366 



CHLOROPLASTS AND CHROMOPLASTS 



CHAP. 14 



In the living cell, the chloroplasts exhibit a negative monoaxial double 

 refraction (the optical axis being normal to the surface of the chloro- 

 plast). Figure 44 illustrates this phenomenon, while the position of the 

 index ellipses is shown by figure 45. The negative double refraction 

 disappears upon imbibition with glycerol, and is thus a movphic bire- 

 fringence, which can be explained by a laminar structure, the planes of 

 the laminae being oriented normally to the short axis of the chloroplasts. 



rA 



6 



(a) 



(b) 



Fig. 45. — Morphic birefringence of chloroplasts (after Menke 1938). (a) Position 

 of index ellipses in a chloroplast of Mougeotia; (b) same in a chloroplast of a higher plant 

 in front view and in profile. A is the optical axis. 



Further proofs of this structure were derived by Menke (1938) from 

 observations of dichroism. The dichroism of the chloroplasts in the 

 natural state is weak but detectable, particularly in monochromatic red 

 light where the birefringence has its maximum (at 681 m^u). Chloro- 

 plasts fixed by Menke and Kiister (1938) with osmic acid and then 

 impregnated with gold chloride exhibited a stronger dichroism. They 

 appeared bluish when their short axes were parallel to the plane of 

 polarization of light, and orange when they were perpendicular to this 

 plane. This result can be explained by the assumption of submicro- 

 scopical laminae upon which gold is deposited in thin layers. 



When the chloroplasts are imbibed with glycerol, the negative double 

 refraction caused by the laminar structure disappears (because of the 

 equalization of the refractive indices of the laminae and of the interstices). 

 Instead, a positive double refraction appears which must be an intrinsic 

 property of some regularly arranged anisotropic molecules. Menke and 

 Frey-Wyssling ascribed it to an array of elongated lipide molecules. 

 Dried chloroplast matter shows a positive double refraction; extraction of 



