148 Plant Tissue Culture 



Flask cultures and tube cultures are suitable for 

 experiments in which gross increase and habit of 

 growth are the features of chief interest and in 

 which, when further details are required, they can 

 be obtained by removing the cultures from the 

 original containers where, for example, they are 

 to be fixed and sectioned. These are, in general, 

 characteristics of the initial phases in the develop- 

 ment of any field of investigation. But these 

 methods do not permit the observer to follow the 

 behavior of individual cells except under condi- 

 tions which entail a more or less serious disruption 

 of the continuity of growth. Plant tissue cells do 

 not migrate along a solid substratum (Chambers, 

 1923, 43 ; Pf eiffer, 1933, 450 ; Scheitterer, 1931, 109, 

 and Gautheret, 1937, 282, notwithstanding), since 

 the cellulose pellicle interferes with the adhesive 

 qualities of the cells. It is, therefore, not possible 

 to use flasks of the Carrel-flask type for micro- 

 scopic examination under growing conditions. 

 For this purpose, hanging-drop cultures (see 

 Parker, 1938, 26), in spite of numerous objection- 

 able characteristics, appear to be best. This type 

 of culture is second only to the flask type in im- 

 portance in carrying out plant tissue cultures. 



In the typical hanging-drop culture a drop of 

 nutrient, either liquid or semi-solid, is placed on a 

 cover glass, the cover is inverted over some form 



