9.2b Plant Transport Systems

11/04/2013 § 1 Comment

Translocation in phloem

The phloem tissue inside plants is what transports sugars and amino acids. Phloem cells use energy to transport the substances, therefore the process is called active translocation. Areas where sugars and amino acids are loaded into the phloem are called sources, which include photosynthetic tissues like mature green leaves and green stems. Areas where the sugars and amino acids are unloaded and used are called sinks, which include growing roots and leaves or developing fruits and seeds.


Plants that live in dry areas, like deserts, are called xerophytes. These plants have numerous strategies to help them survive in these habitats, such as increasing the amount of water absorbed and reducing transpiration. Cacti are an example of a xerophyte, and they have leaves so reduced in size that they end up looking like little spikes. A cactus’s stomata would open at night, when the temperature is much cooler, therefore transpiration is a lot slower.

Essay Questions

  1. The main part of growing plants are roots, stems and leaves. Draw a plant diagram to show the arrangement of tissues in the stem of a dicotyledonous plant.
  2. Draw a labelled diagram showing the tissues present in a dicotyledonous leaf.
  3. Explain the functions of the different tissues of a leaf.
  4. Explain the role of auxin in phototropism.
  5. Outline the adaptations of plant roots for absorption of mineral ions from the soil.
  6. Describe the process of mineral ion uptake into roots.
  7. Describe how water is carried by the transpiration stream.
  8. Explain how abiotic factors affect the rate of transpiration in a terrestrial plant.
  9. List three abiotic factors which affect the rate of transpiration in a typical mesophytic plant.
  10. Explain how wind affects the rate of transpiration from a leaf.
  11. Outline adaptations of xerophytes.
  12. Outline the role of the phloem in the active translocation of biochemicals.
  13. Draw the structure of a dicotyledonous animal-pollinated flower
  14. Describe the metabolic events of germination in a starchy seed.
  15. Explain the conditions needed for seed germination.
  16. Explain how flowering is controlled in long-day and short-day plants.



Page 128, water permeance of waxy cuticle

1. Using the data in Figure 30, describe the relationship between temperature and water permeance. —> The higher the temperature, normally the higher the water permeance. Despite what kind of plant, the relationship is the same for all of them.

2. Discuss the consequences for plants of the effect of temperature on cuticular water permeance. —> We know that as temperature rises, for most plants, the rate of transpiration also rises. When the temperature hits a certain level though, the plants risk denaturing their own enzymes and transpiration would probably start decreasing then. The temperature also affects the humidity surrounding the plant’s waxy cuticle. The plant could lose more water at high temperatures because water permeance is high and the diffusion of water (or osmosis…?) is more likely.

3. Using the data in Figure 30, state the thickness of cuticular wax with

a) the highest water permeance —> 1.3 µm

b) the lowest water permeance —> 1.7 µm

4. Evaluate the hypothesis that the water permeance of the cuticle is positively correlated with its thickness, using the data in Figure 30. —> There are some anomalies that tell us this doesn’t seem so. The thickness of the cuticular wax doesn’t impact the permeance of for water because both the highest and lowest water permeance are somewhere in between 1 and 2 µm.

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