The Living Materials 59 



Summary 



Examination of the history of plant tissue cul- 

 tures shows that only those tissues have so far 

 been successfully cultivated which possess a fun- 

 damentally meristematic character and do not 

 have to undergo any major degree of de-differen- 

 tiation. Plant meristems are, for the most part, 

 of three main types. Apical meristems, particu- 

 larly root tips, have been cultivated extensively 

 and most of the details of technique have been 

 developed through their study. Intercalary meri- 

 stems such as cambium have also been successfully 

 grown, as undifferentiated masses or with their 

 differentiation experimentally controlled. Egg- 

 cells and embryos, insofar as they have been 

 grown in culture, have given rise to complete 

 plants. Such cultures can be of importance in the 

 study of the origin of form and function only 

 insofar as the degree and direction of their dif- 

 ferentiation can be controlled. Other tissues, 

 such as young ovaries, placental tissues, etc., have 

 been studied to a lesser degree and have in some 

 cases proved useful for special purposes. 



