Assignment Task
Question 1
Xylem is the tissue responsible for supporting the plant as well as for the storage and longdistance transport of water and nutrients, including the transfer of water-soluble growth factors from the organs of synthesis to the target organs. The tissue consists of vessel elements, conducting cells, known as tracheids, and supportive filler tissue, called parenchyma. These cells are joined end-to-end to form long tubes. Vessels and tracheids are dead at maturity. Tracheids have thick secondary cell walls and are tapered at the ends. It is the thick walls of the tracheids that provide support for the plant and allow it to achieve impressive heights. Tall plants have a selective advantage by being able to reach unfiltered sunlight and disperse their spores or seeds further away, thus expanding their range. By growing higher than other plants, tall trees cast
their shadow on shorter plants and limit competition for water and precious nutrients in the soil.
The tracheids do not have end openings like the vessels do, but their ends overlap with each other, with pairs of pits present. The pit pairs allow water to pass horizontally from cell to cell. Phloem tissue is responsible for translocation, which is the transport of soluble organic substances, for example, sugar. The substances travel along sieve elements, but other types of cells are also present: the companion cells, parenchyma cells, and fibers. The end walls, unlike vessel members in xylem, do not have large openings. The end walls, however, are full of small pores where cytoplasm extends from cell to cell. These porous connections are called sieve plates. Despite the fact that their cytoplasm is actively involved in the conduction of food materials, sieve-tube members do not have nuclei at maturity. The activity of the sieve tubes is
controlled by companion cells through plasmadesmata.
Plants have tissues to transport water, nutrients and minerals. Xylem transports water and mineral salts from the roots up to other parts of the plant, while phloem transports sucrose and amino acids between the leaves and other parts of the plant. Xylem and phloem tissues are found in groups called vascular bundles. The position of these bundles varies in different parts of the plant. In a leaf, for example, the phloem is usually found closer to the lower surface.
– Root
Xylem vessels are tough and strong, so the vascular bundles are in the centre of the root to resist forces that could pull the plant out of the ground.
– Stem
The stem has to resist compression (squashing) and bending forces caused by the plant’s weight and the wind. The vascular bundles are arranged near the edge of the stem, with the phloem on the outside and the xylem on the inside.
– Transpiration stream
Transpiration is the evaporation of water at the surfaces of the spongy mesophyll cells in leaves, followed by loss of water vapour through the stomata.
– Rate of transpiration
If the rate of transpiration increases, the rate of absorption of water by the root increases too. Factors that affect the rate of transpiration also affect water uptake by the plant. If water is scarce, or the roots are damaged, a plant may wilt.
– Leaf adaptations
The leaves in hot or dry environments may be adapted to reduce transpiration. For example, their stomata may open at night and close at midday. This is the reverse of what usually happens, and it means that the stomata are closed when the rate of transpiration would be greatest.
– Investigating transpiration
The rate of transpiration can be investigated by measuring the decrease in mass due to water loss, or by measuring the volume of water absorbed.
– Sugar sources are plant organs such as leaves that produce sugars.
– Sugar sinks are plant organs such as roots, tubers (underground stems), and bulbs (swollen leaves) that consume or store sugars.
Sources are the structures that produce photosynthates for the growing plant. The sugars produced in the sources, such as leaves, must be delivered to growing parts of the plant. These sugars are transported through the plant via the phloem in a process called translocation. The points of sugar delivery, such as roots, young shoots, and developing seeds, are called sinks.
Seeds, tubers, and bulbs can be either a source or a sink, depending on the plant’s stage of development and the season.
The products from the source are usually translocated to the nearest sink through the phloem. For example, photosynthates produced in the upper leaves will travel upward to the growing shoot tip, while photosynthates in the lower leaves will travel downward to the roots.
Intermediate leaves will send products in both directions. The multidirectional flow of phloem contrasts the flow of xylem, which is always unidirectional soil to leaf to atmosphere. the pattern of photosynthate flow changes as the plant grows and develops. Photosynthates are directed primarily to the roots during early development, to shoots and leaves during vegetative growth, and to seeds and fruits during reproductive development. They are also directed to tubers for storage.
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