168 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



aid of the absorbed substances held by the soil (Sect. 22). Moreover, 

 roots may exert a direct solvent action upon the particles of soil with 

 which they are in contact, by means of the carbonic acid produced by 

 respiration. Czapek finds that roots for the most part do not evolve 

 any perceptible amounts of other free acids, but that they are able to 

 excrete acid potassium phosphate, which may exercise a solvent action 

 either directly or by inducing double decompositions with chlorides or 

 nitrates. 



In such cases, it must be directly determined whether or not a par- 

 ticular root may not always, or under special conditions only, be able to 

 excrete free acids for solvent purposes. Various fungi can indeed evolve 

 large quantities of organic acids, and the production of acid depends largely 

 upon the way in which they are nourished, as well as upon other conditions, 

 for the amount of acid produced may be increased by continuous neutraliza- 

 tion (Sect. 86). The energetic corroding powers of lichens are probably 

 due to the excretion of free acid, and it is well known that other secretory 

 products, such as enzymes, are frequently employed to convert insoluble 

 substances into soluble ones which can be absorbed, or to enable a parasitic 

 or saprophytic organism to penetrate into the interior of living or dead 

 bodies (Sect. 65). Similar secretory powers have been developed by the 

 roots of many parasitic and saprophytic phanerogams, and hence it would 

 not be surprising to find that certain typical flowering plants are able to 

 excrete fixed acids in considerable amount, and thus render soluble the 

 mineral constituents which they require. 



There is no doubt that the characteristics of the root-system, as just 

 described, have not in all cases the same importance, for in a water-culture 

 irregular currents constantly occur, and these suffice to bring fresh supplies 

 .of dissolved material continually into contact with the roots. According to 

 the external conditions, a plant will be either sufficiently provided in this 

 manner, or will require to obtain a greater or less amount of its nutriment 

 by the active spoliatory means above described. 



The retention of water. A general account of the causes leading to the 

 retention of water by the soil has already been given, and it is not intended here 

 to give a detailed account of all the factors of importance in this respect, such as 

 the quality, consistency, or fineness of division of the soil. Various other circum- 

 stances influence the actual amount of water present in the soil. Thus, when 

 free fluid water is present near to the surface, the soil immediately above will be 

 fully saturated with capillary water (= full capacity), whereas, when the level of the 

 free water of the soil is lower, there will be little or no capillary water in the surface 

 layers (= lowest or absolute capacity)'. When fully saturated and all superfluous 

 water allowed to drain away, sandy soils contain least, humus soils most, water. 



1 Cf. A. Mayer, AgricuHurchemie, 1895, 4. Aufl., Bd. II, p. 135. 



