128 CHEMISTRY OF PLANT LIFE 



mal salts have a significant effect in regulating the acidity or alka- 

 linity of plant juices, or body fluids, and so determining the nature 

 of the enzymic activities and colloidal conditions of the biological 

 systems (see Chapters XIV and XV). It is probable that other 

 organic acids, such as formic, acetic, oxalic, and succinic acids, 

 in plants and sarco-lactic acid, in animal tissues, perform similar 

 regulatory roles; but there seems as yet to be no indication as to 

 why different acids should be used for this purpose by different 

 species, or organisms; or as to the methods by which they perform 

 their specific functions, whatever these may be. 



In plants, the organic acids are usually in solution in the sap. 

 When the plant ripens, they generally disappear, either being neu- 

 tralized by calcium, or other bases, and deposited as crystals in 

 the leaves or stems, or else used up in the synthesis of other organic 

 compounds. Small proportions of these acids are usually present 

 in mature seeds, and the percentage increases materially during 

 germination, indicating that they play an important role in insur- 

 ing the proper conditions for the conversion of the reserve food of 

 the seed into soluble materials available for the nutrition of the 

 young growing plant. 



BIOLOGICAL SIGNIFICANCE OF FRUIT ACIDS, ETC. 



The occurrence of organic acids, or their derivatives, which 

 have pronounced odors or flavors, in the flesh surrounding the 

 seeds of fruits, in the endosperm of vegetable seeds, or in the tubers, 

 etc., of perennial plants, thus making them attractive as food for 

 animals and men, undoubtedly serves to insure a wider distribu- 

 tion of the reproductive organs of these plants; a fact which has 

 unquestionably had a marked influence upon the survival of spe- 

 cies in the competitive struggle for existence during past eras and 

 in the development and cultivation of different species by man. 

 Indirect evidence that the proportion of these attractive com- 

 pounds present in certain species may have been considerably 

 increased by the processes of " natural selection " in the past is 

 furnished by the many successful attempts to increase the per- 

 centage of such desirable constituents in fruits or vegetables by 

 means of artificial selection of parent stocks by skillful plant 

 breeders. 



