The Unicellular Organisms. S 



most important characteristics of living matter; and in all 

 living beings the same phenomena are observable, in addition 

 to respiration, which is the taking in of oxygen.^ 



By these characters living may be distinguished from non- 

 living matter. This living matter, " the physical basis of life," 

 or protoplasm, as it is usually called, is a viscous semifluid 

 substance with granules interspersed. A certain vagueness of 

 meaning has at times attached itself to the expression proto- 

 plasm. It must not be regarded as a substance of definite 

 chemical composition. It is a mixture of various substances, 

 whose exact relations cannot, from the nature of the case, be 

 accurately ascertained. For in order to manipulate it the 

 protoplasm must be killed; and dead protoplasm is an 

 altogether different thing from living protoplasm. In dead 

 protoplasm the actual elements which compose it can, of course, 

 be accurately enumerated; these have been found to be 

 carbon, hydrogen, oxygen, nitrogen, sulphur, and a few 

 others, such as calcium, phosphorus, potassium, sodium, 

 magnesium, and iron. The first five are combined to form 

 various proteids, i.e. albumens, globulins, etc. But any 

 account of the chemical nature of the substances would take 

 too long a space, and requires a detailed treatment at the 

 hands of a chemist. Attention, therefore, will be directed 

 only to the bare outline given above, and to the fact that 

 protoplasm is not a chemical but a morphological expression 

 for a complex substance exhibiting the properties already 

 referred to. 



Recent microscopical research into the nature of protoplasm 

 has revealed the fact that it has a definite structure, that its 

 particles are disposed in a regular fashion ; but the interpretation 

 of the observed facts has differed greatly. The two principal 

 views of the constitution of protoplasm are known respectively 

 as the " network theory " and the " foam theory." According 

 to the first view, the protoplasm is disposed in a network of 

 denser protoplasm, the meshes of which are filled by the more 



' This function is separated from the others, since the talcing in of free 

 oxygen (= respiration) is not quite absolutely universal. Thus there are 

 the anaerobiic bacteria, which not only do not take in free oxygen, but are 

 killed by it. They must obtain their oxygen from compounds. 



