174 PHOTOSYNTHESIS 



(about 19,000 Lux) photosynthesis was inhibited 65 per cent. The same 

 concentration of cyanide with weaker Hght (1800 Lux), which normally 

 utilized slightly more than the carbon dioxide that was produced by respira- 

 tion, did not inhibit photosynthesis. Warburg has made use of these 

 observations in his theoretical deductions. Further investigation of the sub- 

 ject with other methods and plants seems desirable. 



Age: For a long time it was thought that leaves showed active photo- 

 synthesis only after they had attained a certain size and condition of de- 

 velopment. While different species of plants vary greatly in their rate 

 of development it is highly probable that the foregoing conclusion is 

 erroneous. Undoubtedly the development of the capacity for photosyn- 

 thesis depends very much upon the conditions of temperature and light 

 under which the plants are growing, yet it appears from the resuhs of 

 Willstatter and Stoll that photosynthesis is quite high in leaves which 

 are just unfolding. Young leaves have an exceedingly high rate of res- 

 piration which decreases to one-quarter of this rate when the leaf ma- 

 tures. The results of photosynthesis measurements depend in many in- 

 stances upon a proper determination of respiration and such results vary 

 according as to whether they are calculated on the basis of area, fresh 

 weight or dry weight. Willstatter and Stoll's experiments show that 

 the rate of photosynthesis after about nine days of growth is almost 

 constant for equal areas of leaf surface. Calculated on the basis of dry 

 weight and in some cases on the basis of fresh weight, the rate of photo- 

 synthesis decreases with age. On the basis of chlorophyll-content there 

 is a decided decrease in photosynthetic activity. It is important, there- 

 fore, to consider carefully on what basis photosynthesis measurements 

 are determined and also the methods which are employed in making such 

 measurements. 



Willstatter and Stoll also compared young leaves with those of the 

 previous year of Laurus nobilis and of Taxus baccata and found that 

 on the basis of fresh weight there was little difference, while on the basis 

 of dry weight the older leaves had a lower photosynthetic rate. In the 

 autumn, when the leaves are just beginning to change in color, on the 

 basis of dry or fresh weight as well as on the basis of area, they show a 

 decided decrease in photosynthesis. It is, however, hazardous to draw 

 any very general conclusions regarding the effect of age and develop- 

 ment on the photosynthetic activity of plants. The marked individual 

 and specific peculiarities of different species together with their adaptive 

 modifications resulting in differences of structure as well as composition 

 of the leaves become evident in photosynthesis. In general, functional 

 activity decreases with age, but owing to the complication of factors 

 which is involved in producing this diminished activity, a quantitative 

 expression of the causes underlying it is as yet impossible."^'^ 



^'Ewart, ;. c, 452. Kreusler, Landii>. Jahrb., 14, 913 (1885). Willstatter and 

 Stoll, /. c, 86. 



