Introduction 9 



as "tissue cultures." (See Harrison, 1928, 18). 

 (See also references 1-41.) 



But it is not always given to one individual 

 both to formulate the broad outlines of a new tech- 

 nique and to put that technique into actual prac- 

 tice. Haberlandt was faced at the start with a 

 series of major difficulties inherent in plant 

 anatomy and morphogenesis which it has actually 

 taken workers a third of a century to learn to 

 overcome. These difficulties are chiefly three. In 

 the first place, most cells of a plant are not bathed 

 in any free, complete nutrient medium. The 

 xylem sap lacks many of the organic constituents 

 needed to maintain life. The phloem sap is in 

 direct contact with only a very few specialized 

 cells. Most plant cells must obtain a large part 

 of their nutriment by diffusion through neighbor- 

 ing cells. There is no "natural" nutrient which 

 can be extracted from a plant and used for the 

 cultivation of its cells and for subsequent analysis. 

 In the second place, most plant cells, with rare 

 exceptions, are surrounded by a rigid pellicle. 

 This prevents the cells from seeking and engulfing 

 food, thus greatly restricting the forms of food 

 which they can use. It prevents their adhering 

 satisfactorily to any solid substratum. And, since 

 excision involves rupturing this pellicle and expos- 

 ing the protoplast naked to the surrounding 



