Factors Affecting Growth and Reproduction 209 



The plants with a restricted period of sunHght grow to a height of 

 4 to 5 feet ; the other plants grow to be 7 or 8 feet tall. Evi- 

 dently long days favor vegetative growth in this plant ; short 

 days favor reproduction. 



Under similar conditions radishes respond very differently. 

 They continue to develop a thickened root throughout the grow- 

 ing season and do not form flowers when the daily period of 

 illumination is shortened to 7 hours. With twice that amount 

 of sunlight these plants bloom in about i month. 



These two plants are each typical of many species whose 

 vegetative development and reproduction are determined by the 

 length of day. It is also probable that the length of day is the 

 important factor that makes the beet a biennial in the latitude of 

 Kansas and an annual in the latitude of Alaska. 



Quality of light. The quality of light is also an important 

 factor in growth. You have seen that when a beam of light is 

 separated into its constituent rays, as in a rainbow, they form a 

 series of colors running in order through red, orange, yellow, 

 green, blue, indigo, and violet. The red rays have the longest 

 wave lengths, while the violet rays have the shortest wave lengths. 

 Ultra-violet light has still shorter wave lengths. Under natural 

 conditions the longer light waves of the red end of the spectrum 

 are most important in photosynthesis. The shorter wave 

 lengths of the violet and ultra-violet rays are most important in 

 inhibiting vegetative growth. Ultra-violet rays are sometimes 

 used to kill bacteria, and they no doubt have similar dele- 

 terious effects upon the protoplasm of green plants. They 

 are rapidly absorbed by the atmosphere and by clouds, and 

 this probably has something to do with the difference of vege- 

 tative growth at low and high attitudes and in clear and foggy 

 climates. 



Indirect effects of light. The various effects of light upon 

 growth and reproduction are a result of the physical and chemical 

 effects of light upon the numerous physiological processes in the 



