20 AERATION AND AIR-CONTENT. 



always giving off C0 2 , and C0 2 , especially in the concentrated solu- 

 tion which must be momentarily formed in the cell- wall of the root- 

 hairs, has an appreciable solvent effect upon the majority of the 

 minerals composing the soil. This C0 2 alone is capable of giving 

 rise to such solutions as are required for the nutrition of the plant. 

 As the direct evidence is also adverse to the idea of an excretion of 

 acid, the principle of not seeking remote causes would lead us to 

 attribute to CO 2 only, the long-recognized solvent power of the 

 plant upon the soil. 



Kunze (1906) has shown that corrosion studies and culture experi- 

 ments in powdered stone prove that the higher plants can not obtain 

 necessary nutrients from unweathered stone. Moreover, plants 

 which are marked by vigorous excretion of acid, such as white cab- 

 bage and vetch, show a stronger development, due to the energetic 

 decomposition of the soil than those with more or less of this quality, 

 e. g., mustard and sainfoin. Rapidly growing plants of relatively 

 short vegetation period show marked excretion, e. g., Cucurbita, 

 Helianthus, Phaseolus, Zea, etc. Among the grasses, Secale and 

 Avena show strong excretion, Hordeum and Triticum less. More- 

 over, while the ability of the grasses to dissolve soil nutrients through 

 root excretions is small or lacking, this is compensated by the greatly 

 branched root-systems and their energetic transpiration. The 

 marked excretion found among the Boraginacece is explained by the 

 vigorous development of the plant-body, relative short vegetative 

 period, and the dry habitat. Since only a small amount of acid or 

 none at all is produced by a great number of plants, and since in 

 many cases C0 2 plays only a subordinate part in breaking down the 

 soil particles, it is assumed that many higher plants must possess 

 another means of producing nutrient solutions in the soil, as by the 

 aid of soil fungi. 



Stoklasa and Ernest (1908 : 64), in a study of the chemical nature 

 of the root secretion, conclude that carbon dioxid is the sole gas 

 secreted in normal respiration. The injurious action which the 

 lack of oxygen in the soil produces upon the root is evident, and 

 appears especially in soils crusted over or supersaturated. The 

 unsatisfactory aeration of the soil always produces certain patho- 

 logic phenomena of the plant, which can only be connected with the 

 improper oxidation of the products of the decomposition of the 

 carbohydrates and proteins. 



Griiss (1907 : 69) studied the chemical changes involved in 

 wounded potatoes and found the oxidizing and diastatic enzymes 

 increased. Wismewski (1912 : 1045) found that the rest-period of 

 buds of Hydrocharis morsus-rance was shortened by wounding. 

 Muller-Thurgau and Schneider-Orelli (1910 : 309; 1912 : 386) ob- 

 served increased respiration in potato tubers and bulbs of Conval- 

 laria after the warm-bath, as did Iraklionow (1912 : 515) in potato 



