TISSUE CULTURE 87 



conditions (Fig. 66), After the root is quite Jong, 1 cm. of its 

 tip may be snipped off and put in a fresh solution where it con- 

 tinues to grow as before. Side rootlets are formed just as in the 

 natural condition. But always only rootlets, never more than 

 this and never undifferentiated tissue. P. R. White has kept the 

 embryonic tissues of young roots actively growing for 32 months 

 (and they are still growing at the time of this writing) but again 

 always rootlets are formed and never fully developed roots. 

 Tissue culture generally implies the growing of cells which show 



Fig. 66. — A detached 16-cm. root which has, in seventeen days, developed in 

 culture from an extirpated 1-cm. root tip. 



little or no differentiation, but this is not always necessarily 

 the case; thus, kidney and pancreas keep to type. White 

 regards his plant cultures as much tissue cultures as are those of 

 kidney and pancreas which show differentiation, because fully 

 developed organs, i.e., mature roots, are never formed (secondary 

 growth, e.g., of xylem, is not produced). 



There is another type of plant culture which has a resemblance 

 to tissue culture, namely, that of myxomycetes, the slime molds. 

 Only recently has it been possible to grow this primitive form of 

 life continuously in the laboratory. It is grown on agar con- 

 taining oatmeal. The culturing of slime molds is not very 

 different from the growing of any fungus, but as it can be grown 

 vegetatively without interruption much like tissue cells in 

 culture, it is worthy of mention here, if for no other reason than 

 that it, even more than animal tissue cultures, offers an abund- 

 ance of undifferentiated protoplasm for physiological study 

 (Fig. 1). 



