DEVELOPMENT OF PLANTS 159 



acted upon by an entirely different class of bacteria that carry 

 on the work to a further stage of decay. So you can think of 

 many different kinds of bacteria each taking up the work where 

 it was left by its predecessor and carrying it a step fartluT. In 

 this way the complex compounds that make up the Hvinj^^ organ- 

 ism are broken down into very simple ones, such as water (H2O) 

 or gases as carbon dioxide (CO2), ammonia (NH3), methane 

 (CH4), sulphureted hydrogen (HoS), nitrogen (N), etc. It is 

 not to be understood that all of these simpler substances arise 

 only in the final stage of decay. Some of them may arise very 

 early in the process, as ammonia and carbon dioxide, or they 

 may be set free at later stages in the decay. Some of these 

 gases are the cause of the foul odors associated with decay and it 

 should be added that certain of the rather simpler products of 

 decomposition, termed ptomaines, are exceedingly poisonous. 

 The ptomaines found in fish, cheese, ice cream, are decomposition 

 products formed by the bacteria of decay. Unfortunately these 

 substances are without taste or odor. 



This process of decay is of importance from two standpoints. 

 First, it prevents the accumulation of organic matter upon the 

 earth. Were it not for these changes every plant and animal 

 would remain unchanged after death and so prevent further 

 continuation of life. Second, we see that the successive steps 

 in decay result in the formation of simple compounds that are 

 either directly or indirectly utilized by plants in the formation 

 of their tissues. Thus H2O and CO2 are used by the plant in 

 forming sugars and starches which furnish the materials that 

 go to make up the cell walls and also assist in building up the 

 living substance. So also the N and the S that we have seen 

 set free as elements or simple compounds are utilized in the 

 formation of the living matter of the plant, though not directly, 

 as we will see below. 



3. Construction or Synthesis. — In sharp contrast to tiie alK)ve 

 mentioned forms stands a rather small group of bacteria that 

 possess the remarkable power of taking certain of the simpler 

 products of decomposition and adding other elements to them. 

 These forms, therefore, are quite different in thi-ir work from other 



