368 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1940 



hairs. In general, these experiments may be considered as unsuccess- 

 ful and it was not until a few years ago that the French botanist 

 Gautheret succeeded in obtaining actively growing cultures of cam- 

 bial tissue of various plants. These may be rogarded as possibly the 

 first examples of true plant-tissue cultures. 



Without attempting any further review of earlier studies reference 

 will be made to only one recent research on plant-tissue cultures other 

 than roots. It has been found by 'Wliite that small fragments of tis- 

 sue excised from a plant callus may be cultured on a semisolid agar 

 medium. Such cultures are illustrated in plate 3, figures 1 and 2. 

 Except for the production of an occasional scalariform cell no evi- 

 dence of cellular differentiation or of polarity is exhibited by callus 

 cultures on such a medium. If, however, a culture that has been 

 maintained in an undifferentiated state during 10, 25, or 40 weekly 

 transfers is then placed in a liquid nutrient (the same solution minus 

 the agar), a marked transformation occurs within a few weeks (pi. 

 3, fig. 3). Growing points differentiate at the surface of the cell 

 mass and develop into stems and leaves in exactly the same manner 

 as in similar calluses when attached to the plant. 



The simple expedient of immersing the tissue mass under about 8 

 millimeters of liquid instead of exposing it on the surface of the me- 

 dium has thus resulted in a j^rofound alteration of the developmental 

 behavior. It has not as yet been determined just what factor is 

 responsible for this effect; the most obvious difference between the 

 two media is the restricted oxygen supply in the liquid culture. The 

 significant point in this work is that the capacity for differentiation 

 of the cells has not been lost but can be suppressed or evoked by 

 simple technical manipulation. There is hereby opened up another 

 avenue for the investigation of the developmental potentialities of 

 the cell. 



In this short review it has been attempted to call attention to the 

 outstanding problems and achievements of the study of plant-tissue 

 cultures. With the establishment of a secure experimental foundation 

 one may confidenth'^ venture to predict a rapid and fruitful develop- 

 ment of this phase of botanical research. 



