IB Biology Sub-topic C3.1 Notes
Plant growth
You previously learned how water and solutes move through xylem and phloem. The circulation of these is incredibly important to sustain plant cell metabolism and growth, especially because they endlessly grow.
Whilst human cells continually undergo mitosis to growth this stops sometime after puberty. After this, stem cells divide by mitosis to replace cells that have died or been sloughed off. Therefore, humans have a definite size limit and thus undergo determinate growth.
On the other hand, plant cells carefully organize their cells so that some remain pluripotent stem cells and can continue to grow the stems and roots out. Therefore, plants have an indefinite size limit and thus undergo indeterminate growth.
The structures in plants responsible for this are called apical meristems, found in the tips of stems and roots. During one mitotic division, one pluripotent stem cell remains whilst the other differentiates and migrates to its tissue. This organization allows plants to retain their puripotency in the meristem.
There are three types of meristem in plants that form different tissues:
- Protoderm meristem - this forms epidermal tissue.
- Procambium meristem - this forms vascular tissue
- Ground meristem - this forms pith issue.
Auxins
However, plants do not grow uncontrolled. Their division by several hormones, the most of important of which is auxin. Auxin controls growth, leaf development, and flower formation. The most prevalent one is indole-3-acetic acid, which functions to:
- Promote cell elongation in the apical meristem.
- Inhibit residual stem cells found in auxiliary buds.
This acts to promote vertical growth of stems but prevent the growth of leaves, called apical dominance. In roots, cytokinins are the more active hormone.
Phototropism
The regulation of shoot and root growth primarily occurs in response to external factors, a phenomenon called a tropism. You are expected to know about phototropism, plant growth in response to light. There are two types of phototropism:
- Positive phototropism - the growth of shoots towards light.
- Negative phototropism - the growth of roots away from light.