46 FUNDAMENTALS OF CYTOLOGY 



Streaming movements of various kinds appear to be of general occur- 

 rence in protoplasm. They have been studied chiefly in amebas, Plas- 

 modia of slime molds, dividing animal eggs, and the highly vacuolate 

 cells of plants. The movement may involve the entire protoplast, the 

 cytoplasm streaming as one mass and carrying the various inclusions 

 with it, or only localized portions may be concerned, the other regions 

 showing no visible change. No complete explanation of this fascinating 

 phenomenon has yet been given. With the aid of the motion-picture 

 camera it has been found that the streaming observed in the slime mold 

 results from a rhythmic contraction and relaxation of the protoplasm, 

 and the force involved has been measured. Contraction and relaxation 

 in protoplasm are now attributed mainly to a folding and unfolding of 

 linear protein molecules (see page 48). The energy necessary to proto- 

 plasmic streaming is evidently derived from respiration, but the manner 

 in which this energy is utilized in producing the movements is unknown. 



Protoplasm as a Colloidal System. — The physical properties of proto- 

 plasm are largely dependent upon the fact that it is a complex colloidal 

 system. Matter is in the colloidal state w^hen it has the form of numerous 

 small particles, the resulting properties being most characteristically 

 displayed when the particles are between about 0.1 and 0.001^ in at least 

 one dimension. This is below the reach of the ordinary microscope. 

 Such particles are molecular aggregates except perhaps in the case of 

 extremely large molecules. In a colloidal system at least two phases are 

 essential : a medium which constitutes the continuous phase and a second 

 substance dispersed as particles within it. The phases may be liquid, 

 solid, or gaseous, and they may have any chemical composition so long as 

 they are dissimilar enough to remain physically distinct. The chemical 

 constituents of a given phase are called com-ponents. The important 

 feature of all colloidal systems, which vary greatly in minute structure, 

 is that the phases lie in contact with each other over a surface of enormous 

 extent, even in a minute cell. This fact means much when it is remem- 

 bered that many reactions are promoted b}^ forces acting at surfaces. 



It is, of course, the fluid colloidal systems that are of particular 

 significance in biology. Such a system is known as a sol if it flows readily 

 and as a gel if it does not. It may be made to pass from one state to the 

 other, often by relatively small alterations in temperature, electrical 

 charge, or the degree of hydration. Thus a sol may become a gel (gela- 

 tion, pectization), and the gel may again become a sol (solation, peptiza- 

 tion). Irreversible coagulation may also occur. In a fluid colloidal 

 system most of the continuous phase is free and easily removable, but 

 some of it may constitute a denser solvation layer at the surface of the 

 other jjhase and strongly resist forces tending to remove it. When the 

 layer consists of water, the colloidal particle is said to be hydrated. 



