The xylem and the phloem make up the vascular tissue of a plant and transports water, sugars, and other important substances around a plant. What is commonly referred to as ‘sap’ is indeed the substances that are being transported around a plant by its xylem and phloem.
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The separation between plants that have veins and plants that do not is one of the great divides within the plant kingdom. This separates plants into vascular and non-vascular plants. Most plants have xylem and phloem and are known as vascular plants but some more simple plants, such as mosses and algae, do not have xylem or phloem and are known as non-vascular plants.
The phloem carries important sugars, organic compounds, and minerals around a plant. Sap within the phloem simply travels by diffusion between cells and works its way from leaves down to the roots with help from gravity. The phloem is made from cells called ‘sieve-tube members’ and ‘companion cells’.
Sieve-tube members are living cells that create chains of cells running the length of the plant. Angiosperm sieve-tube members have porous ends called ‘sieve plates’ that allow sap to move diffuse easily from cell to cell.
The cells of sieve-tube members are missing some important structures such as a nucleus, ribosomes and a vacuole which is where companion cells come in.
The companion cells run adjacent to sieve-tube members and are connected by a number of channels called ‘plasmodesmata’. Companion cells are not lacking in any vital organelles and their nucleus and ribosomes serve both the sieve-tube member and itself. The companion cell can sometimes also deliver sugars and other substances into the sieve-tube members from neighboring cells.
The xylem is responsible for keeping a plant hydrated. Xylem sap travels upwards and has to overcome serious gravitational forces to deliver water to a plant’s upper extremities, especially in tall trees.
Two different types of cells are known to form the xylem in different plant groups: tracheids and vessel elements. Tracheids are found in most gymnosperms, ferns, and lycophytes whereas vessel elements form the xylem of almost all angiosperms.
Xylem cells are dead, elongated and hollow. They have secondary cell walls and ‘pits’ (areas where the secondary cell wall is missing).
Tracheids are long thin cells that are connected together by tapered ends. The tapered ends run alongside each other and have pits that allow for water to travel from cell to cell.
Their secondary cell walls contain lignin – the compound that creates wood. The lignin in tracheids adds structural support to the xylem and the whole plant.
Vessel elements are shorter and wider than tracheids and are connected together end-on-end. The ends of the cells contain what are known as ‘perforation plates’. The perforation plates have a number of holes in their cell walls which allows for water to travel freely between cells.
Xylem and phloem in leaves
Photosynthesis in leaves requires a lot of water from the xylem and produces a lot of sugar for the phloem. The xylem and phloem enter a plant’s leaves via their petiole – a short stalk that connects a leaf to a branch.
With the exception of lycophytes, veins divide multiple times in a leaf which creates a good spread of veins and makes it easier to collect sugars and deliver water to photosynthesizing parts of the leaf. Vascular tissue also provides structural support to leaves.
Xylem and phloem in stems
Xylem and phloem travel entire length of stems in discrete threads called ‘vascular bundles’. In eudicots, vascular bundles are arranged in a ring within the stem. Each vascular bundle is orientated with the xylem on the interior and the phloem on the outside of the xylem.
In monocots, the vascular bundles are scattered throughout the stem rather than being arranged in a circle.
Xylem and phloem in roots
The xylem and phloem are grown within the central section of a root called a ‘stele’. In eudicots, the xylem usually forms a cross of cells within the stele which runs the length of the root. Four independent phloem strands grow between each bar of the xylem cross.
In monocots, the center of the stele is composed of pith. The phloem and xylem form a weak circular pattern within the pith of the stele. Phloem and xylem grow around the inner layer of pith with phloem cells on the outside of the xylem.
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Vascular bundles from stems meet at the base of the stem to merge with the root stele.