Ch 8 Phloem Stuff

Maximum Height in Plants:
- The tallest plants, such as redwoods, can reach heights close to 420 feet; however, the actual heights are around 380 to 390 feet.
- The competitiveness of height is primarily influenced by evaporation (pulling water up) and gravity (pulling it down).
Evaporation vs. Gravity:
- Evaporation must exceed the gravitational force pulling down on water.
- Extreme height increases the pressure beneath which water is pulled, risking plant snapping if the upward force is too strong.
Water Movement in Plants:
- Water is pulled through plants via negative pressure created mainly in leaves, not directly by gravity.

Balancing Gas Exchange and Water Conservation:
- Stomata allow for gas exchange but can lead to water loss, particularly in arid conditions.
- Plants have several adaptations to mitigate water loss while still facilitating gas exchange.
Stomatal Structure:
- Sometimes, stomata are recessed into the leaf epidermis, minimizing direct airflow that can enhance evaporation.
- This structural change allows for gas exchange without excessive water loss due to airflow.
Comparison to Evaporation:
- An analogy comparing a fan blowing air over spilled water to recessed openings is presented, illustrating how airflow influences evaporation rates.

Trichomes:
- Small hair-like structures on plant leaves, similar to fur, slow down air movement, contributing to reduced water loss.
Photosynthesis Mechanisms:
- Different types of photosynthesis (C3, C4, and CAM) affect water efficiency in plants:
  - C3 Plants:
    - Open stomata during the day for gas exchange, but risk water loss.
    - They can waste time fixing oxygen instead of just carbon dioxide.
  - C4 Plants:
    - Sequester carbon dioxide and minimize oxygen fixation, reducing water loss while maintaining photosynthesis efficiency.
  - CAM Plants:
    - Open stomata at night to absorb CO2, store it as malate, and then close during the day to prevent water loss while still conducting photosynthesis using stored malate.

Understanding Phloem:
- Phloem is the vascular tissue responsible for transporting sugars (via bulk flow) produced primarily in the leaves to other parts of the plant (sinks).
Source-Sink Dynamics:
- Sources: Areas (like leaves) where sugars are produced.
- Sinks: Areas (like roots and fruits) where sugars are utilized.
- Phloem loading and unloading mechanisms help manage sugar distribution effectively based on plant needs.
Phloem Cell Types:
- Two key cell types from phloem:
  - Sieve Tube Elements: Long, hollow cells that allow for sugar transport.
  - Companion Cells: Assist with loading and unloading sugars into sieve tubes and provide metabolic support.

Sugars Initiation:
- Sugars produced in mesophyll cells diffuse into companion cells and from there into sieve tubes.
Pressure Flow Mechanism:
- As sugars are loaded into sieve tubes:
  - The solute potential decreases, causing water to influx from neighboring xylem cells, generating pressure.
  - Sugary water is then pushed from high-pressure areas (source) to low-pressure areas (sink).
Water Dynamics:
- If sink cells remove sugars, it lowers the sugar concentration, encouraging water to leave and maintaining pressure differences essential for flow.

Alarm Signals:
- When plants are damaged (e.g., eaten by caterpillars), they can send signals through phloem to trigger a defensive response in other parts of the same plant or neighboring plants.
Mycorrhizae Relationship:
- Mycorrhizal fungi form symbiotic relationships with plant roots, aiding in nutrient and water absorption while receiving sugars in return.
- Fungal networks can connect multiple plants, allowing for sharing resources and signaling damage to neighboring plants in cases of disturbance.
Plant Behavior and Cooperation:
- Plants can exhibit a kind of cooperation by sharing nutrients with closely related plants or those in distress, often referred to as "talking trees."
- This behavior enriches plant communities and highlights the interconnectedness within ecosystems.