THE CHLOROPLAST 109 



ordered of these is the smectic, where the molecules are arranged in 

 equidistant parallel layers. Robinson (1958) has shown that poly- 

 adenylic acid and polyuridylic acid, when mixed under the right 

 conditions, show a specific interaction, forming crystals. Robinson 

 and Ward ( 1957 ) have also shown that when the L and D forms of 

 the synthetic polypeptide, polybenzoylglutamate, are mixed in 

 dioxan, they behave as a liquid crystal. Although the solution was 

 birefringent, no regular orientation was observed with the polar- 

 ization microscope; but after a short time, orientation appeared on 

 the walls of the capillary and spread toward the center with a very 

 regular orientation. 



If digitonin is caused to flow through a capillary, it becomes 

 birefringent when observed through crossed polaroids; a similar 

 phenomenon is observed with chloroplastin. This is due to the 

 orientation of the particles in the flowing stream. The chlorophylls 

 then become oriented within the digitonin micelles. If chloroplastin 

 is allowed to stand, no birefringence is observed. However, a liquid 

 crystalline phase will separate out; under certain conditions, bire- 

 fringent rodlike fibers (tactoids) form. These observations are simi- 

 lar to those made on tobacco mosaic virus (Lauffer et at, 1949). 



Quantitative theories have been proposed to explain liquid crys- 

 tals as being the result of anisotropic interactions between long, rod- 

 like molecules. These theories have the essential feature that long 

 rods can be packed economically in a given space only if they are 

 aligned. Hence, a concentrated solution of rodlike molecules tends 

 to form liquid crystal structures, but in dilute solutions, isotropic 

 structures occur (Zimm, 1959). For a more complete discussion 

 of liquid crystals, reference should be made to the Discussions of 

 the Faraday Society (1958) and Transactions of the Faraday So- 

 ciety (1933'). 



These studies suggest that chloroplastin possesses properties of 

 a liquid crystal and because of these properties, structural as well as 

 photochemical integrity is maintained. Its behavior then is similar 

 to that of the in vivo chloroplast. The lamellar structure may be an 

 efficiency mechanism rather than a critical functioning device. How- 

 ever, this does not exclude the probability that a liquid crystal type 

 of matrix is necessary for orientation of the active pigment complex. 



