chap, ii Photosynthesis 289 



aquatic plants not only absorb the free carbon dioxide 

 which is in solution in the water, but obtain a further 

 supply from dissolved carbonates and bicarbonates. The 

 observation was made almost simultaneously by Hassak 

 and was confirmed by Darwin and Pertz in 1896. 



The exhalation of oxygen accompanying or following 

 the appropriation of carbon dioxide was not made the sub- 

 ject of such close inquiry. The chief experiments made 

 were directed to establishing satisfactory tests for the 

 presence of the gas. In 1879 Hoppe-Seyler introduced 

 haemoglobin as a reagent. This substance" absorbs even 

 traces of free oxygen with great avidity and presents dis- 

 tinctive spectra in its presence and its absence. In 1890 a 

 further test was devised by Beijerinck, depending on the 

 ready change of colour produced by traces of free oxygen 

 in solution of indigo carmine. Biological methods were 

 devised as well as chemical ; in 188 1 Engelmann showed 

 that Bacterium termo is actively motile in the presence 

 of small quantities of the gas and perfectly quiescent in 

 its absence. In 1891 Beijerinck found that the luminosity 

 of certain bacteria may be used to determine whether or 

 no green Algae are exhaling oxygen, the gas causing them 

 to glow with increased intensity. 



The Chlorophyll Apparatus. Sachs claimed to be the 

 first to express the opinion that chlorophyll is the organ 

 to which the appropriation of carbon dioxide must be 

 attributed. He put forward the view in his Experimental- 

 physiologie in 1865 in the following form : ' It is a striking 

 phenomenon in the history of vegetable physiology that 

 the chlorophyll has not long ago been distinguished definitely 

 as the instrument of the separation of oxygen.' He rested 

 his opinion on the facts that only organs containing chloro- 

 phyll can effect the decomposition of carbon dioxide and 

 yield oxygen when exposed to light, and that the first 

 recognizable product of the process appears not in any 



