BOTANICAL APPLICATIONS 



Fig. 1. Section through part of a pollen wall (acetolyzed and chlorinated) of Rhododendron 

 ponticum, XHflOO. (Afzelius, by courtesy of Grana Palynologica) 



research. The two main techniques employed 

 have been thin sections and surface repUcas, 

 the former being particularly valuable in 

 plant cytology where internal detail is of 

 interest, and the latter being mainly applica- 

 ble to taxonomic and morphological studies. 

 In addition it is possible to obtain informa- 

 tion in a number of particular cases using 

 direct examination. The various results ob- 

 tained in different fields of botany will be 

 briefly described here. 



Paly no logy. Pollen morphology has been 

 fairly widely studied in the electron micro- 

 scope. The results may be of interest in fields 

 other than botany, for example, in studying 

 the history of post-glacial flora by means of 

 pollen analysis. In addition it may be of in- 

 terest to those working on such problems as 

 hay fever and asthma. 



Two specimen preparation techniques, 

 namely sectioning and replicas, provide dif- 

 ferent pictures of the sporoderm. Thin sec- 

 tions provide a great deal of information 

 concerning its stratification, but in order to 

 obtain a complete picture of the sporoderm 

 of a given pollen grain it is desirable to have 

 a knowledge of the surface topography in 

 addition to sub-surface stratification. Earlier 

 work on pollen grains was confined to the 

 study of thin sections in the electron micro- 

 scope (1, 2, 3). 



It is difficult to prepare sections of pollen 

 cell walls because they are extremely hard. 

 However, advances in the technique of ultra- 

 microtomy have permitted a considerable 

 amount of information to be obtained in this 

 way (4). The stratification of the sporoderm 



Fig. 2. Shadowed carbon replica of the surface 

 of a fresh pollen grain of Rhododendron ponticum, 

 X7000. 



is an extremely complex subject and cannot 

 be discussed in detail here. 



An interesting study of both sections and 

 replicas of pollen grains has been carried 

 out by Miihlethaler (5), who interprets elec- 

 tron micrographs of sections and carbon 

 replicas (6) in terms of existing terminology 

 on pollen morphology. An interesting com- 

 parison between the electron micrographs 

 obtained by different authors of the same 

 type of pollen grain is shown in Figures 1 

 and 2. Figure 1 shows a section through part 

 of an acetolyzed and chlorinated pollen wall 

 of Rhododendron ponticum (4), taken by 

 Afzelius. This can be compared directly with 

 the carbon replica of Rhododendron ponticum 

 taken by the author and shown in Figure 2. 

 It can be seen that the section shows indica- 

 tions of a surface structure which is similar 

 in character to that revealed clearly in the 

 replica. 



The potentialities of the electron micro- 



81 



