131 



seem to be answered in the affirmative by Turian and Kellenberger (1956), 

 whose recent studies, together with those of von Wettstein, are certainly 

 Ihe most penetrating analysis of phuit cell ultra-structure known to liie 

 present author. 



Polarization microscopy. X-ray diffraction, UV -microscopy and 



their possible application 



Polarization microscopy seems to be of little use in the study of acetolyzed 

 exines; a form birefringence has not been revealed and it has equally not 

 been possible to trace the fine lamination of the exine in Lycupodium in 

 this way. Probably the lamellate layer is too thin to provide an effect in 

 the polarization microscope. Freytag (manuscript Sept. 1956) has shown 

 that an outer non-sporopolleninous layer can be demonstrated in some 

 pollen grains by means of polarized light [cf. also electron microscopical 

 studies of a similar layer by Afzelius (1955) and Bradley (1956)]. Likewise, 

 X-ray diffraction does not provide any information about the exine and, 

 therefore, seems to be of no use for obtaining information on the fine 

 lamination in Lycopodium chwatiim. This is possibly due to the fact that 

 the laminated layers are not parallel but curved and undulating (Engstrom, 

 personal communication 1955). Engstrom has investigated dry, non- 

 acetolyzed pollen grains of Almis ghitinosa and spores oi Lycopodium clava- 

 tum by means of low angle diffraction without obtaining positive results. 

 On the other hand, Labouriau and Cardoso, when investigating Li/copor/Zf/m 

 exines by means of wide angle diffraction patterns, encountered a crystalline 

 structure, a fact which seems difficult to explain (Acad. Bras. Cienc. 1948). 



UV-optics have chiefly been used in microspectroscopical investigations. 

 Wilkins (1953) describes the absorption difference between the two types 

 of nuclei in the pollen grains in a UV-micrograph obtained by means of 

 reflection microscopy. 



Interference microscopy 



The light passing through small objects in a microscope will be delayed 

 in comparison to that passing through the surrounding medium, if the 

 object, as is generally the case, has higher refractive index than the sur- 

 rounding medium. The interference microscope aims at measurements of 

 the "optical path difference" (OPD, i.e., the difference in ()i)lical path 

 through the object and an equal thickness of the surrounding medium, 



