238 HISTORY OF BOTANY 



centrated layer on the surface of these bodies," and that 

 the plastid itself " is practically solid." 



THE FUNCTIONS OF CHLOROPHYLL 



As you are well aware, in consequence of the activities 

 of chlorophyll in sunlight, certain products result, oxygen 

 gas is given off from the leaf and carbohydrates appear 

 in it. Following on Priestley's historical discovery and 

 Ingen-Housz's experimental investigations, many writers 

 have dealt with the amount of oxygen exhaled as com- 

 pared with the amount of carbon dioxide taken in, such 

 as, in early days, De Saussure and, in later years, Bonnier 

 and Mangin. The primary difficulty which confronted 

 all the early workers was the separation of the gaseous 

 interchanges connected with photosynthesis from those 

 concerned with respiration. The first who made any 

 successful attempt at disentangling the two processes 

 were Bonnier and Mangin, and you will remember that 

 the net result of their work was to show that the fraction 



CO 



in respiration is less than unity, while the fraction 



rv 



P^- in photosynthesis is greater than unity, and that the 



real photosynthetic coefficient can be obtained only by 

 a comparison of these two fractions. Bonnier and 

 Mangin's work has quite recently been criticised by 

 Maquenne and De Moussey. These authors hold that 

 the real photosynthetic coefficient is approximately 

 unity, but, despite the large number of cases examined, 

 it cannot be said that the exact volume relationships 

 of the two gases have even yet been determined. 



As regards the carbohydrates formed, Sachs in 1862 

 held that starch was the first visible product of photo- 

 synthesis, but Meyer, in 1885, showed that in many 

 plants starch is never formed at all and that such plants 

 produce sugar instead. Ten years later, as the result 

 of Brown and Morris's work, it had come to be believed 



