200 Plant Tissue Culture 



been cultivated in excised roots of tomato (White, 

 1934, 438, 1936, 439). (See Chapter VI.) Dis- 

 eases which cause only moderate local injury and 

 which move rapidly into all parts of the host can 

 be maintained indefinitely in tissue cultures with- 

 out difficulty and without essential change in the 

 methods. Here they produce no macroscopically 

 visible symptoms. By aliquot titration, it is pos- 

 sible to follow the rate of multiplication of the 

 virus as a function of host-tissue multiplication 

 (Fig. 59) and to show that the two rates are inde- 

 pendent of one another. Stanley (1938, 437) has 

 used such cultures as a source of virus in order to 

 demonstrate the essential identity of the chemis- 

 try of virus protein produced in these chlorophyll- 

 free tissues and of that derived from the usual 

 leaf tissues with their high chlorophyll content and 

 activity. Viruses which cause a high degree of 

 local injury and which have a slow migration rate 

 in the tissue are somewhat more difficult to main- 

 tain in culture. A considerable amount of older 

 tissue must be included in every explant along 

 with the usual meristem, since the meristems 

 themselves are often virus-free. Otherwise, the 

 virus may be left behind in the discarded tissue 

 and a strain of healthy tissue segregated for culti- 

 vation. Such healthy strains have been segre- 

 gated from cultures carrying "cucumber mosaic" 



