748 MAN'S PHYSICAL WELFARE Ethylene gas causes a downward curling of the leaves of tomato and tobacco plants. Tomato plants are so sensitive to ethylene that they can be used to detect very minute concentrations of this gas. Ethylene gas anaesthetizes plants as well as animals. It also develops the orange color of oranges.
VITAMINS, ENZYMES, AND HORMONES 749 ene chlorhydrin for twenty-four hours reverses the effect of the above substance, stimulating the sprouting of the potatoes. Winter twigs of pear trees can be induced to leaf out by use of ethylene chlorhydrin or glutathione. One can imagine the practical application of such substances in delaying early budding to avoid frosts. STUDY QUESTIONS 1. What are the physiological catalysts? 2. Differentiate between the three types of physiological catalysts as to (a) their sources, and (b) their functions. 3. Why is research work in the field of physiological catalysts so difficult? 4. Why is it stated that vitamins are plant hormones? 5. What are the steps followed in vitamin research? 6. How may the vitamins be classified? 7. What is meant by the "vitamin-B Complex"? 8. Give the sources of the vitamins included in the "vitamin-B Complex." 9. Why were the vitamins referred to by letters of the alphabet rather than by names in the early period of vitamin research? 10. Give the main food sources and the results of a deficiency of vitamins A, B, C, and D. 11. Why is vitamin deficiency so widespread among the American people? 12. Give a few examples of the rapidity with which vitamin treatments remove symptoms due to vitamin deficiencies. 13. Describe Harry Steenbock's original idea. 14. What is the main function of enzymes? 15. Describe some industrial applications of enzymes. 16. Are enzymes produced by plants? 17. Is an enzyme a hormone? Is a vitamin a hormone? 18. Name three important hormones and describe their effects upon the human body. 19. Why is the research work on hormones of so much value to humanity? 20. Name two hormonelike substances which act on plants and describe their effect on plants. 21. What are the six types of responses of plants to chemical stimuli? 22. Predict some of the possibilities that the future may have in store for the control of living organisms by chemicals.