5.6 Summary

5.6 Summary (ESG7X)

• Anatomy of dicotyledonous plants: Structures discussed in the previous chapter (collenchyma, sclerenchyma, xylem and phloem) are important in carrying out the transport functions.

• Root anatomy and function: The root is important in absorption, anchorage and storage of food. It is made up of the epidermis, cortex, endodermis and stele (consisting of xylem, phloem and pericycle).

• Stem anatomy and function: Stems contain an epidermis, cortex and vascular cylinder (consisting of pericycle, xylem, phloem, cambium and pith). Stems are important for growth, support, storage and transport of water, mineral salts and manufactured sugars.

• Secondary growth: Secondary growth is the thickening of the stem or root as new layers of xylem and phloem are formed by mitosis. It is carried out by cambium and results in stems and roots becoming thicker as the plant ages / matures. Secondary thickening results in the annual rings found in trees that can be used to work out the age of a tree.

• Transpiration: Transpiration is the loss of water from the stomata of plants. It creates a 'suction' or transpirational pull that is important for the movement of water through the plant. Transpiration is affected by environmental conditions e.g wind, temperature, humidity and light intensity. The rate of transpiration is measured using a potometer. In order to prevent excessive transpiration, plants have developed adaptations such as thickened cuticle, position of stomata, hairs on leaves, reduction of leaf size, leaf spines, leaf arrangements and rolling of leaves.

• Translocation: Translocation is the transport of food material (sugars), synthesised in the leaves, to other parts of the plant via phloem. This mode of transport is multi-directional, but requires energy as it occurs by active transport against a concentration gradient. Phloem vessels consist of sieve-tube elements and companion cells which are connected by plasmodesmata. Companion cells act as the regulators and energy stores of the phloem.
• Wilting and guttation are processes of water loss in the plant. Wilting entails excessive loss of water through plasmolysis resulting in excessive cell death, at times resulting in death of the entire plant. Guttation is the release of water via the hydathodes due to high humidity.