96 Plant Tissue Culture 



to be employed. Sucrose crystallizes easily so 

 that even the commercial sugar to be bought in 

 every grocery store is usually as pure or purer 

 than most "C.P. Grade" chemicals, though per- 

 haps less reliable. The use of "C.P. Grade" is 

 to be recommended for all critical studies, as a 

 precaution. "C.P. Grade" sucrose usually con- 

 tains some calcium and magnesium, but the quan- 

 tities of these two elements required for normal 

 growth of plant tissues are so high that the traces 

 likely to come from the sugar are too small to be 

 of probable significance. Its content of copper 

 and zinc may possibly be significant. It does not 

 appear to contain significant quantities of any of 

 the vitamins known to be important for the activ- 

 ity of plant tissues. (Compare Ebeling, 1914, 

 271, 1936, 174; Lewis, 1922, 175.) 



Vitamins and hormones. Four oligodynamic 

 organic substances have been reported to be im- 

 portant for the growth of excised plant tissues. 

 Fortunately, all four of these substances are avail- 

 able as synthetic products of high purity. 



The only universally necessary member of this 

 group seems to be thiamin (vitamin B x ) (Bonner, 

 1937, 198, 199; Robbins and Bartley, 1937, 222, 

 223 ; White, 1937, 231 ; Gautheret, 1938, 69 ; Nobe- 

 court, 1938, 75). Thiamin or its precursors have 

 been shown to be essential for, or at least highly 

 beneficial to, the growth of a great variety of plant 



