84 PLANT GROWTH 



in 1920, on the effect of the length of day to the growth of the 

 plant into a vegetative condition or a reproductive condition. 

 They shortened the day by putting the plants in the dark for 

 a part of the day, or lengthened it by using a small electric 

 light. Many other workers have studied length of day 

 effects, or photoperiodism, as it is called. Many plants have 

 been studied, and among the many discoveries it has been 

 found that a very weak light, less than thirty-watt bulbs at 

 six-foot centers, inadequate for reading, is sufficient to make 

 the plant respond to long-day conditions. Such a light is less 

 than one-five-thousandth of the intensity required for opti- 

 mum photosynthesis but it is all that is necessary to have the 

 effect on the plant of a lengthened day. Even more striking, 

 the weak light must fall on only a few of the young growing 

 leaves to cause the whole plant to show the response to the 

 increased period of light. 



Long-day plants are those that bloom normally in mid- 

 summer or with light for twelve hours or more, such as 

 hibiscus, radish, or lettuce, while with a short day these 

 plants remain vegetative indefinitely. If radishes are planted 

 in the greenhouse in midwinter they will form good roots but 

 will not flower; however, if an electric light is used to extend 

 the light period by four hours each day they will form flower 

 stalks but very small edible roots. Short-day plants bloom 

 normally with a short day, but remain vegetative with a long 

 day. These plants include many of the spring and fall 

 flowers, such as ragweeds, cosmos, and scarlet sage. The 

 photograph (Plate V) shows the relative growth and flower- 

 ing commonly found with long and short days. Other plants, 

 such as the ever-bearing roses, are indifferent to the length 

 of day and will reproduce under all day lengths. The re- 

 sponse to length of day has a profound influence on the 

 development of plants, showing that the functions of growth 

 and reproduction go on with different sets of conditions, but 

 not at the same time. Plants may grow vegetatively to 



