2Plant growth is influenced by a number of external and internal factors. The external factors affecting plant growth are: light temperature humidity oxygen carbon dioxide soil water and soil nutrients pressure (altitude) gravity
3Plant HormonesMost plant hormones are made in minute quantities by actively dividing tissues at the tips of roots and stems. Once produced, they are transported to various parts of the plant.The most important plant hormones are;AuxinsGiberrelinsCytokininsEthylene
4AUXINSHormones that affect the plant growth are called auxins. They may stimulate or slow growth, depending on the type of the tissue and the amount of hormone.Auxins are synthesized mainly in shoot meristem.These hormones increase plant growth by stimulating cells to lengthen.In addition they cause cell to differentiate.Auxins also affect the process of abscission – the dropping off of leaves, flowers, or fruits from a plant.
10GIBBERELLINSHormones that affect the plant growth and development of fruits and seeds are called gibberellins.Unlike auxins, they are distributed evenly throughout the plants tissues.They have important effects on stem growth.Commercially they are used to stimulate flowering and to increase fruit size.
17AUXINS and TROPISMThe growth of a plant in a specific direction in response to a stimulus is called a tropism.Plant growth or movement toward a stimulus is called positive tropism, while movement away from a stimulus is called negative tropism.
18AUXINS and TROPISMThe stem of a plant that is growing toward the light is an example of positive phototropism. Roots, show negative phototropism.
25Tropism, negative response to gravity Onion (Allium cepa)
26AUXINS and TROPISMWhen the tendrils of a grapevine wind themselves around the stem of another plant, they are showing thigmotropism- growth in response to touch.
27AUXINS and TROPISMHydrotropism is observed in plants whose roots grow toward water.
28AUXINS and TROPISMThe growth responses seen in tropism are thought to be caused by uneven distribution of auxins in the affected plant parts.
29In the positive photoperiodism of stems, for example, the concentration of auxins becomes higher on the shaded side of the stem than on the lighted side. Thus, the cells on the shaded side grow faster than the cells on the lighted side. The uneven rates of growth on opposite sides of the stem result in bending toward the side less rapid growth. In this case the stem bends toward the light.
38NASTIC MOVEMENTSA plant movement that is in response to a stimulus but independent of the direction of the stimulus is called nastic movement.Most nastic movements involve changes in the internal pressure or turgor pressure of specific cells.
39Rapid movements such as closing leaves involve changes in the turgor pressure in cells
42PHOTOPERIODISMThe flowering of many plants is in response to changes in the length of day over the course of the year.The response of a plant to changes in the length of day or night is called photoperiodism.(Photoperiodism is the non-directional developmental responses to non-directional but periodic light stimuli.)In many types of plants, flowering and other processes, such as leaf abscission, are controlled photoperiodically.
43Short-day plantsLong-day plants Plants that flower during short days were called (or short-night plants). They flower when there are short periods of darkness. These plants usually bloom in the summer. ex: clover, potato, beet, poppy and gladiolus.Plants that flower during short days (or long-night plants).They require long periods of darkness in order to flower.These plants flower in the early spring.ex: Morning glory, tulip, chrysanthemum and aster