262 Plant Physiology 



free acids and acid salts the cell-sap may be acid in 

 reaction. 



These substances may be looked upon usually as the 

 by-products of metabolism, but they may be serviceable 

 and some may function further in general metabolism. 

 The group of fatty acids is well represented among the 

 compounds in plant tissues ; but the acids of commoner 

 occurrence are the related oxalic, malic, tartaric, and citric, 

 all being oxidation products of glycols (dihydroxy-deriva- 

 tives of the paraffins). 



Oxalic acid is of widespread occurrence, and it is most 

 familiar (in the form of calcium oxalate) as the raphides 

 or needle-shaped crystals so common in many vegetative 

 organs. Malic acid is well known in many unripe poma- 

 ceous and stone fruits, but it occurs far more commonly, 

 especially in " fatty ' plants like the stone-crops ; and it 

 is the substance found by Pfeffer and others to be chiefly 

 responsible for the " attraction ." directing motile sperms 

 to the egg-cells of certain ferns. Tartaric acid is readily 

 extracted from the grape, in which fruit it occurs as acid 

 potassium tartrate. Citric acid may constitute from 

 6 to 7 per cent of the juice of lemon, and it is also more 

 abundant in the other species of Citrus than in higher 

 plants generally. 



The production of acids is usually favored by the abun- 

 dance of soluble carbohydrates in the tissues. Submersed 

 in a solution of glucose, for instance, the leaves of Oxalis 

 rapidly increase in acidity. Among the lower plants, 

 fungi and bacteria, the production of organic acid is even 

 more common than with the higher plants, as again referred 

 to under fermentation phenomena. 



