LIVING AND NONLIVING MATTER 17 



involves the breaking down of living matter with the formation of wastes, 

 such as urea, water, and carbon dioxide, and the liberation of kinetic 

 energy, mainly evident as heat and movement. Living matter possesses 

 the power to rid itself of this waste and also to take in new matter and, by 

 adding it to its mass, to repair the loss which it has suffered. All these 

 changes together constitute metabolism. If the material taken in and 

 added to the mass is greater than the amount lost by waste, the result is 

 an increase in the size of the living mass which is called growth. At the 

 same time, mere increase in bulk does not necessarily imply growth. 

 The taking up of water and the swelling that living matter undergoes 

 under certain conditions is not growth. This word is properly applied 

 only when new matter is added to the substance of protoplasm itself. 

 This addition occurs in living matter in a manner different from that in 

 which it occurs in inorganic. The latter usually increases in bulk by 

 additions to the surface, or growth is by accretion; while in the former 

 growth occurs by the introduction of new particles among those already 

 present, which is growth by intussusception. 



4. Reproduction. — Living things have the power to reproduce them- 

 selves by the formation of other masses similar in every respect to the 

 parent mass. Sometimes this similarity is perfect from the first. At 

 other times a fragment from the parent mass gradually assumes the size 

 and form of the parent. Nonliving things do not possess this power. 



5. Irritability. — Living things, generally speaking, have the power of 

 responding to changes in their environment, such changes acting as 

 stimuli. This quality is termed irritability, or reactiveness. The response 

 evidences itself in the movements of animals and results in the various 

 ways in which animals adjust themselves to the conditions of their exist- 

 ence. The organism is not itself modified in any essential respect by the 

 reaction and may, under proper conditions, reassume precisely the character 

 it possessed before the reaction occurred. Nonhving things may also be 

 affected by changes in the environment, but the modification is not in the 

 nature of adjustment, is destructive in its effect, and the thing cannot of 

 itself regain its former character. 



25. Tests of Life. — Living matter, however, is not always to be recog- 

 nized by any characteristics which it possesses. A living seed may 

 appear as inert as any bit of inorganic matter, and some animals may 

 exist dried up and apparently without any of the attributes which belong 

 ordinarily to living things. The test which may be applied in such cases 

 to determine whether or not life is present, or is possible, is to place the 

 object under such conditions of warmth and moisture as experience has 

 shown tend to develop life activities and observe if under these conditions 

 the distinctive phenomena of life are manifested. If they are, the infer- 

 ence is that the object was alive or that the drying up occurred in such a 

 way as not to destroy the organization that is behind all life phenomena. 



