PHYSIOLOGY OF THE DEVELOPMENT OF PLANTS 425 



The same differences may be observed with long-day plants, for 

 example, with wheat. This proves that these differences an; 

 related to the geographical origin of the varieties. Northern 

 wheats prolong very significantly their development when 

 cultivated in southern latitudes with a shorter day, while certain 

 other wheat varieties are indifferent to a shortening of the day. 



The phenomenon of photoperiodism is of great significance in 

 the distribution of varieties of crop plants in different localities 

 and in moving them from one region to another. Long-day 

 plants such as wheat are not hindered in their development when 

 moved to the north, for the prolonged day of northern latitudes 

 does not impede, but accelerates, their development. This 

 makes the culture of such plants easier in northern latitudes and 

 to a considerable degree compensates for the influence of the 

 short northern summer. It is more difficult to move the short- 

 day plants toward the north ; for not only is the summer shorter 

 there, but the long day increases their vegetative period. In 

 such crops, varieties must be chosen for the northern latitudes 

 that react as little as possible to an increase in the length of the 

 day, i.e., that have a lower photoperiodic response. Such 

 knowledge of the photoperiodic reaction of separate varieties is 

 very important. 



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

 of blooming but it has a marked effect also on the development of 

 vegetative organs. The curtailment of vegetative growth by 

 the photoperiod is known as "photoperiodic inhibition" (Mur- 

 neek). Short-day plants, for instance, will develop a much 

 larger leaf surface and in general have more vigorous vegetative 

 organs if the day is prolonged. When cultivated in northern 

 latitudes, they do not mature; but since they produce a large mass 

 of leaves, they may be used for hay and silage. This vigorous 

 development of the leaves is a result of the fact that when bloom- 

 ing is retarded, the organic substances elaborated are directed 

 toward the formation of new leaves, though in many cases photo- 

 periodic inhibition is brought about directly by the length of day 

 (Murneek). For the same reason, long-day plants develop a 

 larger leaf surface and a larger total green weight with a short 

 day, although the shortening of the day by decreasing the num- 

 ber of hours for the photosynthesis should reduce the accumula- 

 tion of organic substances. With an excessively short day, e.g., 



