74 Introductory Biology 



small step in advance. Progress is a series of such consecutive, progres- 

 sive advances. What one scientist discovers may be the stepping-stone 

 for the discoveries of other scientists. 



What are some of the physical properties of protoplasm when viewed 

 with high magnifications of the microscope? Protoplasm usually ap- 

 pears to be a colorless, odorless, jellylike material, with granules and 

 globules of various shapes and sizes, which is constantly varying in ap- 

 pearance and consistency. Many colorless structures are rendered more 

 visible by the application of various dyes which stain certain parts and 

 not others. Protoplasm is slightly heavier than water because of the 

 additional substances of which it is composed. It is a somewhat viscous 

 semifluid which under certain conditions may display internal, flowing 

 ("streaming'') movement. Protoplasm diflfers in consistency at different 

 times and also in appearance. The same protoplasm may appear quite 

 different when studied by different methods. When the inherent varia- 

 bilities of protoplasm and the different methods of studying it are taken 

 into consideration, we may have at least partial explanations for the 

 differences of opinion among the various investigators as to the physical 

 structure of protoplasm. 



Theories Regarding Physical Structure 



Some of the theories which have been proposed from time to time re- 

 garding the physical structure of protoplasm are (1) granular (gran' u- 

 lar) (L. granum, grain), in which aggregates of minute granules are 

 distributed in a liquid medium (Fig. 24, A); (2) fibrillar (fi -bril' ar) 

 (L. fibrilla, small fiber), in which small fibers are present in a liquid 

 medium (Fig. 24, B) ; (3) reticular (re -tik' u lar) (L. reticulum, little 

 net), in which the fibers appear as a network embedded in a liquid 

 material (Fig. 24, C) ; (4) alveolar (al'veolar) (L. alveolus, small pit, 

 or hollow), in which a foamlike mass of minute, spherical bubbles are 

 embedded in a more viscid medium (Fig. 24, D) ; (5) colloidal (kol- 

 oid' al) (Gr. kolla, glue; eidos, form), in which the many complex sub- 

 stances of protoplasm are present in a finely divided, or colloidal condi- 

 tion (Fig. 24, £). 



Which of these theories is correct? Or, are they all correct in part, 

 depending upon the method of investigation used and the particular 

 characteristic displayed by the specific protoplasm being studied at a 

 certain time? The chemical and physical changes which are constantly 

 going on in protoplasm probably explain the different appearances of 

 the different protoplasms and even the variations in appearance in the 

 same protoplasm from time to time. 



