l82 NUTRITION AND METABOLISM 



as to prevent the intruding of microorganisms. The body of plants 

 and animals is surrounded on the outside by tough and dry layers of 

 epithelial cells, and the cavities of the animal body also have their 

 protective membranes. Microorganisms cannot enter the tissues 

 if these membranes are perfectly sound, and we know that, as a rule, 

 the tissues of healthy plants or animals are free from bacteria. Thus, 

 a healthy apple or potato or egg will not be infected and decomposed 

 by microorganisms if handled carefuUy, meat will begin to decom- 

 pose on the outside, and the inner parts may be still good when the 

 outer layer is already in a state of decay. 



In the plants, each cell is surrounded by its special cell membranes 

 which are a barrier to infecting organisms. If we prick the skin of a 

 healthy apple with a pin infected with yeast, the infection will not 

 spread though we know that yeast will grow most abundantly in cider; 

 in the apple, however, it has no means of spreading from one cell to 

 the other. Molds possess this means; they can puncture cell walls, 

 and forcing their way from one ceU to the other, they will soon 

 bring about the rotting of the entire fruit after it once becomes 

 infected. This protection seems especially necessary in the plant's 

 roots which are greatly exposed to injury from insects and other animals 

 in the soil and surrounded by billions of n?icroorganisms. They are 

 attacked only by fungi which can force their way from cell to cell, 

 or by bacteria which can dissolve the membranes by means of enzymes, 

 and thus cause a softening of the root tissue. The bacteria causing the 

 various rots of vegetables belong to this type. 



There is, then, a great variety of factors deciding the type of 

 decomposition of organic matter in nature, and by knowing the chemical 

 composition as well as the structure and other physical conditions, 

 it is possible to foretell which group of organisms is most likely to 

 attack the compounds in question. 



.Another quite important factor, the temperature, will be dis- 

 cussed in more detail in one of the following chapters. 



Rotation or Elements in Nature 



All organic matter on earth is undergoing continuous change. Or- 

 ganisms grow and decay. The same carbon and nitrogen atoms which 

 constitute the organic world of to-day constituted it thousands of 



