THE NATURE OF PHOTOSYNTHESIS 141 



photosynthesis, because the rate here is determined by the chlorophyll- 

 content, namely, the amount of radiant energy which the leaf is capable of 

 utilizing. 



On this basis the difference in the temperature coefficients of the two 

 types of leaves becomes intelligible. With the leaves low in chlorophyll- 

 content, a light reaction was determining the rate, resulting in a low Qio- 

 In the other case, the leaves of high chlorophyll-content, this factor was 

 relatively less in minimum concentration, and the influence of tempera- 

 ture on the enzymatic reaction was more clearly expressed. Willstatter 

 and StoU also found that the temperature coefficient of photosynthesis 

 was lower at higher temperatures. 



It is not surprising that the temperature coefficient of photosynthesis 

 should vary from that of simple chemical reactions. The rate of photo- 

 synthesis is influenced by a number of factors. As an example of this 

 we may mention the lower solubility of carbon dioxide at higher tempera- 

 tures which may result in a lessened supply of this gas in the chloroplasts. 



An interesting phenomenon discovered by Miss Henrici and described 

 in the section of the Time Factor is related to the effect of starch forma- 

 tion on the photosynthetic rate. The formation of starch in the leaf 

 is greatly affected by temperature. The presence of starch in the chloro- 

 plast also is of influence on the rate of photosynthesis. Temperature 

 can, therefore, influence the rate of photosynthesis in this rather indirect 

 manner of affecting the starch formation in the chloroplasts. The 

 threshold of photosynthesis in alpine shade plants and lichens is con- 

 siderably below 0°. This is probably due to their low freezing point 

 arising from the high osmotic pressure of the cell sap which contains no 

 starch but much soluble sugar. Even when ice crystals appear in the 

 interior of the cells photosynthesis still occurs ; this is especially true of 

 the lichens. 



e. Chlorophyll. 



That chlorophyll is of paramount importance in the photosynthetic 

 process was concluded by the first investigators of the phenomenon. Only 

 those portions of plants containing chlorophyll are capable of reducing 

 carbon dioxide to carbohydrates. Frequently plants are colored red or 

 brown by an admixture of other pigments, but these usually also con- 

 tain chlorophyll. There are lower organisms capable of reducing carbon 

 dioxide, certain bacteria which do not contain chlorophyll, but the re- 

 actions here are very different from those taking place in the chlorophyl- 

 lous plants. Some of these bacteria do not require light as a source of 

 energy for the carbon dioxide reduction, but are apparently capable of 

 accomplishing the reduction by means of chemical energy as, for ex- 

 ample, the oxidation of hydrogen or ammonia. In those plants which 

 use light as a source of energy for the photosynthetic reaction, chlorophyll 

 is an essential component. There exists, however, some difference of 



