SUBSTANCES GIVEN OFF BY ROOTS 161 



On the other hand, when the soil water is so plentiful that air is 

 excluded from the soil, root growth is retarded and absorption 

 thereby hindered. For this reason wet lands have to be drained. 



In retarding growth as well as slowing down osmosis, a low 

 temperature of the soil retards absorption. This fact is related 

 to winter killing, which is sometimes due to the fact that the roots 

 can not absorb water from the cold or frozen soil about them as 

 rapidly as it is lost from the shoot. 



There are other factors, such as the presence of alkalies and 

 certain acids in the soil which hinder absorption by their injurious 

 effects on roots. 



Root Pressure. — The absorptive action of roots sometimes 

 manifests itself in forcing water through the stem, acting much 

 like the pump which forces the water through the city's water 

 mains. This pressure exerted by roots is known as root or sap 

 pressure, and is one cause of the so-called bleeding of plants when 

 they are injured. In most plants of the temperate regions, root 

 pressure is only evident in the spring when the plants are not 

 losing much water by evaporation and are gorged with sap. 

 When Grapes are pruned in the spring, they usually bleed pro- 

 fusely. A vigorous European Grape will sometimes bleed a liter 

 per day. A Maple tree may exude from 5-8 liters in a day. 



Measurements show that sap pressure is often several pounds 

 to the square inch. In the following table the pressure is recorded 

 in millimeters of mercury (760 millimeters of mercury = 1 at- 

 mosphere or about 15 lbs. of pressure to the square inch). 



Red Currant 358 



Sugar Maple 1033 



Black Birch 2040 



European Grape S60 



Substances Given off by Roots. — Roots give off carbon diox- 

 ide. The milky appearance of lime water in which roots are 

 grown is evidence of this fact (page 97). In the soil the carbon 

 dioxide unites with the water, forming carbonic acid, which has 

 an important dissolving action upon the soil minerals. This fact 

 is demonstrated by the etching effect roots have when grown in 

 contact with the surface of polished marble. 



There is much evidence that roots also give off oxidizing en- 

 zymes whereby the poisonous substances of the soil are oxidized 

 to harmless forms. 



