104 Carbon Assimilation. 



only sugars they found were sucrose, glucose, fructose and maltose. 

 Pentoses were tested for, but not found. It is to be regietted that 

 Brown and Morris do not state definitely what tests they applied for 

 the various sugars, as tlieir results have been accepted without ques- 

 ion by most later workers. The presence of cane sugar seems quite 

 definitely established, as leaf extracts after treatment with invertase 

 increase in reducing power and change in optical activity, and this 

 change is notverydifferentfromthat which would resultifthe increase 

 in reducing power were due to the inversion of cane sugar into glucose 

 and fructose. They were also able to show tlie presence of maltose 

 by obtaining maltose phenyl-osazone from leaf extracts. Similai-ly 

 glucose phenyl-osazone is produced, but no evidence is given as to 

 why it is concluded that ^/-glucose and tZ-fructose are the only 

 hexoses present, beyond the fact that they could obtain no other 

 phenyl-osazones. It should be noted that ^/-mannose gives the 

 same osazone as rf-glucose and rf-fructose, while the I forms of these 

 three hexoses, which are always stated to be absent from the leaf, 

 give a phenyl-osazone of the same crystalline form and the same 

 melting point as the d forms of the sugars (see e.g., Tollens, 1914). 

 More recent work, however, has never succeeded in revealing the 

 presence of any hexoses other than rf-fructose and (^-glucose, though 

 on the other hand it must be admitted that no definite evidence 

 has so far been brought forward in favour of the absence of all 

 other hexoses. For example. Parkin (1911) was unable to obtain 

 any osazone from extracts of snowdrop leaves other than glucose 

 phenyl-osazone, and hence concludes that galactose and mannose 

 are both absent. This argument is satisfactory for galactose, but 

 it does not hold for mannose, as that sugar gives the same osazone 

 as glucose and fructose. 



Moreover, the recent work of Davis, Daish and Sawyer (1916) 

 has cast grave doubt on the presence of maltose in leaves, which 

 these authors find only present in estimable quantity as a result of 

 enzyme action in leaves not instantaneously killed. Contrary too, to 

 Brown and Morris, these later workers conclude that free pentoses 

 are present in leaves. 



They base this conclusion (Davis and Sawyer, 1914) on the 

 fact that leaf extracts contain substances soluble in 80% alcohol 

 which are not precipitated by basic lead acetate, which are 

 unfermentable by ordinary yeasts and which exercise a cupric- 

 reducing power after other sugars have been fermented away. 

 There are of course many sugars which would fulfil these conditions, 



