142 Plant Tissue Culture 



best time for subculturing. A one-week culture 

 period has been chosen as best for routine work. 

 The average growth rate throughout the year, of 

 cultures grown in the laboratory without special 

 temperature control, is about 7 to 8 mm. per cul- 

 ture per day, being greater than this in summer 

 and less in winter (Fig. 26). Cultures begin to 

 branch on about the 3rd day and may have as 

 many as 30 branches by the end of a week. Some 

 of the decrease in linear increment after the 5th 

 day may be due to distribution of material into the 

 branches instead of its concentration in a single 

 axis. However, if, instead of using tips as inocula, 

 non-apical segments bearing branch primordia 

 such as are employed as stock cultures are used, 

 the branches, although at first somewhat retarded, 

 will when established increase at rates character- 

 istic for the particular species and for the nutrient 

 used, but independent of the number of branches 

 developing on the culture. This behavior makes 

 it clear that the rate of growth of a branch (and 

 presumably of the main axis as well) is not deter- 

 mined by the number of branches present and that 

 the retardation of growth rate after the 5th day 

 is not entirely due to diversion of nutrient mate- 

 rials to newly formed branches. 



While this laboratory has found a one-week cul- 

 ture period to be best adapted to the problems 



