The irregular distribution of virus particles in systemically infected plants is well known for some viruses. Limasset & Cornuet (1949) showed that tobacco mosaic virus (TMV) concentrations increase as a function of the distance from the shoot apex, and Solberg & Bald (1963) demonstrated a downward-increasing concentration gradient from the apical meristem for TMV in systemically infected Nicotiana glauca Grah.