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Figure 36-3 Page 793 Branch Root system Shoot system Stem Apical bud Axillary bud Node Leaves Node Internode Lateral roots Taproot The shoot system produces carbohydrates (etc.) by photosynthesis. These solutes are transported to the roots in the phloem tissue: Translocation The root system removes water and minerals from the soil environment. These solutes are transported to the shoot in the xylem tissue: Transpiration Water and Minerals Transpiration Carbohydrate etc. Translocation

Figure 36-3 Page 793 Branch Root system Shoot system Stem Apical bud Axillary bud Node Leaves Node Internode Lateral roots Taproot Because these pathways involve solutes in water passing in the adjacent tissues of a narrow vascular bundle, this is a circulation system! Transpiration and Translocation The water is moving up the xylem, and down the phloem, making a full circuit! Water and Minerals Transpiration Carbohydrate etc. Translocation

Mendocino Tree (Coastal Redwood) Sequoia sempervirens Ukiah, California 112 m tall (367.5 feet)! This tree is more than ten times taller than is “theoretically possible” based solely upon the length of the column of uncavitated water. How could this be achieved?

Transpiration in a tall tree has at least 3 critical components: Evaporation: pulling up water from above Capillarity: climbing up of water within xylem Root Pressure: pushing up water from below

©1996 Norton Presentation Maker, W. W. Norton & Company Transpiration: root pressure (osmotic “push”) guttation Solutes from translocation of sugars accumulate in roots. Water from the soil moves in by osmosis. Accumulating water in the root rises in the xylem. Water escapes from hydathodes. This is not “dew” condensing!

Transpiration: root pressure (osmotic “push”) The veins (coarse and fine) show that no cell in a leaf is far from xylem and phloem (i.e.water and food!). The xylem of the veins leaks at the leaf margin in a modified stoma called the hydathode. These droplets are xylem sap. Root pressure accounts for maybe a half-meter of “push” up a tree trunk.

Capillarity: maximum height of unbroken water column The small diameter of vessels and tracheids and the surface tension of water provide capillary (“climb”). Cohesion of water, caused by hydrogen bonds, helps avoid cavitation. A tree taller than 10.4 m would need some adaptations to avoid “cavitation” atmospheric pressure keeps water in tube gravity pulls water down vacuum created 10.4m glass tube water

Transpiration: evaporation (“pull”) 76 cm vacuum mercury water mercury Water evaporating from a porous clay cap also lifts the mercury! Transpiration can lift the mercury above its normal cavitation height!

Lower pressure High pressure Lower pressure Apical Bud Leaf Root H+H+ sucrose H+H+ H+H+ Translocation is Bidirectional Transpiration is Unidirectional Translocation is Bidirectional XYLEM PHLOEM

Figure 36-3 Page 793 Branch Root system Shoot system Stem Apical bud Axillary bud Node Leaves Node Internode Lateral roots Taproot Root Pressure: Water moves into the root because of solutes from phloem. Pressure pushes the water up the stem. Water and Minerals Transpiration Carbohydrate etc. Translocation Transpiration Capillarity: Water climbs in the xylem cell walls by adhesion. Water molecules follow by cohesion. Evaporation: Water evaporates from mesophyll into atmosphere. Water molecules are pulled up the xylem by virtue of cohesion.

Figure 36-3 Page 793 Branch Root system Shoot system Stem Apical bud Axillary bud Node Leaves Node Internode Lateral roots Taproot Pressure = Bulk Flow The pressure gradient forces phloem sap away from leaves to all sinks (bidirectionally). Water and Minerals Transpiration Carbohydrate etc. Translocation Root (etc.) = Sinks Solutes removed from phloem by active transport. Water follows by osmosis, reducing pressure. Leaf = Source Photosynthesis produces solutes. Solutes loaded into phloem by active transport. Water follows by osmosis, increasing pressure.