just that one side of the stem. With one side of the stem growing faster than the other, the
plant begins to bend.
You might have noticed that plants tend to bend towards the light. This is an example of
a tropism where light is the stimulus, known asphototropism(Figure10.32). To obtain
more light for photosynthesis, it’s advantageous for leaves and stems to grow towards the
light. On the other hand, roots are either insensitive to light or actually grow away from
light. This is advantageous for the roots since their purpose is to obtain water and nutrients
from deep within the ground.
A seed often starts out underground in the dark, yet the roots always grow downwards into
the earth and not toward the surface. How do the roots know which way is up? Gravit-
ropismis a growth towards or away from the pull of gravity. Shoots also exhibit gravit-
ropism, but in the opposite direction. If you place a plant on its side, the stem and new
leaves will curve upwards. Again, the hormone auxin is involved in this response. Auxin
builds up on the lower side of the stem, elongating this side of the stem and causing it to
bend upwards over time.
Plants also have a touch response, calledthigmotropism.If you have ever seen a morning
glory or the tendrils of a bean plant twist around a pole, then you know that plants must
be able to detect the pole. Thigmotropism works much like the other tropisms. The plant
grows straight until it comes in contact with the pole. Then the side of the stem in contact
with the pole grows slower than the opposite side of the stem. This causes the stem to bend
around the pole.
Table 10.3:TropismsType of Tropism Stimulus
Phototropism light
Gravitropism gravity
Thigmotropism touchSeasonal Changes
Along with detecting differences in light or gravity, plants also are able to detect the seasons.
Leaves change color and drop each autumn in temperate climates (Figure10.33). Certain
flowers, like poinsettias, only bloom during the winter. And in the spring, the winter buds
on the trees break open and the leaves start to grow. How do plants detect time of year?
Although you might detect the change of seasons by the change in temperature, this is not
the primary way by which plants detect the change of seasons. Plants determine the time
of year by the length of the day. Because of the tilt of the Earth, during winter days there
are less hours of light than during summer days. That’s why during the winter it may start
getting dark very early during the evening and even stay dark while you’re getting ready for