THE PLASTIDS 77 



changes in viscosity that are knoAvn to occur in cytoplasm, it accounts satis- 

 factorily for its hydrophilic properties, and it suggests a possible solution to 

 the perplexing problem of how so many diverse processes can go on in the 

 same cytoplasm at the same time. Many of these reactions may occur at 

 the interfaces between the various fibrillar units and the liquid constituents of 

 the system. Since different fibrillar units are undoubtedly of different chemi- 

 cal composition, different chemical and physical reactions could take place, 

 some at one interface and some at another. Furthermore, it is possible that 

 some reactions occur within the liquid component and others within the more 

 solid fibrillar units of the cytoplasm. 



The Plastids. — All living cells of the higher plants contain prominent 

 cytoplasmic bodies known as plastids. These structures are usually ellip- 

 soidal in shape and are frequently conspicuous because of the presence of 

 pigments. The color of the pigments in the plastids has been used as the 

 basis for their classification but this system is highly artificial since a single 

 plastid may be colorless, green, red or yellow at different periods of its exis- 

 tence. 



In meristematic and embryonic cells the plastids first appear as very tiny 

 granules in the cytoplasm that range in size down to the limit of visibilit}^ 

 (Randolph, 1922). These granules gradually enlarge and differentiate until 

 mature plastids are produced. In growing and in mature cells plastids fre- 

 quently multiply by simple fission. 



In the algae the chloroplasts exhibit a wide range of size and shape but 

 in the higher plants they show a remarkable similarity. The mature chloro- 

 plast of the higher plants is typically a slightly flattened ovate spheroid with 

 the longer axis ranging between 4 /j and 6 /x in length. The number 

 of plastids is not constant in different cells nor in the same cell at different 

 stages of its development. 



The structure of chloroplasts has been more thoroughly investigated than 

 that of other kinds of plastids. Most investigations indicate that the chloro- 

 plast consists basically of a proteinaceous matrix or stroma which is probably 

 surrounded by a membrane. In at least some chloroplasts the chlorophyll 

 apparently occurs in small disk-shaped granules called grana which are dis- 

 tributed throughout the stroma (Weier, 1938). The chlorophyll is ap- 

 parently associated with both proteins and lipids. The yellow pigments 

 carotene and the xanthophylls also occur in the chloroplasts. 



The chromoplasts contain red or )^ellow pigments and are frequently 

 very different in size and shape from the chloroplasts. Usually they are very 

 slender spindle-shaped or needle-shaped bodies. They occur both singly and 

 grouped in bundles. The irregular angular outline of these plastids contrasts 



