Stems and Plant Transport Chapter 33

Stem Functions Support leaves and reproductive structures Allow leaves to absorb sunlight needed for photosynthesis Flowers and fruits are located in areas accessible to insects, birds, or air currents Provide internal transport Water and dissolved nutrients up from ground Sugars down from leaves Produce new living tissues Buds  leaves, reproductive structures In some plants additional functions: Asexual reproduction Photosynthesis Starch storage

External Stem Structure Woody stems (terms to know) Terminal bud Bud scales Axillary (lateral) buds Node and internode Scars: Bud scale scars Leaf scars Bundle scars Lenticels

Types of Stem Growth Primary growth Secondary growth In all plants Increase in length Occurs at the apical meristems Secondary growth Only in woody plants Increases in girth Occurs at the lateral meristems

Internal structure – herbaceous dicots Epidermis – outer protective layer Cuticle – waxy coating on epidermis Stomata – permit gas exchange Cortex – may contain photosynthetic cells; also provides support Vascular bundles – arranged in a circle around the central core Xylem – located toward the stem’s interior Phloem – toward the outside Pith – in the center of the stem and functions mainly in storage

Internal structure – herbaceous monocots Vascular bundles are scattered throughout Do NOT have lateral meristems and do NOT produce wood or bark Palm trees and bamboo – monocots that produce wood-like tissue from primary growth

Woody plants: secondary growth The result of two lateral meristems: Vascular cambium produces - Secondary xylem (wood) and secondary phloem (inner bark) Cork cambium produces Cork cells and cork parenchyma  periderm Heartwood – the older wood in the center of the tree; cells are plugged with pigments and other wastes Sapwood – the younger, functional secondary xylem that is conducting water and minerals

Transportation of water In general, water and dissolved minerals travel upwards from roots in one direction Water moves by being pulled to the top of the plant Water potential – the free energy of water Pure water has a water potential of 0 Dissolved solutes decrease the motion of the water molecules and therefore decrease the free energy More solutes = negative number Water moves from a region of higher free energy (less negative number) to a region of lower free energy (more negative number)

Water Potential Water potential of soil varies: Dry soil = very low potential (very negative) Wet soil = higher potential (but still a negative number) Water potential in root cells is also negative: Due to the presence of dissolved solutes in the cells Under normal conditions, the water potential of the root is more negative than the surrounding soil This means water moves by osmosis from soil into the root

Water potential… Water potential of the air is extremely negative Water potential gradient from least negative (soil) through the plant into the most negative (air) Most scientists think that the tension-cohesion model explains the movement Water sticks together and is pulled up the xylem similar to water moving up a straw

Translocation of food Dissolved sugars move in all different directions in phloem cells Sugar made during photosynthesis is converted to sucrose (table sugar – glucose + fructose) before being loaded into phloem cells Sugar moves from source to sink Pressure-flow hypothesis - movement in phloem is due to pressure gradient High pressure at source  low pressure at sink