Plant hormones are chemical messengers that regulate plant growth and development. They cause substantial changes even in small concentrations. The main hormones are auxins, which promote elongation and proper development. Auxins are produced by plant shoot tips and cause phototropism by stimulating uneven growth - cells on the shaded side grow more, causing the shoot to bend towards light. Experiments by Darwin and Went showed that differences in auxin concentration between shoot sides cause this phototropic response.

1. Presentation 18 – Plant Hormones (for Li, K, Sr)
• Chemical messengers produced by one plant organ affect cells
in another location within the plant
• Very small concentrations can cause substantial changes in the
plant
• Most often used to regulate the growth and maturity of the
plant
• Picture on the left shows a potato grown in the dark. Because of
the absence of light, an imbalance of growth hormones causes
many sprouts to grow out of the potato in many different
directions. This causes the potato to rapidly “eat up” the starch
remaining in the tuber.
• Picture on the right is a dramatic representation of a plant shoot
growing towards the light (candle). You may have seen this in
plants that grow in shaded areas on potted plants in window
sills.
• One important function of plant hormones is to coordinate
growth responses called tropisms
• Plant organs respond to stimuli by growing towards them
(positive tropism) or way from them (negative tropism)
• Phototropism is response toward light: shoot tips of plants are +
phototropic while the root tips are – phototropic
• Thigmotropism is response to mechanical pressure or “touch,”
some plant organs like tendrils (modified stems) are +
thigmotropic, wrapping around other plants or trellises
• Gravitropism/geotropism is response to the force of gravity.
Shoot tips are – gravitropic, root tips are + gravitropic. In the
picture on the lower right, no matter how you orient the seed
when planting it, the roots will always grow downward.
• Phototropism has been much studied since the 1800’s.
• How do plants curve towards the light? This involves a
difference in growth of the cells on both sides of a plant shoot.
(A coleoptile is a special name for the covering of a grass shoot).
The cells on the side of the plant that faces away from the
source of light should elongate more that the cells on the side
of the plant that faces the light source. This causes the plant to
appear to “bend” towards a given source.
• Experiments by Charles and Francis Darwin as well as FW Went
showed that this happens because of the distribution of a plant
hormone called auxin.

2. • The Darwins discovered that the substance that caused the
bending was produced by the plant’s shoot tip even if the
bending itself occurred on the shoot, a short distance away
from the tip.
• Went discovered that the substance that caused the bending
was a chemical called “auxin,” (from the Gr. word “to
grow/increase”). Auxin was produced by the shoot tip of plants
in response to light. When a shoot tip was cut and placed on a
porous material, like a block of agar, auxin would diffuse into
the agar block.
• Went wanted to demonstrate that it was the difference in the
concentration of auxin that caused the difference in growth.
He took the agar block with the auxin and placed it on various
positions on top of shoots that had their tips cut off.
• In the drawing above, the first two shoots were the – controls.
They showed no growth differences. The third had the auxin-
agar block placed evenly on top. It too showed no growth
difference. This is because the auxin was distributed evenly on
both sides of the plant shoot. The fourth and fifth shoots had
the auxin-agar block placed a little bit off-center, the fourth
had it placed on the right, the last had it placed on the left
FW Went’s Experiment side. This caused the shoots to curve as auxin caused the cells
on the side it was on to elongate more.
growth/elongation
“fountain of youth”
hormone
elongation and
proper development
stress hormone
maturation hormone causing
fruit ripening and leaf aging
• There are five major classes of plant hormones (and many more minor classes). This table summarizes their origins,
locations, and major functions. All of these hormones work together to ensure proper growth and development and
proper response to stimuli. Familiarize yourself with this table. Look up the meanings of the encircled words.