THE BIOLOGY OF THE SOIL 93 
absorption diminishes as the concentration of the external 
water increases. (Compare Halophytes, p. 88; and 
Chalk-Plants, p. 89.) The critical point is reached when 
the outside water and the cell-sap are of the same strength. 
Absorption then ceases entirely. 
2. The second membrane through which the water 
has to pass to reach the central sap is the living protoplasm, 
which forms a film lining the inside of the cell-wall. 
Whereas the cellulose-membrane is permeable to all 
liquids of any degree of concentration, the protoplasmic 
membrane allows only certain solutions to pass through 
it, and these only in a very dilute condition. One is a 
living membrane, and the other is not, and it is the living 
membrane which controls the intake of water, and deter- 
mines in what proportion the various substances present 
in soil-water shall enter the plant. This is known as 
selective absorption. Thus, if two solutions of equal 
strength—one a salt of soda, and the other a salt of potash 
—are presented to a cell, a greater proportion of potash 
will pass through the protoplasm into the interior than 
of the soda. This happens to serve the plant very well, 
since potash is the more valuable salt. 
The Qualities of Humus-Soils.—All soils supporting 
vegetation contain humus. Humus in quantity forms 
peat, and the qualities of peat depend upon : 
. The origin of the peat. 
. The nature of the plants forming it. 
. The amount of water contained in it. 
The amount of soil mixed with it. 
The amount of air contained in it. 
6. The amount of lime in it. 
7. The presence or absence in it of fungi, soil-bacteria, 
and earthworms. 
The destruction of organic matter and its conversion 
into humus is the work of minute non-green vegetable 
microbes, known as bacteria. The first stage of the 
process, putrefaction, is effected by bacteria which are 
able to live without air—that is, without oxygen. These 
are called anaerobic bacteria (Gr. a, an, not; aér, air). 
The further stages of decomposition are brought about by 
a succession of soil-bacteria, which cannot thrive without 
oxygen (a’robic bacteria). 
The complete decomposition of organic matter by these 
Om OO bo 
