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cereals, which will blossom sooner, the longer the day and the 

 shorter the night. By illuminating them with electric light during 

 a part of the night or all night long it is possible to make them bloom 

 in midwinter. These are called "long-day" plants. Plants of this 

 type have a shorter vegetative period in the northern latitudes, 

 where the days in summer are very long and where the length of 

 day overbalances the retarding effect of low temperature. These 

 plants show the highest rate of development under conditions of 

 continuous illumination. 



The effect produced on plants by the relationship existing 

 between the duration of night and day is known as " photoperiod- 



Fig. 145. — Tobacco plant, having grown the same length of time with (1) a "short" 

 day, and (2) a "long" day {redrawn after Garner and Allard). 



ism." It is of interest to note here that the nature of the plant's 

 response to the length of day is to a considerable degree dependent 

 on the origin of the plant. In the majority of cases, tropical 

 plants are short-day plants, while those of the temperate zone 

 belong to the group of plants requiring a long day. Hence, when 

 transported to the tropics many of our plants will never blossom. 

 The length of day is not only important in determining the time 

 of flowering, it has also a marked effect on the development of 

 vegetative organs. Thus, for instance, long-day plants will be 

 conspicuously vegetative if the day is shortened to 12 hr. The 

 plants will develop a much larger leaf surface and, in general, will 



