420 PLANT PHOTOPERIODISM 



associated with nucleic acid synthesis. The nucleic acid associated with 

 photoperiodic induction — if indeed there be one — must be a special 

 and specific nucleic acid. We know indeed that there are no gross 

 changes in overall nucleic acid content in Xanthium during photo- 

 periodic induction (Lockhart, 1955). We have seen that it is unlikely 

 that nucleic acid synthesis is specifically associated with floral hormone 

 synthesis in the leaves of Xanthium, although the participation of 

 nucleic acid in the transformation process of the bud has not been 

 excluded. Let us therefore consider for a moment a second kind of 

 induction, that associated with vernalization. In the case of vernaliza- 

 tion, as in photoperiodic induction, a particular treatment is applied 

 to the plant which then "remembers" that it has once had this treat- 

 ment and flowers forever afterward in response to it. In the case of 

 vernalization the cold treatment, which is the effective environmental 

 factor, must be perceived by the growing point itself. Aach and 

 Melchers have shown (1957), in the case of vernalization, that just 

 as in the case of photoperiodic induction, no new macromolecular 

 entities can be detected even by delicate serological techniques during 

 the course of the vernalization treatment. Inhibitors of nucleic acid 

 synthesis similarly do not inhibit the progress of vernalization in cold- 

 treated buds. There is nonetheless some basis for the supposition that 

 the metabolism of compounds intimately associated with the nucleic 

 acids may be related to vernalization. Highkin (1955) has shown, for 

 example, that the effects of cold treatment on vernalizable pea seeds 

 may be in part replaced by the nucleoside, guanosine. We have no 

 quantitative estimate of the fate of the guanosine which is effective in 

 this chemical vernalization. Although it may participate in the synthe- 

 sis of a specific and unique type of nucleic acid, it has not been possi- 

 ble to show that this is the case. We are almost as much in the dark 

 with regard to the chemical events of transformation in the vernalized 

 growing point as we are in the case of transformation in the photo- 

 periodically induced growing point. Clearly we are in need of new 

 approaches, new techniques, new thoughts. And it is the hope of this 

 reviewer that the thoughts and experiments outlined above may aid us 

 in a final coming to grips with the chemical processes of the inductive 

 process. 



