9.3 Growth in Plants

Essential idea: Plants adapt their growth to environmental conditions. Discuss: What environmental conditions affect plant growth and how do plants adapt?

Plants exhibit indeterminate growth Indeterminate growth = growth that continues throughout entire life Undifferentiated cells in the meristems of plants allow indeterminate growth.

Meristems perpetually embryonic parts that make new cells for plant growth Types of meristems: Lateral Apical

Apical Meristems tips of roots and buds; primary growth (height of plant) Mitosis and cell division in the shoot apex provide cells needed for extension of the stem and development of leaves.

Lateral meristems Cylinders of dividing cells along length of roots and stems Produce secondary growth (girth of plant); Types: Vascular Cambium Between vascular bundles On inside produces secondary xylem (sapwood)– major component of wood On outside, produces secondary phloem Cork Cambium In bark, produces cork cells of outer bark Note: heartwood has primary (original) xylem

Plant hormones control growth in the shoot apex. A hormone is a chemical message sent from one region that can alter activity in another region Guidance: • Auxin is the only named hormone that is expected.

Plant shoots respond to the environment by tropisms. Tropism= directional growth response to external stimulus Phototropism = growth towards light Gravitropism = growth in response to gravity Both of these are dependent on a hormone called auxin. Aim 6: Investigations into tropisms could be carried out

Auxin efflux pumps can set up concentration gradients of auxin in plant tissue. Auxin influences cell growth rates by changing the pattern of gene expression. Note: IAA is an auxin

Nature of science: Developments in scientific research follow improvements in analysis and deduction—improvements in analytical techniques allowing the detection of trace amounts of substances has led to advances in the understanding of plant hormones and their effect on gene expression.

Application: Micropropagation of plants using tissue from the shoot apex, nutrient agar gels and growth hormones. • Application: Use of micropropagation for rapid bulking up of new varieties, production of virus-free strains of existing varieties and propagation of orchids and other rare species. Some key steps: Take tissue samples and sterilize them (these pieces are called explants) Place in sterilized growth media with plant hormones.

Theory of knowledge: • Plants communicate chemically both internally and externally. To what extent can plants be said to have language?