Factors Affecting Growth and Reproduction 205 



Exposure to full sunlight increases the rate of transpiration to a 

 point where the water content of the plant tissues is reduced 

 below that necessary for the greatest amount of growth. Vege- 

 tative organs, in general, do not require intense illumination for 

 their greatest development, because only a small fraction of the 

 sunHght is used in photosynthesis, and their growth is favored 

 by moisture both in the soil and the air. Just how much the sun- 

 light should be reduced to promote growth varies with different 

 plants and in different geographic regions. A reduction of 20 

 to 50 per cent is favorable to vegetative growth in many plants. 

 In moist regions, as for example along northern coasts, the 



Short day 



Long day 



Fig. 119. Effects of length of day on tobacco plant. Both plants were grown in a green- 

 house during the winter. The plant at the right received, in addition to daylight, electric 

 light from sunset to midnight, while the plant at the left received the natural daylight only. 

 This is a typical short-day plant. When exposed to long days, this variety will grow 15 feet 

 or more in height and produce upward of 100 leaves. (Garner and A Hard, u.s.d.a.) 



