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PLANT GROWTH AND TROPISMS; 

 CARBON DIOXIDE FIXATION 

 AND TRANSLOCATION OF 

 PLANT SUBSTANCES 



(Readings: Weisz, pp. 253-263. S.P.T., pp. 183-185; 57-63. Villee, pp. 107- 

 113; 126-127. Review Exercise X on "Photosynthesis." G. Wald, "Life and 

 Light," Sci. Am. 201, No. 4, 92-108, Sept. 1959, Reprint No. 61.) 



All organisms respond to stimuli, though not 

 all of them with as swift integration and motions 

 as provided by the neuromuscular systems of 

 higher animals. Plants, for example, from uni- 

 cellular molds to flowering plants, respond to a 

 variety of stimuli with appropriate motions. 



When we plant seeds in the ground, for ex- 

 ample, we pay no attention to how they are 

 oriented, yet the stems always grow upward and 

 the roots downward. Similarly, in any situation 

 in which light comes regularly from one side, 

 plants tend to bend toward the light. 



These responses are obviously highly ad- 

 vantageous, directing the organs of the plant 

 where they can do the most good. Such directed 

 motions in response to directional stimuli are 

 called tropisms. (If the entire organism, rather 

 than one of its parts, moves toward or away from 

 the stimulus, this is sometimes called a taxis.) In 

 the case of growing upward or downward, the 

 force is gravity, and the direction is the center of 



the earth. We speak of such responses as geo- 

 tropisms, and distinguish the directions toward 

 and away from the center of the earth as positive 

 and negative. So one describes the growing 

 downward of roots as positive geotropism, the 

 growing upward of shoots as negative geotrop- 

 ism. Similarly, bending toward the light is posi- 

 tive phototropism, whereas bending away from 

 the light would have been called negative photo- 

 tropism. 



Since they lack contractile tissues, plants per- 

 form these motions by differential growth. Light, 

 for example, inhibits the axial growth of shoots. 

 Hence the side toward the light grows more 

 slowly than the shaded side, with the result that 

 the shoot bends toward the light. Some of the 

 lower invertebrates that are attached as are 

 plants exhibit similar tropisms. The hydroid 

 Eudendrium, for example, a coelenterate, bends 

 toward light by differential growth, just as does 

 a plant. 



109 



