254 INTERMEDIATES IN REDUCTION OF CO2 CHAP. 10 



unsaturated alcohols (similar to those listed in table 10.11) is revealed 

 by a characteristic pleasant smell. 



The volatile constituents of green leaves also were studied by Maze 

 (1920), but on a much smaller scale. He distilled, under reduced pres- 

 sure, leaves of 29 plant species and identified the following products: 

 ethanol, C2H5OH (L = 1.5), and acetaldehyde, C2H4O (L = 1.25), in 

 most species; glycolaldehyde, CH2OHCOH (L = 1.0), and lactaldehyde, 

 CH3CHOHCHO (L = 1.16), in the leaves of poplar; acetoin, CH3- 

 CHOHCOCH3 (L = 1.25), in the leaves of green corn and peas, particu- 

 larly if gathered in the evening. 



In this connection, we may also recall the observations of Meyer 

 (1917, 1918) on the occurrence of "oil droplets" in the chloroplasts of 

 certain leaves and algae (c/. Chapter 3). As mentioned on page 43, 

 Meyer interpreted these droplets (which may be nothing else but the 

 grana, recently recognized as normal constituents of most chloroplasts) 

 as an "assimilatory secretion." He did not determine the chemical 

 composition of this "secretion," but compared its properties (volatility 

 with steam, solubility in ether, insolubility in water, capacity to blacken 

 silver nitrate in alkaline solution, smell, etc.) with the properties of the 

 compounds isolated by Reinke, Curtius and Franzen, and concluded that 

 they are nearest to those of hexenaldehyde. He suggested that hexen- 

 aldehyde is a component of the "assimilatory secretion" (only a compo- 

 nent, because the quantity of hexenaldehyde found by Curtius and 

 Franzen was much too small to account for the whole of the " assimilatory 

 secretion"). 



Meyer's "assimilatory secretion" has since apparently not been 

 investigated. However, Wieler (1936) made a renewed attempt to 

 identify chlorophyll grana in chloroplasts with oil droplets. He suggested 

 that the silver nitrate reduction by the chloroplasts (Molisch reaction, 

 page 360) can be due to their content in hexenaldehyde; but this sugges- 

 tion was opposed by Dischendorfer (1937). 



Hexenol, hexenaldehyde, hexenic acid, and similar compounds are 

 naturally suspect of being related to hexoses and this makes them 

 interesting from the point of view of photosynthesis; and the same can 

 be said of Maze's acetoin. Since, however, both hexenaldehyde and 

 acetoin are " o verreduced " (L > 1), it is highly improbable that they 

 may serve as intermediates of photosynthesis; they are more likely to be 

 its by-products. 



Nye (cf. Spoehr, Smith, Strain and Milner 1940), in a re-examination 

 of the role of hexenaldehyde in leaves, found evidence that it is formed 

 during the grinding of leaves. Whole leaves, or leaves which have been 

 killed with hot water, toluene, or chloroform before grinding, yielded 

 little or no hexenaldehyde upon distillation. If the grinding was carried 



