144 Plant Tissue Culture 



tures may be built up by a recurrent cyclic re- 

 utilization of materials leached out of the older 

 inactive or even dead tissues. The increment 

 figures obtained at the end of a long period may 

 thus give no true measure of the level of meta- 

 bolic activity of the tissue involved. It is doubt- 

 ful whether the advantages of a long culture 

 period are not greatly outweighed by its disad- 

 vantages. 



Culture conditions required. The requisite 

 characteristics of the nutrient have been discussed 

 elsewhere. Cultures are apparently for the most 

 part relatively insensitive to light and can be 

 placed either in darkness or in diffuse light 

 (Robbins and Maneval, 1924, 258; White, 1932, 63, 

 1937, 265; Malyschev, 1932, 53, 54; Gautheret, 

 1935, 15). They should never be exposed for long 

 periods to direct light as they are extremely sen- 

 sitive to high temperature and to temperature 

 changes and could easily become overheated 

 (White, 1932, 63, 1937, 265) . It is highly desirable 

 to have the temperature of the culture chamber 

 and transfer room rather accurately controlled. 

 Root tips of wheat have an optimum temperature 

 of about 27° (White, 1932, 63), tomato nearer 30° 

 (White, 1937, 265). The range between optimum 

 and maximum is, however, very narrow and a 5° 

 rise above these temperatures, if maintained for 



