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The shoot system produces carbohydrates (etc. ) by photosynthesis. These solutes are transported to the roots in the phloem tissue: Shoot system Figure 34. 1 Node Internode Apical bud Axillary bud Carbohydrate etc. Node Leaves Branch Translocation Stem Transpiration Lateral roots Root system The root system removes water and minerals from the soil environment. These solutes are transported to the shoot in the xylem tissue: Translocation Water and Minerals Taproot

Dicot circulation: stem anatomy Dicots start with one ring of bundles… Figure 34. 19 Let’s take a closer look at the vascular tissues © 1996 Norton Presentation Maker, W. W. Norton & Company

Dicot stem anatomy: vascular bundle phloem fibers Support of Stem functional phloem Translocation vascular cambium Cell Divison: More Xylem and Phloem xylem Transpiration As a dicot grows, how does it add vascular capacity to become a tree? © 1996 Norton Presentation Maker, W. W. Norton & Company

Because these pathways involve solutes in water passing in the adjacent tissues of a narrow vascular bundle, this is a circulation system! Shoot system Figure 34. 1 Carbohydrate etc. Node Leaves Branch Stem Transpiration Translocation Lateral roots Root system Transpiration and Translocation The water is moving up the xylem, and down the phloem, making a full circuit! Node Internode Apical bud Axillary bud Water and Minerals Taproot

Compare Figure 35. 5 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? http: //www. nearctica. com/trees/conifer/tsuga/Ssemp 10. jpg

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”) Compare Figure 35. 8 Solutes from translocation of sugars accumulate in roots. guttation Water from the soil moves in by osmosis. Accumulating water in the root rises in the xylem. This is not “dew” condensing! Water escapes from hydathodes.

Transpiration: root pressure (osmotic “push”) Compare Figure 35. 8 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. http: //img. fotocommunity. com/photos/8489473. jpg

Capillarity: maximum height of unbroken water column glass tube vacuum created gravity pulls water down atmospheric pressure keeps water in tube water 10. 4 m 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”

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

This is a cross-section of a “typical” leaf: Syringa vulgaris (lilac) soil mineral entry evaporative cooling means the solute concentration increases!

Lower pressure PHLOEM XYLEM Transpiration is Unidirectional Compare Figure 35. 17 sucrose H+ Apical Bud Translocation is Bidirectional High pressure Leaf sucrose H+ Translocation is Bidirectional Lower pressure sucrose H+ Root

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

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