48 



However, in all the experiments where the final result has been 

 favorable to the action of light T have convinced myself that a cloudy 

 sky for twelve hours always showed itself in the amount of the 

 absorption of oxygen in such a manner that the examination of these 

 figures, noted day by day, would almost serve to show the state of 

 the atmosphere during the day which preceded the observation. A 

 very conclusive instance of this action is given us by experiment 

 No. 4 of the second series, in which the state of the sky being care- 

 fully observed it showed very marked changes. 



(8) The accelerating influence exercised upon seeds exposed to the 

 action of light during the day did not stop at night; it continued to 

 act in the dark with an equal, sometimes even with a greater intensity. 

 I will cite as examples experiments Nos. 3, 4, 6, 7, and 8, wdien obser- 

 vations made twice a day, morning and evening, allowed of examin- 

 ing the fact I state. How can we explain this persistent action of 

 light? One hypothesis only can be admitted. A portion of the 

 action of the light absorbed by the grain during the day is stored up 

 by it and used by it at night to accelerate its respiration. The i)roof 

 of this is that the diiferences of elevation [or quantities of absorbed 

 oxygen] shown in the morning by the instruments for seeds kept in 

 the dark are always below those shown by the instruments and plants 

 in the light. The influence of the light, then, continues for a certain 

 time, at least several hours, even after the light itself has ceased to 

 act; on the other hand, however, this action is not exerted immedi- 

 ately. There is one other phenomenon that we have demonstrated 

 by our experiments. Suppose the sky to be very clear; the differ- 

 ences in favor of light are only apparent after two or three days 

 and become much more marked toward the end of the experiment; 

 that is to say, in proportion as the daily action of sunlight is more 

 and more frequently repeated. 



(4) I should also call attention to still another peculiarity, viz, 

 that the difl'erences in the quantities of oxygen absorbed in the dark 

 and in the light were generally much greater at the beginning of 

 these researches than in the later experiments, and particularly in 

 those of the second series. The temperature appears to me to be the 

 only element that varied in these experiments. There must there- 

 fore be a more intense respiratory action exercised by light at low 

 temperatures, and this influence would become weakened at high tem- 

 peratures. This fact would be in entire agreement with the demands 

 of physiology. It is easy of comprehension that a scarcity of heat 

 should be counterbalanced by the action of light, which furnishes 

 for the reaction of the respiratory organs the force that they could 

 not obtain from an insufficient temperature. On the contrary, when 

 the heat is intense the intervention of the light is no longer neces- 

 sary, the first cause bcMiig suffirieut to excite the process of germina- 

 tion in the protoplasm of the seeds. 



(5) This action of light seems to difi'er a little accordmg as it acts 

 upon seeds containing albumen or those without albumen. In the 

 case of the albuminous seeds of the castor-oil plant the advantage was 

 much more apparent in favor of those exposed to the light, which 

 advantage appeared to me much less decided for the seeds without 

 albumen, such as the haricot bean. Nevertheless, as the experiments 

 were not invariable in their results, the cause of the variations ob- 



