PROTOPLASM AND CELLULAR ORGANIZATION 



15 



servations on naked living protoplasm. A few 

 bodies can be seen in living protoplasm, but 

 most of the structures are practically color- 

 less. This makes it necessary to treat it with 

 dyes which stain certain parts. Many differ- 

 ent dyes have been employed and numerous 

 methods have been devised for the study of 

 protoplasm. While most of these result in 

 the death of the protoplasm, the structure is 

 probably not changed very much. 



When examined with a microscope, pro- 



toplasm usually looks like a grayish jelly in 

 which may be embedded granules and glob- 

 ules of various sizes and shapes. It differs 

 under various conditions; usually it is about 

 the consistency of glycerin, somewhat vis- 

 cous but capable of flowing. Protoplasm may 

 exist as a sol that streams easily, or as a 

 more solid gel; under certain conditions it 

 may change from a sol to a gel, or a gel to 

 a sol, and back again; this is the unique prop- 

 erty of a colloid. 



sc»«m»b.Ci^ 



Figure 4. Colloidal states. Ultramicroscopic structure of a sol and a gel (diagrammatic). Left, 

 a sol state. The colloidal particles are represented as circles of different diameters and the water 

 particles (molecules) as dots. Such a solution has the physical properties of a liquid. Right, a gel 

 state. The colloidal particles adhere together to form a continuous network. Such a substance 

 has the physical properties of a semisolid substance (jellylike), which tends to be elastic. The 

 arrows show that the sol and gel states are reversible under appropriate conditions. 



Many minute granules can be seen in 

 protoplasm with the aid of a microscope. 

 When the protoplasm is in a liquid or sol 

 state, the granules may be observed moving 

 about. This is known as Brownian move- 

 ment, having been discovered by an English 

 botanist, Robert Brown, in 1827. This type 

 of movement is due to invisible particles 

 striking against larger granules. It also oc- 

 curs in water and other liquids and is not 

 necessarily a sign of life. 



Certain knowledge of the fine structure 

 of protoplasm has been contributed by the 

 physical chemists. They tell us that proto- 

 plasm is a colloid and all life is associated 

 with the colloid state. Many of the prop- 

 erties of protoplasm depend on the fact that 



it is a colloid, a mixture in which compara- 

 tively large but still invisible particles are 

 suspended in a liquid medium, that is, they 

 do not settle out. The particles are estimated 

 to range in size from O.OOOI to O.OOOOOI mm. 

 in diameter. Colloid suspensions often have 

 a sticky, gluelike consistency; this accounts 

 for the name, which comes from a Greek 

 word that means glue. Changes in proto- 

 plasm from a sol to the gel condition and 

 back again may be explained on the basis of 

 the distribution of the colloid particles. If 

 the particles are more or less evenly distrib- 

 uted in a liquid medium, as in Fig. 4, the 

 mixture flows easily and is in the sol state, 

 but if the particles are arranged so as to 

 form a meshwork, with the liquid medium 



