-60- 

 for orchid viruses. The time period for the 

 polymerization of LR White is also much shorter than for 

 Lowicryl . In addition, this embedding medium possesses 

 another advantage of being very easy to use. 



The protein A-gold technique is simple, sensitive, 

 reliable, and gives specific labelling of high reso- 

 lution. When compared with other immunocytochemical 

 techniques, it has repeatedly provided superior results 

 (Bendayan, 1984; Beesley et a_l . , 1982). It also dis- 

 plays several advantages: antibodies raised in different 

 mammalian species can be used; the preparation of 

 colloidal gold and the formation and purification of the 

 protein A-gold complex are quite simple procedures; and 

 the use of gold particles as the electron-dense marker 

 allows for easy identification of the labelled structures, 

 and makes the technique suitable for double labellings 

 (Roth, 1983). The reliability of the technique and its 

 wide range of applications in electron microscopy have 

 been clearly demonstrated (Bendayan, 1984). It is usually 

 very difficult to identify the spherical virus particles 

 scattered within the cells in ultrathin sections. This 

 study has shown that with the immunogold technique, 

 spherical virus particles can be differentiated. In BYMV- 

 infected pea tissues, there are some thin plate inclusions 

 which may be misidentif ied as BYMV cylindrical inclusions. 

 The composition of thin plates cannot be recognized by 



