XIV.] LIVING MATTER AND ITS EFFECTS. 233 



polishing material. But, even where there is no special 

 deposition of mineral matter in the plant-cells, the walls of 

 the cells themselves are commonly formed of a compact 

 membrane which may offer considerable firmness of tex- 

 ture. The cell-membrane consists of the material called 

 cellulose, which differs essentially from the inclosed proto- 

 plasm, in that it contains no nitrogen, but rather resembles 

 starch in chemical composition. In woody plants, the cell 

 walls become very much thickened ; and the accumulated 

 woody matter, which is insoluble in water, contributes to the 

 strength and support of the vegetable structure, and decays 

 but slowly. Hence, the accumulation of remains of plants 

 may, under proper conditions, yield deposits of considerable 

 durability. 



Accumulations of partially decomposed vegetable matter 

 form the substance known as peat or tinf. This is pro- 

 duced only under certain conditions of moisture and tem- 

 perature ; damp ground, in a temperate climate, being the 

 situation most favourable to its formation. In this part of 

 the world, the principal peat-forming plants are certain 

 mosses known to botanists under the generic name of 

 Sphagnum. The stems of the bog-moss die away in their 

 lower part, while the upper portion continues to grow freely. 

 The interwoven dead portions form a tangled mass, which 

 holds water like a sponge and favours the growth of the 

 moss above. Remains of other plants become mixed with 

 the mosses, and contribute to the formation of the peat, 

 while trunks of trees occasionally get embedded in the 

 bog. Muddy matter is likewise washed in during floods, 

 and helps to consolidate the felted mass and to produce a 

 deposit of considerable firmness. The rate at which the 

 peat grows varies greatly under different conditions, but 

 some notion of the rate may be gained from the fact that 



