4 , PHOTOSYNTHESIS 



the ultimate result of photosynthesis is an increase in cell 

 material. This material includes protein and lipids as well 

 as carbohydrate. The assimilatory quotient for the forma- 

 tion of protein has an average value of — o-8 and that for 

 lipid — 0-7. Small departures from unity for the assimilatory 

 quotient for the over-all process may thus be attributed to 

 the formation of a greater or lesser amount of products 

 other than carbohydrate. 



TABLE 1.1 



THE ASSIMILATORY QUOTIENT IN A VARIETY OF PLANTS 



Measurements made with a high light intensity and a high concentration of 

 carbon dioxide. {From Willstdtter and Stoll (1918), Barker (1935), <^"^ ^he 



authors' data) 



Plant 



Sambucus nigra , (leaf) 

 Pelargonium zonale ,, 

 Ilex aquifolium „ 



Leucobrywn glaucum ,, 

 Chlorella pyrenoidosa (culture) 

 Nitzschia closterium ,, 



Assimilatory quotient 



0-96 to — i-oo 

 0-98 to —0-99 



— I 00 



— 099 



-0-90 to —0-98 



— 0-96 



It can be shown experimentally that a plant can grow to 

 maturity when supplied with a single carbohydrate as the 

 sole organic carbon source and this would suggest that 

 a substance equivalent to a simple sugar, triose or hexose, 

 might be the primary product of photosynthesis. Albino 

 seedlings, which are devoid of chlorophyll, will grow only so 

 long as their initial reserve material lasts, and when this is 

 exhausted they die. If, hbwever, these seedlings are grown 

 under sterile conditions in a medium containing a single 

 carbohydrate the plants continue to grow and reach maturity. 



Photosynthesis results in the production of the order of 

 10^^ tons of organic carbon per year (Rabinowitch, vol. i). 

 From this figure it can be calculated that on the average 

 each carbon dioxide molecule must be incorporated into 

 plant structure every 200 years and that the oxygen of the 

 air is renewed approximately every 2,000 years. 



