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Plant Hormones. How do we know they exist?. Why do the shoots grow up, but the roots grow down?. Plant Hormones. Phototropism experiments by Charles and Francis Darwin:. Plant Hormones. Boysen-Jensen’s experiments:. Mechanisms of bending:. Plant Hormones.

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plant hormones
Plant Hormones

How do we know they exist?

Why do the shoots grow up,

but the roots grow down?

plant hormones2
Plant Hormones

Phototropism experiments by Charles and Francis Darwin:

plant hormones3
Plant Hormones

Boysen-Jensen’s experiments:

Mechanisms of bending:

plant hormones4
Plant Hormones

Frits Went discovered the hormone Auxin. Concentration of auxins in different parts of roots

and shoots determines growth rates of the cells… Since distribution of auxins is affected by

both light and gravity, auxins are involved in plants’ phototropic and gravotropic growth


plant hormones5
Plant Hormones

Major plant hormones and their action:

Abscisic acid closing of stomata; seed dormancy

Auxins elongation of shoots and roots, gravotropism,


Cytokinins promotion of sprouting of lateral buds

Ethylene ripening of fruit

Gibberellins germination of seeds and sprouting of buds;

elongation of stems; stimulation of flowering;

development of fruit

plant hormones6
Plant Hormones

Hormones can work with

each other to regulate various

aspects of plant biology.

Here, auxin and cytokinin work

together in regulating lateral

bud formation… the sprouting

of lateral buds depends on the

ratio of cytokinin to auxin…

plant hormones7
Plant Hormones

Plants use starch-filled organelles called statoliths to detect gravity… this is very similar

to how jellyfish detect gravity, and in some ways, it is even similar to how

humans keep themselves balanced…

plant hormones8
Plant Hormones

A hormone called florigen both stimulates and inhibits flowering. Since flowering is

sometimes a function of day length, pigments called phytochromes are involved in

“measuring” day length. The ratio of different forms of this pigments change as a function

sunlight exposure and can thus be used by the plants to set their “internal biological clocks”.

plant communication
Plant Communication

The Venus flytrap catches flies as a source of nitrogen.

Insects are attracted to the nectar in the trap, but when they trigger sensory “haris”

within the trap, they are captured…

What is the mechanism?

plant communication11
Plant Communication

The mechanism is based on “irreversible, differential growth”.

Triggering of the hairs creates an electrical potential that pumps H+ into outer cell walls.

The acidic condition created by the H+ induces enzymes to loosen the cellulose fibers of

the walls, causing water to rush in… the “growing” or swelling of the outer cells then

causes the rapid closing of the “trap”…