129 



Fragmentation and replica-making can be combined with shadowing by 

 means of metal-evaporation, although this treatment is too rough in studies 

 of cytoplasm etc. (silver, or aluminium, is often used in light microscopy, 

 palladium in electron microscopy). The thickness of the metal film must 

 be calculated with care, as must also the angle at which the metal atoms 

 are to strike the material (cf. micrographs in Wyckoff 1949, p. 110, 121, 

 and 227-232). In high resolution electron microscopy the method has now 

 almost lost its importance since sufficient contrast is preferably obtained 

 by using suitable electron stains. 



A particularly penetrating evidence concerning the structure of the wall 

 will, no doubt, ultimately be derived from studies of ultra-thin sections. 

 Various types of fine structure in the resistant part of pollen and spore 

 walls have recently been described by various authors. At a resolution of 

 about 30 A (cf. EMG, Figure 142, page 77) acetolyzed sporoderm seems 

 to consist of small granules, 50-60 A in diameter. They are either arranged 

 in lamellae, with the granules in a single layer, or grouped into an amorphous 

 type of structure. The lamellate fine structure can be transformed into the 

 latter type of structure by means of oxydation, as has been proved in 

 Lycopodiiim clauatum (Afzelius 1956). Hitherto, attention has mainly been 

 centred on the fine structure of the exine. Electron microscopy may, 

 however, also help to reveal the fine details in non-acetolyzed sporoderms 

 and the interior of pollen grains and spores, as shown by some electron 

 micrographs by D. von Wettstein reproduced in PI. IV and V. The following 

 constitutes von Wettstein's description and interpretation of his E]\IG:s: 



The spore wall of Funaria hygrometrica is divided into three layers, intine, 

 inner exine, and outer exine, each of which is structurally homogeneous 

 and cannot be subdivided. The intine consists of a three-dimensional frame- 

 work of fibrillae (probably cellulose). In properly fixed material the cyto- 

 plasm of the spore is in close contact with the intine and obviously not 

 separated from it by a membrane (PI. IV, Fig. c). The outermost layer 

 of the cytoplasm seems to consist of a framework of fibrillae related in 

 size, but not in chemical composition, to that of the intine. If, in the course 

 of fixation, the cytoplasm is separated from the intine, some sort of mem- 

 brane appears in the micrographs. The inner layer of the exine in Funaria 

 displays a higher electron density than the intine. A kind of fibrillar struc- 

 ture seems to be present although obscured by a substance between the 

 fibrillae. The intine/exine limit is marked by a fine and dense zone about 

 50-100 A in thickness (PI. IV, Fig. c). In PI. IV, Fig. b, the exine seems to be 

 separated from the intine by a special, comparatively thick, layer. Ob- 

 viously, a corresponding layer is lacking in Figs, a and c in the same plate. 

 These, however, exhibit radial, strictly perpendicular sections through the 

 wall, whereas Fig. b exhibits an oblique section, as can be inferred from the 



