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Tropism- The growth of a plant in a certain direction in response to a stimulus [Example: Gravity, Sunlight or random
suface.]
Phototropism is the response of plants to light. Positive phototropism, growth towards light, enables many growing embryos
and new growth of mature plants to absorb the sunlight they needed for photosynthesis.
A second type of tropism, gravitropism, is growth in response to gravity. Roots are positively gravitropic, meaning that
the root tips grow downward. Gravitropism is adaptive becuase roots grow where they are more likely to get water and nutrients.
New growth on plants stems is negativly gravitrophic, meaning that the tips grow upward towards the light source.
A third type of growth response is called thigmotropism, or response to touch. An example of thigmotropism is the coiling
of the stems of a plant such as the morning glory. Thigmotropism enables a plant to grow on a surface that can support
it. Wherever the stem touches the support, the cells on the opposite side grow longer, causing the stem to coil around,
or grasp hold of, the support.
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