ABSORPTION OF WATER 149 



The influence of a low soil temperature on the absorption of 

 water by the plant, however, must not be overestimated. In 

 hardy plants, as for instance winter cereals, low temperatures 

 interfere less with the intake of water. These plants grow and 

 develop perfectly well in late fall and early spring, in spite of 

 periodical frosts. Their unhindered absorption of water from the 

 soil is indicated by an abundant guttation. 



Oxygen of the air is another external factor influencing the 

 rate of water absorption by the root. In order to develop exuda- 

 tion pressure, as well as for their growth, roots require a supply of 

 oxygen. On too compact or submerged soils, therefore, plants 

 develop poorly, or perish. This is frequently observed in small 

 depressions of the ground where water stagnates. It is not the 

 excess of water itself that is injurious to the plant, since it develops 

 perfectly in water cultures. It is rather the lack of oxygen result- 

 ing from submersion that is harmful. Plants in water cultures 

 develop successfully only when an air current is passed through 

 the solution or the latter is changed often. Experiments with 

 potometers have resulted in a delay of absorption, where the water 

 was saturated with carbon dioxide or nitrogen, thus removing the 

 oxygen. The inhibiting influence of carbon dioxide was more 

 marked than that of nitrogen, the former apparently acting as a 

 specific poison. 



Besides the accumulation of the poisonous carbon dioxide, lack 

 of aeration may lead to other consequences having an indirect 

 effect on the root system. The various bacterial processes of the 

 soil, for instance, may be profoundly altered. Oxidation may be 

 replaced by anaerobic fermentation. Poisonous products of 

 anaerobic decay may accumulate in the soil, which in American 

 literature have been termed soil toxins. All these substances 

 poison the roots of plants and check water absorption. The 

 usual herbaceous vegetation does not develop, therefore, on very 

 swampy soils. In these places grow special plants, whose roots 

 are in a way immune to the soil toxins. Moreover, submerged 

 and bog plants have air spaces in their organs. These air spaces 

 help to supply with oxygen the roots and rhizomes, which are 

 usually surrounded by mud. 



Owing to this adaptability, bog plants thrive in soils which are 

 entirely lacking oxygen and poisonous to other plants. The abun- 

 dant guttation, so characteristic of many of these plants, shows 



