WILLIAM S. HILLMAN 685 



ous kinds (2, 16. 17). It is probably in this manner, rather than 

 through action in the external medium, that EDTA is acting in the 

 systems in question. On the other hand, the pH-activity relationship, 

 consistent with a chelation phenomenon, is the opposite of what 

 Avould be expected for the uptake of a weakly dissociated acid; it 

 might, of comse, be due to an effect of external pH on the internal 

 pH of the tissues involved, and thus on internal chelation. 



The effects on photoperiodic response might be due to an action 

 on iron metabolism or internal iron availability. The work of Smith, 

 Mcllrath, and Bogorad (15), at least part of which is strongly con- 

 firmed by results such as those in Table 1, suggest that iron is in- 

 timately involved in photoperiodic induction. One of the first steps 

 in the process might be a reaction of the receptor pigment itself 

 or some closely associated compound with iron or an iron complex. 

 This sort of reasoning, based only on the results presented here, might 

 apply to other metals as well. Iron seems most likely simply because 

 of the results quoted, because of its wide biological role, and its 

 relatively high affinity for most chelating agents. The suggestion is 

 no more than sheer speculation at present, but indicates a direction 

 in ^s'hich evidence might be sought. 



One should recall that EDTA in the presence of iron can be de- 

 composed by daylight (2, 5) . It is questionable whether this reaction 

 bears any relationship to the effects studied here, since, at least in 

 L. perpusilln, very low energies of red light are sufficient to control 

 the EDTA-induced short-day behavior (7) . 



In conclusion, a few general remarks may be made on the use of 

 aseptically cultured species of Lemnaceae in studies of photoperiodism. 

 The ease of controlling so many factors, which is an advantage offered 

 by these plants, also in a sense raises difficulties. Where factors can 

 be controlled, they must be controlled, and studied. A consideration 

 of the ^vork covered here suggests innumerable questions as to the 

 nature and relationships of various nutritional, medium, light, and 

 temperature effects, most of which have not even been discussed. A 

 great number of experiments will be required before the responses of 

 even the two plants mentioned are completely described, and the 

 foundations for more intensive biochemical investigations have been 

 laid. 



In addition, there are other useful plants in this group. Preliminary 

 experiments with a strain of L. paucicostata recently raised from seeds 

 sent by Professor P. Maheshwari indicate a short-day response differ- 

 ing from that of L. perpusilla 6746 in its relation to both chelating 



