2 Plant Tissue Culture 



and behavior. The sociologist asks it at the level 

 of races and nations, tribes, sects, and societies. 

 The chemist asks it at the level of "substances" 

 and the physicist at the level of material bodies 

 great or small, be they galaxies or particles of 

 light. The geneticist and taxonomist ask it con- 

 cerning the segregation and delimitation of char- 

 acters of related organisms. With regard to 

 single organisms, we ask ' ' How is it that the size, 

 form, texture, rate of development, function of the 

 various parts are determined and limited? "Why 

 does an anterior limb-bud give rise to a hand while 

 a posterior limb-bud gives rise to a foot? Why 

 does a cell of a given type, when chance places it 

 in one region of the body, become an element of 

 the glandular epithelium of the kidney, while in 

 another part of the body the same type of cell 

 appears as a constituent of the iris? Why does 

 the sub-epidermal cell of a foliar leaf become a 

 center of food synthesis, while in a floral leaf it 

 becomes a megaspore-mother-cell with a repro- 

 ductive function? Why do the cells surrounding 

 a wound in a "normal" individual take part in 

 an orderly granulation, closing the wound, form- 

 ing well-limited scar tissue, organizing a harmon- 

 ious replacement of the injured regions, while in 

 an individual carrying a cancer, even at some dis- 

 tant part of the body, cells of the same region may 



