Plants respond to environmental stimuli through tropisms. A tropism is plant growth in response to a stimulus. Examples are phototropism where plants grow towards light and gravitropism where shoots grow upward and roots downward in response to gravity. Plants also detect seasonal changes through changes in day length, influencing processes like flowering, leaf loss, and leaf color changes. Short-day and long-day plants have different flowering responses depending on night length. Deciduous trees shed leaves seasonally while evergreens retain leaves year-round with protective cuticles.

1. Plant Responses to
the Environment

2. Plant Tropisms
•When you hear
the word
“tropism”,
what do you
think of?

3. Maybe it reminds you of
the words “tropical
island”

4. Or
“Tropical
rainforest
?”

5. Or maybe you’re just a tired
student who is sick of hearing
about plants and just wants a
stinking answer?!?!
I swear, if she makes
me take notes on plants
again, I will throw this
book at the wall and run
for the hills!

6. So here are the facts…
• Plant tropisms
– Plants respond to an
environmental stimulus by
growing in a particular direction
– Growth in response to a
stimulus is called a tropism

7. • Like a 7th-grader’s attitude, a tropism
can be positive or negative
I better hurry up
and get this
report
finished..then I’ll
have more time
for math Forget that book
problems! report, I’m going
to IM my friends
all night!
POSITIVE NEGATIVE

8. • Light Tropism
– When you place a houseplant so
that it gets light from only one
direction, such as a window,
what happens?
– You may have noticed that the
plant will start bending towards
the light

9. • This is known as phototropism

11. • Gravity Tropism
– Plant growth also changes in
response to the direction of gravity
– This change is called gravitropism
– If a plant is placed on its side or
turned upside down, the shoot tips
have a negative gravitropism (away
from the center of the Earth) and
the roots have a positive
gravitropism (towards center of
Earth).

12. This plant is on its side, and the shoots still
grow upward!

13. Seasonal Changes
• Plants living in regions with cold winters
can detect changes in seasons because
the days get shorter and nights get
longer
• The opposite happens when spring and
summer approach
• Plants respond to the change in the
length of day

14. Length of Day
– short-day plants flower when night length is
long
– long-day plants flower when night length is
short

15. Seasons and Leaf Loss
– Evergreen trees
shed some of their
leave year-round so
that some leaves are
always on the tree
• leaves are often
covered with a
thick cuticle to
protect leaves
from cold and dry
weather

16. – Deciduous trees
lose all of their
leaves around the
same time each
year (either cold or
dry season)
• loss of leaves
helps plants
survive low
temperatures or
long periods
without rain

17. Seasons and Leaf Color
– leaves of deciduous
trees may change color
before they are lost
– as fall approaches,
green chlorophyll
breaks down and
orange and yellow
pigments are revealed
– they’re usually hidden
by the green
chlorophyll, but they are
always present