38 ELEMENTARY BIOLOGY 



some external force is called a tropism, from a Greek word mean- 

 ing " to turn." The response to gravity is called geotropism, 

 or " earth-turning." We may distinguish the behavior of the 

 root and the shoot by calling the former positive geotropism, 

 and the turning away from the earth, negative geotropism. 



56. How the plant moves. The plant has no muscles, nor 

 any structures that may be compared to muscles. The turning 

 of the root or of the stem is not the same kind of movement 

 as that which takes place when you turn your head or bend 

 your body. The curvature is brought about by a growth. The 

 shoot or the root grows more rapidly on one side, or the growth 

 is stopped on one side, so that it grows in a curved line. 



57. Light. That plants are sensitive to light is well known 

 to all who have had an opportunity to observe either house 

 plants or garden plants. A careful measurement of the growth 

 of plants left in the dark, and of similar plants exposed to 

 daylight, shows very definitely that withholding light from a 

 plant accelerates its growth. But since darkness is a purely 

 negative condition, it would seem that light actually restrains 

 the plant's growth. This is so different from what we com- 

 monly believe, that it is worth studying more closely. 



58. Phototropism. Another response of plants to difference 

 of illumination is shown when we leave them exposed to a 

 one-sided illumination. Such an experiment will convince us 

 that a plant is sensitive to light just as it is to gravity. 



The turning of a plant axis in accordance with the direction 

 of the illumination is called phototropism. Most of our common 

 plants are positively phototropic in the shoot, and somewhat 

 negatively phototropic in the root. 



59. The influence of water. The turning of leaves and 

 stems toward the light and the turning of roots and stems 

 according to the direction of the " earth's pull " are evidences 

 of the living organism's irritability. It has been shown that 

 the plant is also sensitive to various chemicals, and we can 

 determine for ourselves that it responds to water. 



