JOHN BUCK 329 



or, assuming O2 to be the controlling factor, the tracheation of the 

 organ (cf. Heinemann, 1886). 



Dr. Nicol's passing reference to the possibility of inhibiting lumi- 

 nescence by limiting oxygen invites amplification. The case has in one 

 sense been strengthened by the recent evidence that oxygen actually 

 participates in the reaction in which the activated molecule emits the 

 light — formerly it was possible to imagine a more subordinate role 

 analogous to that in hexose resynthesis in muscle. As an essential 

 reactant in light production in almost all organisms, oxygen is of 

 course always a potential trigger at the chemical level, but the pos- 

 sibility of its actually ever normally being limiting is strongly reduced 

 by two circumstances. First, no organism, except possibly the firefly 

 (see below), seems to have any anatomical arrangement even con- 

 ceivably capable of rapidly affecting the oxygen supply to the lu- 

 minous tissues. Second, in all systems thus far investigated (which 

 includes the firefly) luminescence persists at oxygen partial pressures 

 far lower than will support any significant amount of respiration. 

 Dark periods, which are often of great length, would thus involve 

 almost complete tissue anaerobiosis. 



Since oxygen control has often been postulated for the firefly, it 

 may be worth while to examine the evidence thought to favor this 

 view. Oxygen control has been strongly espoused by Snell (1932) 

 and Alexander (1943), but in a critical review of the work (Buck, 

 1948) I concluded that no evidence had been presented which could 

 not be interpreted as an indirect eflfect via the nervous system, rather 

 than a direct limitation of oxygen in the photochemical reaction. 

 Aside from the obvious dependence of light production on the pres- 

 ence of oxygen, therefore, we have only the characteristically profuse 

 tracheation of the light organ, which may of course be concerned 

 with supplying some oxidative precursor or restoration reaction rather 

 than with control, and the circumstantial but close correlation between 

 the ability of certain species to produce sharp flashes and the pres- 

 ence in those species (only) of "tracheal end cells" at the junction 

 of the supply tracheae with the fine tracheal capillaries (tracheoles) 

 which penetrate the photogenic tissue. From the standpoint of pure 

 plumbing the end cells are indeed strategically situated to retard 



