128 



structure of the pollen wall in Clivia miniata (Afzelius 1955) criticizes the 

 use of several terms to indicate different parts of this continuous layer. If 

 attention is paid to fine structure only this criticism seems to be justified 

 in the case of Clivia, and many other pollen types. There are, however, 

 exceptions. The basal part of sexinous rods in pollen grains of Cobaea 

 penduliflora (Erdtman 1952), Ciiciirbita (Miihlethaler 1955), Linum, and 

 some other genera (Erdtman 1956 b) is fixed well into the nexine. Studies 

 of ultra-thin sections, suitably stained, would probably shed more light on 

 these and similar problems, and make possible further contributions to the 

 elucidation of the morphogenesis of the sporoderms. 



Electron microscopy 



There are practically no pollen grains or spores small enough to make 

 possible a study of unsectioned grains or spores in the electron microscope. 

 However, some efforts along these lines have in fact been made (cf. e.g. 

 Burton 1946). Ueno (1949, and personal communication 1956), isolated 

 "viscin threads" of Oenothera odorata and Gaiira lindheimeri etc., and studied 

 them, without further preparation, by means of electron microscopy. 

 These threads, or part of them, probably consist of the same material as 

 the exine, since they are not dissolved if heated in an acetolysis mixture 

 (cf. Erdtman 1952, Fig. 170B:b, p. 292, and Fig. 171 A, p. 293). 



Fragmentation of material, by means of ultra-sonics or other methods, 

 was often carried out before the introduction of ultra-thin sectioning 

 techniques (cf. the spine of a malvaceous pollen grain. Fig. 151 A, p. 262, 

 in Erdtman 1952). It is a rapid method of preparation, but rather rough 

 and unsatisfactory. Many fragments will be too thick and thus instantly 

 charred by the electron beam; they defile the microscope and the method 

 cannot be used in high resolution microscopy. Although now outmoded in 

 electron microscopy, this technique can still be of some use in ordinary 

 microscopical investigations. The interpretation of the micrographs is 

 easier if the material consists of one or a few structural units only, as when 

 dealing with acetolyzed exines. 



Replica methods are often useful in the study of the surface details of 

 pollen grains and spores. Sitte (1953) has modified the wax impression 

 method by Mahl (1942), whereas Bradley (1954, 1957), and Miihlethaler 

 (1955) have used, with much success, a carbon replica method devised by 

 Bradley. Bradley has thus provided new information concerning the colpi 

 in the pollen grains of Lamium album; he has also found that the spores of 

 Riissula verrucosa are covered with a thin amyloid film. 



