PHYSIOLOGY OF THE DEVELOPMENT OF PLANTS 423 



been confirmed by a number of others, showed that in determin- 

 ing the time of blooming very great importance must be attrib- 

 uted to the relative length of day and night. Some plants, e.g., 

 many varieties of soybeans, cotton, beans, asters, and cereals 

 such as millet, sorghum, and rice, bloom earlier, the shorter the 

 days and the longer the nights are. Consequently, under natural 

 conditions these plants will produce flowers only in autumn 

 when the days become shorter. By an artificial decrease of 

 the day, as for instance, if the plants are placed in a dark room 

 during part of the day or are covered with opaque cloth, they 

 may be made to bloom at the beginning of summer. Such 

 plants have been called "short-day" plants (Fig. 133). They 

 mostly are native to southern latitudes where the summer day 

 is comparatively short. There are, however, many plants, e.g., 

 wheat, oats, and other cereals, that will bloom earlier, 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 much earlier. These are called 

 'Uong-day" plants (Fig. 134). 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 over- 

 balances the retarding effect of low temperature. These plants 

 show the highest rate of development under conditions of 

 continuous illumination. 



The response of plants to the duration of night and day is 

 known as ''photoperiodism." It is of interest to note here that 

 the nature of the response of plants to the photoperiod 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 requir- 

 ing a long day. Hence, when transported to the tropics many 

 plants of temperate zones w^ill never bloom. 



The photoperiodic reaction of different varieties of one and 

 the same crop is not always similar. Thus, for instance, the 

 soybeans, as has been indicated, belong to short-day plants. 

 But some of the varieties, e.g., the Biloxi, are considerably 

 retarded in their development by an elongation of the day. 

 Thus with a 16-hr. day as compared with a 12-hr. day, their 

 vegetative period doubles. Other varieties, e.g., the Mandarin, 

 show httle response to such a change and bloom simultaneously. 



