HISTORY AND PROBLEMS 9 
one-half the moisture content at the wilting coefficient, or to 
about 5%, somewhat less than Heinrich’s hygroscopic coefficient. 
During this period growth is suspended and the plant is in a semi- 
active or inactive condition often called estivation. Xerophytes 
have no more ability to grow in dry soil than mesophytes or 
hydrophytes with finely divided root systems such as rice. 
When rain falls, the soil moisture rises from 5%, or even less 
at the soil surface, to 19.6%, or the field carrying capacity. No 
higher percentage can be added and no lower percentage if the soil 
is not confined and there is free contact with dryer soil below. 
Two inches of absorbed rainfall as compared with 1 inch moistens 
twice as much soil but does not increase the moisture 1n percentage 
of the dry weight of the soil. On the diagram growth begins again 
at g. 
Hydrophytes cannot grow in the mesophytic and xerophytic 
habitat because of lack of abundant water, and mesophytes and 
xerophytes cannot grow in a hydrophytic habitat because of a lack 
of oxygen since they are adjusted to live only in the presence of 
air in the soil and around their stems and leaves. A liter of water 
at 20° C can contain the same amount of CO, as a liter of air at 
the same temperature but only 1/65 as much oxygen. In hydro- 
phytes the major problem is to secure oxygen, and mesophytes 
and xerophytes cannot do this in a hydrophytic environment. 
Xerophytes have the principal problem of not succumbing to 
desiccation and are especially adjusted to prevent harmful des- 
iccation. Mesophytes are not adjusted to hydrophytic or to 
xerophytic conditions. 
In measurements of the water economy of mesophytes in con- 
tainers, one of the difficulties is to maintain the water content 
in the container at the proper percentage range, that is, never 
below the wilting point and never above the field carrying ca- 
pacity. When water is added to a container the water is raised 
to field carrying capacity, and if the proper amount of water is 
added, practically all the soil mass is moistened to this point. If 
half enough water is added half the soil remains dry. If more 
water is added swamp conditions are produced and none will drain 
out until complete saturation is reached. The plants will probably 
