218 TEXTBOOK OF PLANT PHYSIOLOGY 



attached acid radical. Under the influence of lipase, lecithin, like- 

 wise, gives rise to fatty acids. Phosphoric acid is split off from 

 glycerine by a special enzyme, glycero-phosphatase. 



Lecithins are found in all living cells, animal as well as plant. 

 They are not reserve substances, but enter into the composition 

 of the protoplasm as a necessary integral part of it. There is 

 reason to believe that they accumulate especially on its surface, 

 in the so-called plasma membrane. Overton, as has already been 

 noted, attributes an outstanding function to them in the deter- 

 mination of osmotic properties of this membrane. Some authors, 

 for instance, Palladin, likewise attribute to lecithins and to other 

 closely related substances, the so-called phosphatides, an important 

 role in the oxidation processes during respiration. Finally, having 

 the capacity to form easily complex compounds with a great variety 

 of substances, particularly with proteins, lipoids may have an 

 important role in the synthetic processes of the cell. All these, 

 however, are only suppositions of a hypothetical nature. There 

 are no exact experimental data extant which would indicate the 

 significance of lipoids and phosphatides. 



Dibasic organic acids are extremely widespread in the 

 cell sap of plants. They are in the free condition, as well as 

 in the form of salts. The most common of them are: oxalic 

 acid, COOH— COOH; malic, COOH— CH 2 — CHOH— COOH; 

 and tartaric, COOH— CHOH— CHOH— COOH . Of tribasic acids 

 the most widespread is citric acid, COOH — COH (CH2 — COOH)2. 

 Many organic acids are found in the fleshy leaves of succulents, 

 as well as in such plants as the sorrel, Oxalis, etc. Immature fruits 

 are also rich in these acids. For some time, the view was held that 

 acids originate through the imperfect oxidation of sugars during 

 the respiration process, but this view has been disputed. It 

 appears probable that the organic acids arise from the products 

 of disintegration of proteins, the amino acids, through the splitting 

 off of the amino groups (compare Art. 71). The presence of organic 

 acids is responsible for the acid reaction of the cell sap, which is 

 shown by the majority of plants. In some plants the pH value 

 may reach 1.6 to 1.3. In the presence of calcium ions, which 

 are always found in plants, oxalic acid produces crystals of calcium 

 oxalate of different forms: single, clusters, and raphides. The 

 appearance of these crystals is usually a sign of senescence of the 

 cell, they being especially abundant in dying tissues. The almost 



