Robert B. Wi throw 395 



cules which may then be oxidized in the presence of molecular oxygen. 

 Several amino acids and indoleacetic acid have been shown by Galston 

 to be readily photooxidized by riboflavin. One interesting feature about 

 such a sensitized photooxidation is that the photosensitizing pigment 

 is not itself rapidly destroyed in the process. It merely absorbs a quan- 

 tum of light, becomes activated with excess energy, and transfers the 

 energy to a neighboring molecule as activation energy or re-emits the 

 energy as fluorescence, depending upon the opportunity for suitable 

 collision. The activated neighboring molecule then uses the energy for 

 reaction with molecular oxygen or degrades it to thermal energy. It is 

 possible that riboflavin and other fluorescing pigments may carry out a 

 wide variety of photochemical oxidations which may account for many 

 of the rather obscure effects of light on plant cells such as light-modified 

 permeability, viscosity, and protoplasmic streaming. The recent work 

 of Goodwin and Kavanagh (iy) on fluorescing substances in roots in- 

 dicates that plant cells may contain a number of fluorescent substances 

 capable of entering into such photooxidation systems. 



It is impossible at the present time to evaluate the over-all physiologi- 

 cal significance of light effects on permeability, photooxidations, and 

 other such processes. Further investigations may show that these play 

 a more significant part in the metabolic activities of the shoot than is 

 now assigned to them. As far as roots growing in soil are concerned, 

 however, these phenomena are obviously unimportant as directly con- 

 trolling processes but they may exercise definite indirect effects. 



PHOTOPERIODISM 



Photoperiodism involves photochemical and thermal reactions initi- 

 ated in the leaf which through the translocation of one or more hor- 

 mones control the course of development of the terminal shoot meri- 

 stems and in turn markedly influence the rate of shoot growth in many 

 species. Higher plants fall into four classes as regards their flowering 

 response to photoperiod on normal daily cycles: (a) long-day plants 

 which flower on photoperiods longer than a certain critical day length, 

 but remain vegetative on shorter photoperiods; (b) short-day plants 

 which flower on photoperiods shorter than a certain critical clay length; 

 (c) intermediate plants which flower only on photoperiods within a 



