any inducing treatment, when leaves from a 

 plant that has received such treatment are 

 transplanted to it. Moreover, the spreading 

 of induction follows along the sieve tube ele- 

 ments of the phloem. 



The phytochrome system is important not 

 only in relation to the photoperiodic re- 

 sponses of the plant, but also in relation to 

 other responses. Certain seeds, for example, 

 will not germinate unless first exposed to 

 light, particularly red light, and this light- 

 induced activation can be abolished by a 

 subsequent illumination with infrared light. 

 Moreover, red-infrared reversals of growth 

 rate and curvature in developing plant 

 embryos have been observed. It seems prob- 

 able, therefore, that the phytochrome system 

 triggers most of the photic responses of 

 plants, including positive and negative pho- 

 totropic growth movements. 



Practical Importance of Auxins and Other 

 Growth Substances. Auxins are now being 

 used to initiate the development of roots in 

 plants and now many commercially valuable 

 plants are being grown from auxin-treated 

 cuttings. Promising results are also being 

 obtained in other directions. Auxins are used 



The Responses of Higher Planfs - 267 



to foster the "talcing" of grafts, and auxin- 

 treated seeds (in certain species) germinate 

 more successfully than untreated ones. With 

 sugar beets, for example, the yield has been 

 increased by about 60 percent, although this 

 effect is probably due to a stimulation of the 

 growth of the seedlings rather than to a 

 direct action on germination. Auxins may 

 also prove to be important in the develop- 

 ment of fruits. In most species, the fruit fails 

 to develop properly unless pollination has 

 occurred; but in the tomato at least, the ap- 

 plication of auxins has yielded well-developed 

 seedless fruits from unpollinated flowers. 



Another economically important use of 

 auxin preparations is to prevent the prema- 

 ture dropping of fruit in apple and pear 

 orchards; and more recently, a new synthetic 

 auxinlike compound (2,4-dichlorophenoxy- 

 acetic acid, or 2,4-D) has been employed 

 quite widely as a "weed killer" on lawns and 

 golf courses. In proper concentrations, appar- 

 ently, 2,4-D has litle effect upon the grasses, 

 but it exerts a lethal action on broad-leaved 

 plants (for example dandelions) by over- 

 stimulating oxidative metabolism in leaves 

 contacted by the spray. 



TEST QUESTIONS 



1. Explain two essential differences between 

 growth movements and turgor movements. 



2. Analyze the mechanical response of the sensi- 

 tive plant in terms of the receptors, conduc- 

 tors, and effectors of the system. Why is this 

 movement called a turgor movement? 



3. Describe and explain the experiments that 

 first demonstrated the existence of a growth 

 substance, or auxin. 



4. How do roots and stems differ in regard to 

 their responses to the auxins? 



5. A. What is a tropism? 



B. Describe precisely how the behavior of 

 auxin helps to explain: (a) the positive pho- 

 totropisms of stems; (b) the negative photo- 

 tropisms of roots; (c) the positive geotropisms 

 of roots; (d) the negative geotropisms of 

 stems; (e) other tropisms. 



6. Specify four practical uses of auxins and re- 

 lated compounds. In each case explain the 

 basic action. 



7. Identify and briefly discuss: (a) gibberellins; 

 (b) coconut milk factors; (c) kinetin; (d) un- 

 saturated lactones. 



8. Define the terms: (a) photoperiodism; (b) 

 short-day plants; (c) indeterminate plants. 



9. Summarize the evidence that indicates that 

 photoperiodism is controlled by a reversible 

 photochemical reaction. 



10. Identify phytochromes P660 and P730. 



11. Explain how the phytochromes are affected 

 by: (a) white light; (b) red light; (c) infrared 

 light; (d) darkness. 



12. Explain how short-day and long-day plants 

 differ in relation to the operation of the 

 phytochrome system. 



