9.3: Growth in plants

Annuals die every year. Life span limited to one year e.g. corn, wheat, lettuce, rice Biennials take two years to complete life cycle e.g. Parsley, wild carrots Perennials live longer than two years, death usually due to environmental factors or disease e.g. daisies

Plants show indeterminate growth – they grow throughout their lives. Humans display determinate growth, we stop growing at a certain point. Dicotyledonous plants have two types of meristematic tissue (growing part of the plant): Apical – tip of roots/stems. Called primary growth Lateral – growth in the thickness of the plant. Secondary growth. Again there are two types of lateral meristems: - Vascular cambium – between xylem & phloem in vascular bundles, inside the bundle secondary xylem are produced. Outside the bundle secondary phloem are produced. - Cork cambium – cork cells produced on the outer bark

Apical Meristem

Lateral Meristem

Tropism What do plants respond to? Light, chemicals, gravity, water, touch Plants demonstrate positive phototropism What are the advantages of this behaviour? What causes this behaviour? Auxins!

Auxins are found in: 1.seed embryos 2.Meristems of apical buds 3.Young leaves The hormone can only work if the plant cells contain an auxin receptor. Auxin increases a plants flexibility, allowing it to move towards the stimulus. Auxin is redistributed in the plant, not produced. The redistribution of auxin to cells on the dark side cause those cells to elongate – result – shoot moves towards the light.

What’s the proof? What experiments could be conducted?