4 Plant Tissue Culture 



fundamental hypothesis of the so-called ''cell 

 theory," promulgated a century ago in the words 

 which are set at the head of this chapter (Schwann, 

 1839, 29* that all cells are essentially elementary 

 organisms, theoretically alike and capable of au- 

 tonomous existence. But there is an alternative 

 hypothesis which also rests upon a considerable 

 mass of observed facts, namely, that somewhere 

 in the developmental history of an organism its 

 constituent cells cease to be totipotent, by segrega- 

 tion and loss of certain functions. No mechanism 

 is known by which such segregation or loss can 

 take place, yet it so often appears to have taken 

 place that this alternative hypothesis has today 

 many adherents. 



If all cells of an organism are essentially alike 

 and, within the genetic pattern, totipotent, then 

 the differences in behavior of cells of a given type 

 in different situations in the body must result 

 from the interrelations of these cells with their 

 environments and with other cells in the organism. 

 It should be possible to restore suppressed func- 

 tions by isolating the cell from those external in- 

 fluences which were responsible for the suppres- 

 sion. If, on the other hand, there has been a true 

 segregation and loss of function so that the cells 

 are, in the mature organism, no longer totipotent, 



* Italic numbers refer to bibliography. 



