The microscope image below shows a cross section of part of a stem of a herbaceous (non-woody) plant.
Image courtesy of Berkshire Community College Bioscience Image Library. Adapted and redistributed under a Creative Commons CC0 1.0 Universal Public Domain Dedication license under conditions found at https://creativecommons.org/publicdomain/zero/1.0/deed.en
The fibrous tissue shown in the image in part a) is known as sclerenchyma tissue, and has an important structural role in plants. Other tissue types include parenchyma, sometimes known as pith, which is involved with photosynthesis and storage, and collenchyma, which also has a structural role.
Describe how this range of different tissue types can arise in plants. Avoid repetition of ideas covered in part b).
The stem section shown in the image in part a) is taken from a young plant stem. The diagram below shows how the arrangement of tissue in a plant stem can change as the stem matures. Some of the tissues shown in part a) are also labelled below for comparison.
The change in tissue arrangement is in part triggered by increased expression of a gene known as HCA2, which codes for a transcription factor.
i)
Suggest how increased expression of HCA2 might alter the distribution of tissue within the plant stem.
[3 marks]
ii)
Suggest how scientists could determine the expression levels of HCA2 at any given stage in a plant's life cycle.
The diagram below illustrates a theory of auxin action known as the acid growth hypothesis. In the acid growth hypothesis plant cell wall rigidity is reduced, allowing plant cells to expand and therefore elongate.
Suggest how activated expansin proteins could reduce the rigidity of plant cell walls.
An investigation was carried out into the effect of a respiratory inhibitor on the elongation of wheat seedling shoots. The results of the study are shown in the graph below.
i)
Identify, with a reason, the condition that functions as an experimental control.
[2 marks]
ii)
Use information provided in part a) to explain the results for the seedlings in the presence of auxin alone. Note that marks will not be awarded for content relating to the effect of expansins.
Use the graph in part a) to suggest a possible auxin concentration for the following parts of a plant. Note that your value readings should be given to the nearest accurate value on the scale provided.
In addition to regulating shoot and root elongation, auxin is involved with other aspects of plant growth. The image below shows the appearance of a plant that has been influenced by auxin.
Explain how auxin and any other relevant plant hormones can cause the plant appearance shown in the image.
Current knowledge of plant tropisms has been gained from multiple experiments carried out on plant seedlings. For example, early experiments showed that a plant growth influence (now known as auxin) was produced in the growing tip of seedlings. The image below illustrates three more experiments. Note that gelatin is a permeable material while mica is impermeable.
Identify three control variables that would be required to ensure valid results from all of the experiments shown.
