Xylem: Evapo-transpiration Transport in plants Xylem: Evapo-transpiration

Objectives Describe the structure of xylem vessels & sieve tubes. Discuss how the structure of the xylem vessels suited for its function Describe the process involved in transpiration. Discuss the external factors that affect transpiration

Why do plants need a transport system? To transport food and water throughout the plant from roots to leaves at the top of the plant. These substances are used by the plant to aid in photosynthesis. There are two structures used to transport material, the xylem vessels and phloem tubes. Vascular bundles are xylem and phloem tubes in close proximity to each other.

The position and orientation of vascular bundles vary throughout the plant. The phloem however is always found on the outer part of the bundle

Xylem Xylem vessels transport water and dissolved minerals from the root to the shoot of plants.

Xylem adaptations These are well adapted with: Long & Tubular cells Hollow (no cell contents) Thick bands of cellulose and lignin Water-resistant walls

Direction of movement of substances in vascular tissue.

Xylem tissue

MONOCOTS Root pattern (xylem and phloem) – ring

DICOTS Stem pattern – Ring-shaped xylem and phloem. LARGE CELLS ARE XYLEM Stem pattern – Ring-shaped xylem and phloem. SMALL CELLS ON OUTSIDE ARE PHLOEM

DICOTS Root pattern (xylem and phloem) – X-shape

Mineral and water uptake in root *osmosis

Casparian Strip The Casparian strip controls water movement into the vascular cylinder of the root. Water cannot move between cells. It must move through the cells by osmosis. Why is this important?

Thinking question: What would happen in a root that had no Casparian strip? Why would this be a problem?

Water movement up the xylem Step 1:Capillary action Cohesion and adhesion cause water to “crawl” up narrow tubes. The narrower the tube the higher the same mass of water can climb. Maximum height: 32 feet.

Capillary action Cohesion: polar water molecules tend to stick together with hydrogen bonds. Adhesion: water molecules tend to stick to polar surfaces.

Cohesion-tension theory Cohesion between water molecules creates a “water chain” effect. As molecules are removed from the column by evaporation in the leaf, more are drawn up.

Part 2: Root pressure There is a higher pressure in roots due to the higher conc. of water molecules being absorbed from soil. Water moves form area of high pressure (in roots) to low pressure (in leaves)

Part 3: Transpiration pull due to evaporation Evaporation at the surface of the leaf keeps the water column moving. This is the strongest force involved in transpiration.

Factors affecting transpiration rate Temperature: temp = TR Humidity: Humidity = TR Wind velocity: Wind velocity = TR Light intensity: affects stomatal opening Water content in soil : Less water in soil lead to less available for plants. The cellss get flaccid and in turn, plant wilts. Long term deprivation leads to death of plant.

Stomata control Guard cells around the stomata are sensitive to light, CO2, and water loss. Cells expand in response to light and low CO2 levels, and collapse in response to water loss.

Stomata When stomata are open, evaporation draws water out of the leaf. Gas exchange can also occur to keep photosynthesis and respiration running. When stomata are closed, evaporation cannot occur, nor can gas exchange. What happens to photosynthesis and transpiration?