OCCURRENCE OF PLANT ACIDS IN LEAVES 263 



concentration changes can take place in a plant in the course of the 

 season, or even of a single day. 



We shall quote a few figures to show the range over which the concentration of 

 plant acids may vary in different species. On the one extreme, Franzen and Keyssner 

 (19232) found, in the leaves of Rubus frudicosus (blackberry), in addition to 0.8% lactic 

 acid and 9 X 10-^% succinic acid, only 1.5 X 10"^% malic acid and 3 X 10-^% oxalic 

 acid. On the other extreme, leaves have been observed to accumulate up to 10 or 

 20% citrate, oxalate, or malate. 



The presence of microscopic crystals of acid potassium oxalate in Oxalis acetosella 

 (clover sorrel) was known to Malpighi as early as 1686. The concentration of oxalate 

 in the leaves of common sorrel (Rumex) and of rhubarb {Rheum) is over 1%, and in 

 the leaves of beet, 4% (c/. Czapek 1925, p. 71). The leaves of Begonia semper florens 

 may contain up to 20% oxalate (Ruhland and Wetzel 1926); and in some cacti (e. g., 

 Pilocerus senilis), the concentration of calcium oxalate, increasing with age, can finally 

 reach 90% of the total dry matter. A list of typical "oxalate plants" was given by 

 Bennet-Clark (1933). 



Malic acid was first discovered in fruits, but Vauquelin (1800) found that it is also 

 present in large quantities in succulent leaves, as in those of Bryophyllum. Early 

 determinations of malic acid in leaves were made by de Fries (1884), Warburg (1886), 

 and Ordonneau (1891). However, according to Franzen and Keyssner (1923^), out of 

 235 assays for malic acid in plants only 15 were reliable (5 of them in leaves) and 11 

 probably correct (7 of them in leaves). However, there seems to be little doubt that 

 small quantities of malic acid are present in most, if not all, leaves and algae. Franzen 

 and Keyssner (1923^) found 1.5 X 10"^% malic acid in blackberry leaves; Ruhland and 

 Wetzel (1926) found 0.5% in Begonia semper florens. Klein and Werner (1925) identified 

 it in five species of nonsucculents; Zacharova (1934), in pine needles; Vickery and 

 Pucher (1931) and Pucher, Wakeman, and Vickery (1937), in tobacco leaves; Pucher, 

 Clark, and Vickery (1937i'2), in rhubarb leaves; Pucher, Wakeman, and Vickery (1939), 

 in buckwheat leaves; and Kylin (1931), in brown algae. 



The concentration of calcium malate in some succulents reaches 8% {Agave siciliana), 

 14% {Mesembryanthemum crystallinum), or even 25-50% (certain Crassulaceae) (Czapek 

 1925, pp. 80-82). In addition to succulents, malic acid is present in comparatively 

 large concentrations also in many "oxalate plants," e. g., rhubarb. A list of "malate 

 plants" was given by Bennet-Clark (1933). 



The early assays of citrates in plants, were critically reviewed by Franzen and 

 Helvert (1923) who recognized as reliable only 16 out of 137 published figures; however, 

 small quantities of citrate are undoubtedly present in a majority of green plants. 

 Citrate was found by Vickery and Pucher (1931), Pucher, Sherman, and Vickery (1936) 

 and Pucher, Wakeman, and Vickery (1937) in tobacco leaves; by Pucher, Clark, and 

 Vickery (1937) in rhubarb leaves; and by Pucher, Wakeman, and Vickery (1939) in 

 buckwheat leaves. Wolf (1939), Guthrie (1934), and Borgstrom (1934) showed that 

 citric acid replaces mafic acid as the main product of acid metaboUsm in certain succu- 

 lents; Kleinia neriifolia, for example, accumulates, according to Borgstrom, as much 

 as 17% citrate. 



Oxalate crystals grow steadily in many plants, and obviously represent 

 excretions (although they can occasionally be redissolved). Of all the 

 organic acids, oxalic acid has the lowest reduction level (L = 0.25); it 

 thus contains the least chemical energy and can be discarded without 

 much waste. 



