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 ceU-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 soUs supporting 

 vegetation contain humus. Humus in quantity forms 

 peat, and the qualities of peat depend upon : 



1. The origin of the peat. 



2. The nature of the plants forming it. 



3. The amount of water contained in it. 

 4. . The amount of soil mixed with it. 



5. 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 ; aer, air). 

 The further stages of decomposition are brought about by 

 a succession of soU-bacteria, which cannot thrive without 

 oxygen (aerobic bacteria). 



The complete decomposition of organic matter by these 



