PROTOPLASM 7 



and its environment. The movements of materials through the 

 cell boundaries are believed to be in accordance with well-established 

 physical laws which govern the passage, or diffusion, of liquids and 

 gases through various types of membranes. 



4. Physical Nature of Protoplasm 



Having considered the cellular organization of protoplasm as 

 well as the main structural features of a typical cell, we are prepared 

 to go a step further and indicate certain general facts bearing on the 

 physical nature of protoplasm. As a matter of fact one can speak 

 only in very general terms on this subject for our knowledge at 

 present concerning the ultimate structural organization of proto- 

 plasm is limited. This is due to the fact that protoplasm cannot be 

 subjected to intensive methods of analysis without destroying the 

 primary object of the investigation, namely, the unique quality 

 designated by the term life. A chemical analysis of protoplasm 

 after death shows that a very large percentage consists of water 

 containing various chemical compounds. These compounds when 

 separated into their elements show that about 99 per cent of 

 the material present is derived from the following six elements : 

 carbon, oxygen, hydrogen, nitrogen, calcium, and phosphorus. 

 There is nothing peculiar about these elements. They are of com- 

 mon occurrence, and, therefore, it would seem that in living matter 

 there must be a unique arrangement of these materials which is 

 not found elsewhere in nature. 



Observations on protoplasm show that it varies considerably 

 in its physical appearance. Thus at one time it may appear 

 as a clear, rather thick liquid while at another it appears as a gelat- 

 inous mass. This variation in the consistency of protoplasm, 

 together with certain other characteristics, has led to the con- 

 clusion that it is of a colloidal nature. There are many types of 

 non-living colloids known, all of which are characterized struc- 

 turally by the presence of exceedingly minute particles — in many 

 cases too small to be seen, even with the highest powers of the mi- 

 croscope — dispersed throughout a liquid medium. Variations in 

 the relations between the particles and the liquid medium result in 

 changes in the state of the substance from a liquid (sol) to a solid 

 (gel), and this is apparently what takes place in protoplasm. As 

 might be expected from the structural variations occurring during 

 life, the appearance of protoplasmic material, which has been 



