Plant Growth and Development

Types of Growth  Apical meristem: plant tissue made of actively dividing cells. Primary growth and located at the tip of roots and shoots.  All growth at the apical meristem is called primary growth which increases the height, not the width.  Secondary growth arises from lateral meristems and that increases the width of a plant.

Shoot Primary Growth  Begin with mitosis and once cell division occurs, each cell grows longer.  Then elongated cells specialize (differentiate) into different cell types: parenchyma, epidermal, vascular.  Tissues form stems, leaves, and organs for sexual reproduction.

Root Primary Growth  Root apical meristems produce the cells of the root cap and all other cell types in the root.  There are several zones in the root:  Root cap: protects the meristem as it pushes through the soil.  Zone of elongation: most cells stop dividing but increase in length. Phloem matures and xylem starts to form.  Zone of cell division: cells formed by meristem continue to divide.  Zone of maturation (differentiation): tissues of the root begin to take on specialized roles. Root hairs begin to form.

Secondary Growth  Happens in woody plants after the first season.  Wood is a product of secondary growth from lateral meristem  The vascular cambium is an example of a lateral meristem as it gives rise to secondary phloem and secondary xylem  Cork cambium is another example as it makes the cells that form the bark

Tropisms and Growth Regulators  Tropisms are growth responses to external stimuli.  Positive tropisms are growths towards the stimulus  Negative tropisms are growths away from the stimulus  Phototropism: stems show a positive tropism response to sunlight, where roots show a negative tropism response.  Gravitropism: stems show negative to gravity, where roots show positive.  Thigmotropism: is a change in the direction of growth in response to contact. Climbing vines are an example.

Hormones  There are several groups of plant hormones: auxins, gibberellins, cytokinins, and ethylene.  Auxins  Plants like grasses have a sheath that covers young leaves as the seedling emerges from the ground. This sheath responds to sunlight by bending towards it.  Auxin is released from the apical bud (tip) when exposed to light and travels away from the light and downwards, causing the cells to elongate.  This results in an uneven elongation of cells, and the stem bends towards the light.  Produced in the roots by the apical meristems.  Helps the roots grow with gravity, and works the opposite to stem cells, high amounts of auxin, inhibit elongation.  Help roots grow around rocks.

 Some herbicides contain auxins that cause plants to undergo elongation at an unsustainably rapid rate and die.  Synthetic auxins are used to induce cell elongation in fruits.  If you cut off the apical bud, you can stunt the growth of the plant and possibly cause the plant to grow more flowers, fruit or leaves.

 Gibberellins  Produced in apical meristems AND in leaf chloroplasts  Work with auxins to elongate stems and roots  Promote leaf growth and flowering  Seed germination  Controls fruit development  We spray fruit with gibberellins, to get larger fruit.

Cytokinins  Promote cell division  Found in tissues that are actively dividing such as meristems, young leaves, seeds.  Can also slow cell aging and used on lettuce and mushrooms to keep them from going bad.

Ethylene  Plant growth regulator  Induces changes that protect a plant against environmental stress (leaf drop in the fall and in drought conditions)  Released when a plant has a wound  Stimulates developmental stages like fruit ripening, shoot and root growth, flower opening.  Because they are involved in fruit ripening, farmers and grocers try to control the amount of ethylene produced by adding more when they want them to be ripe and using filters to remove the ethylene when they want to keep them from ripening too quickly.