2 PHOTOSYNTHESIS 



shown to be an autotrophic (from Gk. auto-, self, and trophe, 

 nourishment) organism. It can be grown without soil in 

 water containing the essential mineral salts provided carbon 

 dioxide and adequate illumination are supplied. 



These observations can be easily demonstrated today- 

 using the simplest apparatus. The gas produced upon the 

 illumination of a fresh water alga placed in bicarbonate solu- 

 tion can be shown to rekindle a glowing splint. Many of us 

 must have unconsciously repeated Priestley's experiment 

 when we were children and placed green water weeds in our 

 jars containing tadpoles! The production of organic matter 

 by the illuminated green leaf is most easily demonstrated 

 when starch is formed. After removal of the green pigment 

 by treatment with alcohol and subsequent washing, starch 

 can be detected by staining the leaf with iodine. In this 

 way it can be shown that starch accumulates in many leaves 

 during the day time, i.e. when both light and carbon dioxide 

 are available. If the leaf is suitably masked during illumin- 

 ation it will be seen that the starch is confined to the areas 

 exposed to the light. During the night the starch tends to 

 disappear from the leaf being in part used in leaf respira- 

 tion and in part translocated in the form of sugars, to other 

 parts of the plant. 



The products of photosynthesis, oxygen and organic 

 compounds, are consumed in respiration, so that the two 

 processes may be summarized in one equation, the direction 

 of the change indicating which process is predominating. 



Respiration 



Organic compounds + O2 - v ~ "^ CO2 + H2O (i.i) 



Photosynthesis 



When we attempt to measure photosynthesis in the living 

 plant we observe only the net process — the 'apparent' photo- 

 synthesis. We may, however, measure the rate of respiration 

 in the dark, preferably both before and after the light period, 

 and add the mean of these values to the 'apparent' rate of 

 photosynthesis and thus obtain an estimate of the 'real' 

 photosynthesis. This procedure, which assumes that res- 

 piration continues in the light at the same rate as in the dark, 

 may not always be justified. The experimental evidence so 



