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IS THE PLANT A RUBBERNECK? 



Like other species of plants, the sunflower turns its head toward the sun. The leaves 

 also move, facing east in the morning, south at noon, and west late in the afternoon. 

 In no plants, however, can we find anything to correspond to the muscles by means 

 of which we turn our heads to face now in one direction, now in another 



Fitness The tendency of the root to grow downward will, on the 

 whole, bring the roots of plants into the soil, where the conditions for get- 

 ting water are more favorable. The responses of the shoot to gravity and 

 light are likely, in the long run, to bring the plant into situations favorable 

 to its further development. But it does not follow that everything a plant 

 does is to its advantage. Nor is it clear just how the plant brings about 

 these adaptive movements. 



How Do Plants Bring About Their Adaptive Movements? 



Light and Growth' We know very well that growth depends upon 

 food. We have also learned (see page 138) that green plants make their 

 food in daylight. Yet we can easily establish the fact that seedlings and 

 other plant structures grow more rapidly in the dark than in the light. It 

 is not so easy to show how darkness, which is a negative condition, or an 



iSee No. 1, p. 270. 

 255 



