Chap, ii Photosynthesis 309 



Curtius and Reinke noted its appearance in light and its 

 disappearance in darkness. Reinke, in 1899, said that it 

 was not present in leaves as formaldehyde, but as a sub- 

 stance having the formula C 8 H 2 2 . 



The process we have been discussing was known till 

 recently as the assimilation of carbon dioxide. The term 

 assimilation, from its usage in another connexion, gradually 

 fell into disuse, being replaced at the end of the century 

 by the word photosynthesis, a term introduced by Mac- 

 Millan in 1893. 1 



The first investigations conducted with a view to the 

 determination of the quantities of carbohydrate formed by 

 green leaves were undertaken by Weber in 1878 in Sachs' 

 laboratory. His methods were not altogether satisfactory, 

 but he succeeded in making an approximate estimate. His 

 figures ranged from 4-446 grammes of starch per square 

 metre per hour for Tropceolum majus, to 5-56 grammes for 

 Helianthus annum. He computed his quantities from 

 observations on increase of weight of the leaves, and 

 attributed this increase entirely to starch. 



More accurate researches were conducted by Sachs him- 

 self in 1884, when he introduced his well-known half-leaf 

 method. Selecting large leaves on plants growing in the 

 open air, he cut out and carefully measured portions of one 

 longitudinal half early in the morning, dried them, and 

 took accurately their dry weight. In the evening corre- 

 sponding and equal areas were cut from the other half of 

 the leaf and treated similarly. The difference in weight 

 Sachs took as an approximately correct indication of the 

 amount of starch they had formed, which he reduced to 

 amounts per square metre per hour. His figures for Heli- 

 anthus annuus were 0-914 gramme, a much smaller amount 

 than Weber had obtained six years earlier. In subsequent 



1 See Barnes on 'So-called Assimilation', Bot. Ccntv., 76, 1898, 

 No. 47. 



