PROBLEM 4. How Plants Respond to Stivmli 



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Fig. 2«2 t'ruiged gentians iji the daytivie (left) and at night (right). What might be 

 the stiiutdus that causes this response? (gehr) 



muscles which contract. These bend 

 your arm and pull it away. Secondly, 

 even though the receptors are far from 

 the effectors, the response is fast because 

 impulses travel with great speed in the 

 nerves. Plants have no special receptor 

 cells; thev have no nerves: and they have 

 no muscles. It is not surprising that their 

 responses are slow. How is it that their 

 parts move or bend at all without muscle 

 tissue? 



Unequal growth produces bending. 

 Most bending and turning in plants is 

 caused by unequal s^rowth. If cells on 

 one side of an upright stem grow faster 

 than those on the opposite side the stem 

 must bend in the direction where there 

 is less growth. It has been found that 

 young leaves and stem tips form a 

 "growth" hormone (auxin — awk'sin) in 

 the presence of light. This is transported 

 downward from the leaf or the stem tips 

 and gathers mostly on the shaded side of 



the stem. Whether the hormone causes 

 changes in the cell wall or the proto- 

 plasm is not known. It is known that in 

 certain concentrations it causes cell en- 

 largement and, since it gathers on the 

 shaded side of the stem, the cells there 

 become enlarged, bending the stem to- 

 ward the light. Bending occurs in the 

 growing region of a stem, the portion 

 just back of the tip. Would you be in- 

 terested in working with growth hor- 

 mones? See Exercise 6. 



Gravity also has an effect on the dis- 

 tribution of auxin. In a horizontal stem 

 gravity causes auxin to gather mostly on 

 the lower side. Because of this the tip of 

 a horizontal stem will bend upward. See 

 Figure 281. In horizontal roots auxin 

 also gathers on the lower side. In roots, 

 however, the hormone collects in con- 

 centrations high enough to slow up or 

 inhibit the enlargement of cells. When 

 the lower cells of a horizontal root grow 



