252 INTERMEDIATES IN REDUCTION OF CO2 CHAP. 10 



(20) Butyraldehyde: in hornbeam leaves (next in abundance to hexenaldehyde) , 

 Curtius and Franzen (1912*, 1914); in chestnut leaves, Franzen and Wagner (1918); 

 in oak leaves, Franzen (1920). 



(21) Butenol: in hornbeam leaves, Curtius and Franzen (1912^, 1914). 



(22) Valeraldehyde: in hornbeam, chestnut, and oak leaves, Curtius and Franzen 

 (1912S 1914), Franzen and Wagner (1918), Franzen (1920). 



(23) Pentenol: in hornbeam leaves, Curtius and Franzen (1912^, 1914). 



(24) Citric acid: after oxalic and malic, the most common plant acid. See page 262. 



(25) Aconitic and tricarballylic acid were found by Nelson and Hasselbring (1931) 

 in green wheat, and by Nelson and Mottern (1931) in green barley, maize, oats, and rye. 



(26) Glucuronic acid: in leaves of Scutellaria altissima, Palladin (1916). Other 

 "uronic" acids also occur in plants. 



(27) Ascorbic acid: present in all green plants. See page 269. 



(28) Capraldehyde: present in chestnut and oak leaves, Franzen and Wagner 

 (1918), Franzen (1920). 



(29) a-Hexenic acid: in hornbeam leaves, Curtius and Franzen (1912^, 1914). 



(30) a-Hexenaldehyde: see below. 



(31) a-Hexenol: in hornbeam, chestnut, and oak leaves, Curtius and Franzen 

 (1912*, 1914), Franzen and Wagner (1918), Franzen (1920). 



2. The Volatile Components of Green Leaves 



Our knowledge of the low molecular weight components of green 

 leaves is rudimentary; no attempts have been made to develop in this 

 direction the analysis of the chloroplast matter, Avhose isolation is 

 described in chapter 14. What we know about these compounds is 

 due largely to a series of 29 papers "On the Constituents of Green 

 Plants," initiated by Reinke (1881), continued by Curtius and Reinke 

 (1897), Reinke and Braunmiiller (1899), and Curtius and Franzen (1910, 

 19121-6, 1914^-^ 1915, 1916) and completed by Franzen and coworkers 

 (1918-1923). A few of these papers dealt with the nonvolatile acids 

 in leaves and fruits; but the majority were devoted to a large-scale 

 fractionation of the volatile components. The origin of the investigation 

 was an observation of Reinke (1881), that steam distillation of green 

 leaves yields a compound with the reducing properties of an aldehyde. 

 Curtius and Reinke (1897) proved that it was not formaldehyde, as at 

 first suspected. Reinke and Braunmiiller (1899) determined the "leaf 

 aldehyde" in different species, and observed an increase in its concen- 

 tration during the day, suggestive of a relationship with photosynthesis. 

 Curtius and Franzen (1910, 1912^ distilled 600 kg. of leaves of Carpinus 

 betulus (hornbeam) and obtained enough distillate to identify the aldehyde 

 as the a-hexenaldehyde, CieHioO or CHsCHsCHaCH^CHCHO. 



The concentration of hexenaldehyde reaches 0.35 g. in 1 kg. of fresh 

 leaves of Vitis vinifera, 0.29 g. in Castanea vesca, 0.11 g. in Quercus 

 sessifiora, etc., corresponding to up to 0.1% of the dry weight of the 

 leaves. Altogether, it was identified in 20 species. Subsequently, 

 Curtius and Franzen (1912^-5, 1914^ subjected 1500 kg. of hornbeam 



