PIIOTOPERIODISM 705 



though the range of variation of these two fractions depends upon the 

 plant species. Salvia was prevented from fiowering by supplementary 

 night lighting which caused very little change in either the carbohydrate 

 or nitrogen fractions. Even in tomato there appears to be little relation 

 between the C/N ratio and the setting of fruit. In general, the per- 

 centage of carbohydrate increases and that of nitrogen decreases with 

 increase in length of day, whereas flowering is initiated by a long or a 

 short day, depending upon the species or variety. 



As pointed out by Redington (60) and by Arthur and associates, in 

 seeking an explanation of the action of day length on flowering it is 

 necessary to take into consideration the fact that the photoperiodic 

 response can be definitely localized in the plant. It has been shown by 

 Garner and Allard (24) that with suitable technique flowering in response 

 to a short day may be confined to certain branches or stems of the plant 

 or in particular regions of the individual stem. Knott (37) has presented 

 evidence to indicate that this effect can be sharply limited to the apex 

 of the stem. Rasumov (59) also has confirmed and extended the results 

 relating to localized action of day length. He finds that there is no 

 specific plant organ for receiving the photoperiodic stimulation, which is 

 more readily transmitted downward than upward in the plant. The 

 underground tuber-forming portion of the plant receives the mutually 

 opposed influences of parts receiving a long and a short day, and tuber 

 formation takes place or is suppressed depending on relative intensity 

 of the two influences. Redington suggests that the action of the light 

 on the differentiating tissue must be a direct one and of a photocatalytic 

 nature. Arthur and associates also conclude that the effect of light 

 in initiating flowering may be directly upon the protoplasm of the cells 

 at the growing point without perceptible change in chemical composition. 



The mineral nutrition of the plant in relation to the length-of-day 

 effect thus far has not received much attention. Borodin (10) has 

 reported observations on the influences of variation in the supply of 

 essential nutrients on photoperiodic response in the long-day plant, 

 Mongolian barley, and in the short-day plant, Saratov millet. At a very 

 early stage of development of water cultures of these plants the three 

 elements, nitrogen, phosphorus, and potassium were singly withdrawn 

 in parallel series. Separate groups of these cultures and the controls 

 growing in a complete nutrient solution were exposed to day lengths of 

 18, 12, and 9 hr. The C/N ratios in the plants were determined by 

 analysis at the tillering, shooting, and earing stages. In the long-day 

 plant, barley, nitrogen deficiency increased the C/N ratio and accelerated 

 initiation of the earing stage while phosphorus deficiency produced the 

 reverse effect. Potassium deficiency delayed the earing stage under 

 long-day conditions and the plants perished before reaching the earing 

 stage in the short day. Nitrogen hunger induced ear formation in the 



