44 Plant Tissue Culture 



matic capacity throughout most of the life of the 

 plant (tTlehla, 1928, 115; MacDougal, 1926, 307; 

 MacDougal and Shreve, 1924, 445; Sinnott and 

 Bloch, 1941, 324). Epidermal tissue does so ordi- 

 narily for somewhat shorter periods (Nay lor, 

 1931, 308 ; Naylor and Johnson, 1937, 309 ; Naylor 

 and Sperry, 1938, 310), while such cells as tri- 

 chomes, guard cells, xylem fibers, etc., retain this 

 capacity for only very brief periods (Haberlandt, 

 1913, 294, 1914, 295, 1919, 245, 296, 1920, 297, 1921, 

 354), because of mechanical modifications if for no 

 other reason. It is probable that means may ulti- 

 mately be found for obtaining cultures from most 

 living, nucleated cells. But the initial work in any 

 discipline is quite properly concentrated on the 

 easier materials available and the less promising 

 ones may well await the establishment of more 

 adequate techniques. Kecent work in this field 

 has, therefore, been concentrated on the meri- 

 stems. 



Plant meristems can, for the most part, be clas- 

 sified in three main groups, leaving a rather large 

 residue of those more difficult to classify. 



The fertilized egg is, of course, the meristematic 

 cell par excellence, since there can be no question 

 about its fundamental totipotency. It is, how- 

 ever, an extremely delicate "organism," (Fig. 

 20) normally hedged around by all sorts of protec- 



