452 PLANT PHOTOPERIODISM 



certain critical day length, and those which flower in days shorter 

 than a certain critical day length, is difficult to reconcile on the basis 

 of any simple mechanism. One of the principal kinetic problems 

 presented by plant photoperiodism is the apparent absence of a 

 sufficient number of parameters with which to explain the behavior of 

 these classes of plants. 



The chromosome response indicated as the last type of reaction 

 induced by the red, far-red system is the one which we have been 

 investigating at the Smithsonian (Withrow and Moh, 1957; Moh and 

 Withrow, 1958.) This work shows that the so-called potentiating 

 effect of infrared on chromosome damage in Tradescantia microspores 

 and the root tips of Vicia jaba actually involves a photoreaction that 

 is hmited to the far red in the range of 720 to 780 m/^, peaking at 

 approximately 760 nifi, and that the red is capable of reversing the 

 far-red induction. An interesting feature here is that this is a photo- 

 reaction in which a chromosome response is elicited. The nucleus of 

 the cell is one of the most refractory sites, and the chromosomes are 

 seldom directly affected by external agents, at least not to the degree 

 that the cytoplasmic constituents are influenced. 



Living organisms have developed a remarkable capacity for respond- 

 ing to a tremendous range of light intensity. This range is graphically 

 presented in Fig. 6 on a logarithmic scale from 10^ to 10~^ /^w/cm-, 

 a range of 10^^' in intensity. In foot-candles, the range is roughly 10^ 

 to 10 '^ ft-c. The end of twilight occurs at about 0.4 ft-c, which is just 

 slightly above the lowest limit of flower bud induction which has been 

 shown to be in the range of 0.01 to 0.1 ft-c (Withrow and Biebel, 

 1936). Full moonlight has a maximum value of approximately 0.02 

 ft-c, which is well within the range inducing flower bud formation in 

 the more sensitive plants. The limit of cone vision is an order of 

 magnitude lower. The limit of rod vision for the dark-adapted eye is 

 10~^ jnw/cm^, but the bean or the oat seedling is still capable of 

 "seeing" at three orders of magnitude lower. On an energy basis, of 

 course, the situation is reversed between the dark-adapted eye and the 

 seedling. The seedling "sees" these low intensities as a result of an 

 accumulative photochemical process occurring in the cells in which 

 10' to 10^ sec are required. The eye, on the other hand, is able to 

 see its threshold energy in less than 1 sec, so that from the standpoint 



