728 PART IV. THE PHYSIOLOGY OF PLANTS. 



Organic acids. It is scarcely possible to make any general statement as to 

 either the origin or the fate of these substances which are so largely present 

 in plants. They are probably formed maiuly by normal oxidative catabolic 

 processes, but the formation of some of them (e.g. oxalic acid) is at the same 

 time closely allied with certain anabolic processes in the plant : however, there 

 is no doubt that they are not, as was formerly held, ever formed synthetically. 

 The more complex acids may certainly be looked upon as plastic substances : 

 for, in succulent plants (see p. 725) it has been found that the malic acid, which 

 accumulates in the tissues during darkness, is decomposed by the green parts 

 in the light, with evolution of oxygen and formation of less highly oxidised 

 organic substance (carbohydrate). In fact, the process of assimilation of these 

 acids is essentially the same as that of carbon dioxide, the only difference being 

 that carbon dioxide is more highly oxidised than are these more complex acids, 

 but the conditions of the process are identical in the two cases. These organic 

 acids are of considerable physiological importance in effecting the decomposi- 

 tion of the salts (e.g. nitrates, sulphates, phosphates) absorbed from the soil. 



Amides are formed both in anabolism (p. 720), and in catabolism (p. 725). 



Proteids may all be formed in anabolism ; and the simpler forms (peptones) 

 in catabolism. It is a question whether or not the more complex proteids may 

 not be formed by dissociation of the molecule of protoplasm. 



The waste-products are most probably all formed as the result of 

 catabolic processes ; though their formation is often associated, 

 both as to time and place, with active anabolism. They may be 

 classified into nitrogenous and non-nitrogenous. 



The principal nitrogenous waste-products appear to be the 

 alkaloids (see p. 707). They are probably products of the nitro- 

 genous catabolism of plants ; and it is suggestive that they prin- 

 cipally occur deposited in the cells of deciduous parts, such as 

 leaves, seeds, bark, etc. 



The principal non-nitrogenous waste-products are, water ; free 

 oxygen (green plants in light) ; carbon dioxide, and some other 

 highly oxidised carbon-acids, such as the oxalic ; resins and ethereal 

 oils, tannins, aromatic substances, etc. 



Of these waste-products, some are retained in the cells of the 

 plant, whereas others are thrown off or excreted. The nitrogenous 

 waste-products are deposited either in cells or in the laticiferous 

 tissue: there is practically no excretion of such waste-prod acts by 

 plants. Similarly, those of the non-nitrogenous waste-products 

 which are not gaseous at ordinary temperatures, are retained by 

 the plant. For instance, oxalic acid is deposited in the form of 

 crystals of calcium oxalate either in the cavities or in the walls 

 of the cells (see pp. 108, 113) : the crystals may have either six 

 molecules of water of crystallisation, when they are quadratic; or 



