24 



GROWTH 



are short, and will not flower when the days are short and the 

 nights are long, but continue to increase in length of stem and 

 number of leaves. Others blossom only when the days are short. 

 Tobacco, at least certain varieties, is such a plant. The plants 

 shown in Figure 14 were grown in a greenhouse in the winter 

 when the days are short. The plant on the right was also ex- 

 posed to electric light from sunset to midnight. This plant has 



been grown under such long 

 days to a height of over fifteen 

 feet without the production of 

 flowers. The importance of 

 ultra-violet light in the growth 

 of animals is now well known. 

 In the absence of ultra-violet 

 light the pathological condi- 

 tion known as rickets develops. 

 Cod-liver oil or similar mate- 

 rial containing the necessary 

 vitamin may be substituted 

 for it. 



Food sufficient in both quan- 

 tity and quality is needed. 

 The term food is used here to 

 refer to the carbohydrates, fats, and proteins. The quantity 

 of food needed is usually measured in calories. A three- 

 year-old child needs food equivalent to about IOOO large 

 calories a day, a fourteen-year-old child needs 1800, a full- 

 grown man, 3000-3500 calories. These calories may be sup- 

 plied by any one of the three foods mentioned above or by a 

 mixture of them. In addition, however, to the quantity of food 

 attention must be paid to the quality. Green plants make their 

 own food materials from the elementary materials, carbon di- 

 oxide, water, and inorganic nitrogen, for example, nitrates or 

 ammonium salts. Animals, however, are unable to do this and 

 must be supplied with both carbon and nitrogen in organic com- 



Figure 14. Both these tobacco plants were 

 grown in a greenhouse during the winter. 

 The plant on the left was exposed to the 

 natural winter daylight. The plant on the 

 right received in addition to daylight, elec- 

 tric light from sunset to midnight. This va- 

 riety exposed to long days will grow 15 feet 

 or more in height. After Garner and Al- 

 lard. Courtesy of the United States Depart- 

 ment of Agriculture. 



