706 BIOLOGICAL EFFECTS OF RADIATION 



plants exposed to a 9-hour day although barley normally does not attain 

 the earing stage when exposed to a short day. However, it was found 

 that the C/N ratio fluctuates widely at the beginning of the earing stage 

 under varying nutrition conditions and, in agreement with Arthur and 

 associates, the author concludes that this ratio is not the immediate 

 cause of the change from the vegetative to the reproductive stage, 

 though it may be regarded as an attendant factor. As regards the 

 short-day plant, millet, lack of nitrogen and potassium affected develop- 

 ment as a whole and did not influence the throwing up of the panicle. 

 Only lack of phosphorus, which strongly depressed growth, delayed 

 appearance of the panicle. The conclusion drawn by Borodin would 

 seem to approximately represent the present status of the problem of 

 interrelation of the C/N ratio and the phenomenon of photoperiodism. 



Tincker (74) has made a study of the influence of length of day in 

 combination with variation in the supply of potassium upon the rate 

 of accumulation of carbohydrates in the storage organs of Phaseolus 

 multifiorus, Dahlia, and Stachys tuherifera. The rate of accumulation of 

 these products in the roots and tubers was governed by the light period. 

 Replacement of potassium by sodium was without much visible effect 

 upon the rate of tuber formation, but in a short day lack of potassium 

 caused less dry matter to pass to the underground structures. In all 

 cases potassium accumulated gradually in the tubers. 



In a study of hydrogen ion concentration of the cell sap in relation 

 to the photoperiodic response Garner, Bacon, and Allard (28) showed that 

 transfer of the short-day type of plant from a long day, in which the plant 

 is vigorously vegetative, to a short day causes a sharp though temporary 

 rise in the pH value which usually occurs after 3 to 5 days and is believed 

 to indicate definite transition from the vegetative to the flowering 

 condition. Acidity relations in long-day plants, as represented by 

 Rudbeckia, when exposed to a long day are more or less similar to those 

 found in short-day plants exposed to a short day. Similarly Knott (37) 

 considers that transition from the vegetative to the reproductive stage 

 is associated with decrease in catalase activity in the embryonic tissue 

 involved. In a number of plant species studied Lubimenko and Szeglova 

 (42) conclude that usually the chlorophyll content attains a maximum 

 in an intermediate day length and decreases in either a longer or a shorter 

 day. Murneck (50) found the same relative quantities of the chlorophylls 

 a and b in the leaves of soy beans, cosmos, and salvia exposed to short 

 and long days, and consequently in the reproductive and vegetative 

 stages, respectively, but there was an increase in xanthophyll and 

 carotin under the short-day conditions. 



LENGTH OF DAY AS AN ECOLOGICAL FACTOR 



The results of the extensive experimentation which has been carried 

 out on the numerous ways in which the plant may be affected by relative 



