310 The Physiology of Plants bookiii 



experiments he used leaves detached from the plant, in 

 order to prevent loss by translocation. Under these con- 

 ditions he computed the amount to be about 1-5 gramme 

 per square metre per hour. He computed the carbon 

 dioxide absorbed during the same time at i-8 gramme per 

 square metre per hour. 



The method does not admit of an accurate determination 

 of the activity of the leaf owing to many causes. Its 

 structure and the necessity of selecting only such parts as 

 can be accurately measured ; the translocation of material 

 to the interior if the leaf is left on the stem ; the effect 

 of the accumulation of the products of the photosynthesis 

 if it is severetl from it ; the fact that respiration is pro- 

 ceeding simultaneously : all militate against more than 

 approximate results. It is noteworthy, however, that 

 Brown and Morris with their improved methods obtained 

 not very different results in 1892. 



In 1885 Kreussler found that using air containing ten 

 times the normal amount of carbon dioxide, about -3 per 

 cent., a square metre of the surface of the leaves of Rubus 

 fruticosus decomposed 2-5 grammes of the gas, which 

 is equivalent to the manufacture of approximately 

 1-54 gramme of starch. He illuminated his plants with 

 electric light of about the intensity of diffused day- 

 light. 



More exact investigations were carried out in 1899 by 

 Brown and Escombe. They devised an apparatus by which 

 they could examine and determine the exact quantity of 

 carbon dioxide absorbed by a measured area of a well- 

 illuminated leaf by passing over it a stream of air con- 

 taining a known percentage of carbon dioxide, the stream 

 flowing at a definite rate and at a constant temperature. 

 The stream of gas was kept passing in each experiment 

 for about six hours, and its carbon dioxide content was 

 examined before and after access to the leaf. They found 



