in what is known as the extending from the roots up to the based on purely physical forces transpiration stream. leaves flow along a pressure because the xylem vessels and gradient caused by transpiration. tracheids are lifeless. Where: This occurs because water molecules In the xylem tracheids (dead hollow have such tremendous cohesive • Great forces are required to break the cells), the water moves upwards column of water in xylem - about the forces (due to hydrogen bonding) through the cell until it reaches the same force that would be required to that when water is lost from the top angled end-wall when it must then pass beak a steel wire of the same more is ‘sucked’ up from the lower diameter. through pits into the next tracheid. regions. The lost water is replaced This is somewhat inefficient. • The diameter of the xylem vessels and by the roots absorbing additional tracheids helps determine water flow In the xylem vessels, which also are water from the soil. Because water is rate. dead and hollow, but with no end-walls, attracted to the walls of the xylem the water movement is much faster. cells the forces of gravity are • The rate of water flow upwards in These barrel-shaped cells which have counteracted. trees can vary from 1m to 40m per joined together, end to end, to form a hour. The cohesion-tension theory is continuous tube which can extend supported by most of the uninterrupted for several metres up a • Less than 1% of water reaching the experimental work, however, tree provide less resistance to water leaves of a plant is used in recently it has been challenged but photosynthesis and plant growth. movement up a tree than the tracheids. Most of it is lost in transpiration. the debate goes on and no other proposal has yet been made. The • The water loss from a mature birch How: greatest questions remaining are In 1895 H. H. Dixon and J. Joly tree with an estimated 200,000 leaves related to what happens if the water can be up to 400 litres per day. proposed their cohesion-tension theory. column breaks and a tracheid They said that the principal reason why becomes filled with gas. • At any level in the transpiration water flows up a tree is a combination stream upwards through a plant the of three phenomena: water can leave the xylem and pass Root pressure plays a role in the - the evaporation of laterally to supply the needs of other 1. Transpiration transport of water in the xylem of tissues. water from the leaves, some plants but it does not account 2. Cohesive forces of water molecules • Tracheids (from trachea or air tube) for most water transport. It can and the are so called because in the 17th however, cause guttation under century a scientist (M. Malpighi) 3. Adhesive properties of water. certain atmospheric conditions. thought (albeit wrongly) he had found an important common element in the anatomy of animals and plants. Can you suggest a reason why the • Tracheid cells can measure up to diameter of the trunk of a pine tree 5mm long in a mature pine tree. might vary throughout the day as shown in the graph? • In older trees the xylem in the inner (answer at bottom of page) part of the trunk is dry (heartwood) noon noon noon noon and does not take part in water transport. It occurs only in the outer layer of xylem cells (sapwood) just The Giant Redwoods (Sequoias) of California under the bark. • Large trees have dead xylem tissue at These trees are the largest and volume) is called General Sherman the centre of their trunks so do not among the oldest in the world. and is over 83 metres high and has need oxygen to be carried to the centre Mature specimens are often up to a diameter of 11 metres at its base. - all the living tissue is in the outer 3,000 years old. They grow in the This means that it as high as a 26- layer of a tree so respiratory gases can high mountains in the east of the be carried the short distances by storey building! state - the Sierra Nevada mountains. diffusion. A single, mature giant sequoia can They were discovered by a hunter draw as much as 650,000 litres of • Trees can be aged by counting the A.T. Dowd in 1852 and were then rings of xylem, which are produced water from the soil in one growing annually. brought to the attention of the world. season. Their xylem contains xylem These trees are enormous - with tracheids (with very narrow • Phloroglucinol stains lignin and so their diameters reaching up to 13 lumina) only - they have no xylem when used on a TS of a stem the metres and some have bark up to 60 vessels (because they are xylem in the vascular bundles stains cm thick. The largest specimen (by red. Gymnosperms). k n u r t e h t f o r e t e m a i d e h t g n i w o r r a n s u h t , n o o n t a . e . i t s e t a e r g s t i t a s i n o i t a r i p s n a r t n e h w s d r a w n i d e l l u p e r a s l l a w l l e c m e l y x e s e h t , s l l a w l l e c d n a s e l u c e l o m r e t a w n e e w t e b s e c r o f e v i s e h d a e h t f o e s u a c e b d n a n o i s n e t r e d n u s i m e l y x e h t n i r e t a W This newsletter contains both prescriptive and non-prescriptive material page 2 .
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