Plant Growth Hormone Function and Tropisms
Primary Meristems Whether they are involved in primary or secondary growth, all plant cells and tissues arise from three primary meristems*: • protoderm •ground meristem •procambium *Meristem: plant tissue that remains embryonic as long as the plant lives, allowing for indeterminate growth
Specialized Tissues in Plants • Plant Growth • Meristems – tissues responsible for growth • Undifferentiated cells • Apical Meristem • Produce growth increased length • Differentiation • Cells will assume roles in the plant • Flower Development • Starts in the meristem
Growth Hormones (Regulation) • Plants produce chemical substances called hormones that inhibit or promote growth. • Common plant hormones are…. • Inhibitors • Cytokinins • Gibberellins • Auxins
Gibberellins • Hormones that stimulate cell elongation, premature flowering, and breaking of dormancy.
Hormones that speed plant growth by stimulating cell enlargement. Auxins
Tropisms • A tropism is a growth response involving bending or curving of a plant part toward or away from an external stimulus that determines the direction of movement • Response toward the stimulus is positive, and response away from the stimulus is negative
Phototropism • Phototropism is the growth of stems of plants toward light - it is probably the best known of the plant tropisms - phototropism is caused by elongation of the cells on the shaded part of the plant - so that entire plant bends or curves toward the light • This growth pattern is caused by the hormone auxin- auxin migrates to the shaded part of the plant and stimulates increased cell growth and elongation on the shaded part of the plant
Gravitropism • Gravitropism is a response to gravity - if a seedling is placed on its side, the root will curve to grow downward and the shoot will curve to grow upward • Auxin may play a role in the response of shoots
Thigmotropism • Thigmotropism is response to touch or contact with a solid object - this is most commonly seen in tendrils, modified leaves or stems depending on the species. • the tendrils wrap around any object they come in contact with and so enable the plant to cling and climb.