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Settler. Describe the pathways and mechanisms by which water passes from the soil to the xylem vessels in the root. [6]. Module 3 Exchange and transport. 9.5 Reducing water loss. Starter.
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Settler • Describe the pathways and mechanisms by which water passes from the soil to the xylem vessels in the root. [6]
Module 3Exchange and transport 9.5 Reducing water loss
Starter • Stomata must be open during the day for photosynthesis, so carbon dioxide can enter and oxygen can leave. Water is lost by transpiration from the leaves • Write down one way plants reduce water loss • Discuss
Success Criteria Learning Objectives To understand xerophytes (cacti and marram grass) and understand how they are adapted to reduce water loss To be able to understand how specific plants are adapted Define the term xerophyte and hydrophyte (Grade D – C) Describe the adaptations of plants to the availability of water in their environment (Grade C –B) Explain how xerophytes and hydrophytes are adapted to their environment (Grade B – A)
Xerophytes Xerophytes are plants that have adaptations to reduce water loss or to conserve water. They occupy habitats in which there is some kind of water stress. Examples of such water stress habitats include: • Desert (high temp, low precipitation) • High Altitude & High Latitude ( low precipitation or water locked up as snow or ice) • Rapid drainage (sand dunes)
Xerophytes Some xerophytes are ephemeral : • they have a very short life cycle which is completed in the brief period after rainfall • They remain dormant as embryos inside seeds until the next rains. Some xerophytes are perennial : • They rely on storage of water in specialised leaves, stems and roots.
Xerophyte adaptations Waxy Leaves: • The leaves of this plant have waxy cuticle on both the upper and lower epidermis • The waxy cuticle repels water loss through the upper and lower epidermal cells. If an epidermal cell has no cuticle water will rapidly be lost as the cellulose cell wall is not a barrier to water loss.
Xerophyte adaptations Rolled Leaves / Stomatal Pits / Hairs on epidermis: • The thick waxy upper epidermis extends all the way around as the leaf rolls up. This places the stomata in an enclosed space not exposed to the wind. • The stomata are in pits which allows boundary layer of humidity to build up which also reduces water loss by evaporation. • The hairs on the inner surface also allow water vapour to be retained which reduces water loss through the pores. • The groove formed by the rolled leaf also acts as a channel for rain water to drain directly to the specific root of the grass stem
Left and right Epidermis of the cactus Rhipsalis dissimilis. Left: View of the epidermis surface. The crater-shaped depressions with a guard cell each at their base can be seen. Right: X-section through the epidermis & underlying tissues. The guard cells are countersunk, the cuticle is thickened. These are classic xerophyte adaptations.
Xerophyte adaptations • Dense spongy mesophyll, few air spaces – less surface area for evaporation of water • Leaves reduced to spines e.g. cactus, reduces surface area, less transpiration • Widespread roots – to take advantage of any rain that falls
Xerophyte adaptations Needles as leaves (Firs and Pines): • Like a rolled leaf, study of the internal structure shows it has effectively no lower epidermis. • This adaptation is required as northern climates have long periods in which water is actually frozen and not available for transpiration. Plants in effect experience water availability more typical of desert environments. • This type of adaptation means that conifers have their distribution extended beyond the northern forests to a variety of water stress climates.
Xerophyte adaptations Succulent: • The leaves have been reduced to needles to reduce transpiration. • The stem is fleshy in which the water is stored. • The stem becomes the main photosynthetic tissue.
Xerophyte adaptations • Closing stomata when water availability is low • Low water potential inside cells, achieved by maintaining high salt concentration in cells • Long deep tap root
Hydrophytes • Plants that live in water (submerged, on the surface or at the edges of bodies of water) need adaptations to cope with growing in water or a permanently saturated soil. • Examples include – water lilies, water cress, duckweeds and yellow iris.
Hydrophyte adaptations • Wide, flat leaves that spread across the surface of the water to capture as much light • Very thin or no waxy cuticle, there is no need to conserve water • Open stomata on upper surface of leaves, guard cells are inactive. • Water supports leaves and flowers so no need for strong supportive structures • Small roots, as water can diffuse directly into stem and leaves
Hydrophyte adaptations • Air sacs to enable floating on the water • Aerenchyma – specialised parenchyma (packing) tissue forms in the leaves, stems and roots. This makes them more buoyant and a low resistance internal pathway for oxygen to the tissues, in low oxygen conditions (anoxic) • In mangrove swamps where there is excess water, aerial roots such as pneumatophores grow upwards in air with lenticels
How do hydrophytes transpire? • Hydathodes or water stomata • At the end of a vein, guard cell always leave stomata open to outside • Water appear as beads - guttation
Plenary • Xerophytes are plants living in desert habitats where the conditions are very ……………. . Their leaves show a variety of modifications to ……………….. transpiration. Many have a thick ……………… cuticle. Their leaves may be covered with hairs which trap ……………. ……………. . In some, the leaves are in the form of needles which reduces their surface area. Their ……………. are often sunken into pits in the surface of the leaf or stem. [5]
