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 squareinch. In the following table the pressure is recorded 
in millimeters of mercury (760 millimeters of mercury = 1 at- 
mosphere or about 15 Ibs. of pressure to the square inch). 
Red Currant suc osnx ol eadktedeaphe en Pace eee neh etianiee 358 
Supar Maplescn : can squdie saeeten eo aiuameew ede Dewees 1033 
Black Birch sinu(e-e nds vauver «pacts Cee eds Reais re ee OES 2040 
Huropean: Grape shay. weees See ca Gas oe eae vee eee eee ee 860 
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. 
