PROTOPLASM AND THE CELL 



53 



General Characteristics of Protoplasm and the Material of the Cell 



To begin with, it may be said that this substance has a variable 

 degree of fluidity under different conditions. The range of this 

 variation may be from semisolid to semiliquid. It is viscid and 

 gelatinous in consistency. It is more or less granular, nearly color- 

 less, and more or less translucent; however, it is never perfectly 

 transparent. The trauslucency causes a mass of it to have a lustrous 

 gray appearance. As a constituent of protoplasm there is always 

 a considerable percentage of water, which conditions the degree of 

 viscosity. 



WT. 





Km / •" 

 •• '■"■ ..' *'■ ■ 



J'y- '''■■. 



: i->. -v ♦•"•<.*. '-^J'.' S ■■■•■■ .'.•■..■■ 







' ... i- --•■.-.,■.■ -- f 



. >v^^._.^.J^"^-?'^•%^,^ 



* ' * - . 



Fig. 19. — structure of living protoplasm as seen in tlie starfish. (From Wilson, 

 The Cell, published by The Macmillan Company.) 



Protoplasm is in a colloidal state of the emulsoid type. A colloid 

 is a substance of gelatinous nature, permeable by crystalloid solutions, 

 and diffusing not at all or very slowly through animal or vegetable 

 membranes. In the emulsoid, or colloidal emulsion, the substances 

 are distributed through the more watery or dispersion medium. 

 A colloid is identified by the presence of particles which are groups 

 of molecules dispersed through a more fluid or watery phase. These 

 particles, of course, are larger than molecules, but they are too small 

 to be seen with the ordinary microscope. It is possible for water and 

 substances in solution to enter protoplasm from without, and this is 

 reversible. With loss of water from the dispersion medium the dis- 

 persed particles of the colloid become congested by loss of general 

 fluidity. This condition is known as the gel state. When there is 

 increased water in the dispersion medium and the particles move with 

 greater ease in the more fluid medium, the colloidal state tends to be- 



