THE NATURE OF PHOTOSYNTHESIS 



149 



phyll-content of the leaves increases with age and that the photosynthetic 

 activity also increases, the two are by no means parallel. 



In autumn, with the change in color of the leaves, the conditions be- 

 come very complex. In general there is a decrease in photosynthesis (g. 

 CO2 per hour) when this is calculated on the basis of leaf area, dry or 

 fresh weight. With the yellowing of the leaves there is also a decrease in 



TABLE 26 



Photosynthesis of Leaves from the Same Plant but in Different Stages of 

 Development, at 25°, 5 Per Cent CO2, About 48,000 Lux, from 

 THE Determinations of Willstatter and Stoll. 



the chlorophyll-content. Under conditions of maximal photosynthesis 

 (25°, high CO2 and light intensity) the activity of the leaves turning yel- 

 low, on the basis of chlorophyll-content (i.e. the Pc) is not very different 

 from the normal leaves. This is due to the fact that synchronous with the 

 decrease in chlorophyll there is a decrease in the activity of the protoplasmic 

 factor. As a consequence the Pc also decreases with the yellowing of the 

 leaves. In some cases the two factors, chlorophyll-content and protoplasmic 

 activity, do not decrease at the same rate so that it may occur that the Pc 

 of autumnal leaves is higher than in midsummer. Willstatter and Stoll 

 observed all possible variations in the Pc with the yellowing of the leaves, 

 rising, constant and falling values. Such values, it must be recalled, repre- 

 sent the photosynthetic activity on the basis of chlorophyll-content. In 

 absolute terms, the amount of carbon dioxide reduced in leaves turning 

 yellow is a tenth or less than that of normal leaves. 



We have already mentioned experiments in which the photosynthesis 



