58 Plant Tissue Culture 



certain plant extracts (Fig. 35). White (1932, 

 89) and Moebius (1922, 105) have attempted to 

 grow immature ovules and Tukey (1937, 114) and 

 Borger (1926, 92) attempted to grow excised 

 placental tissue without marked success. There 

 are, in additon to these meristematic regions, cells 

 scattered throughout the plant body, such as the 

 medullary ray cells, the "xylem parenchyma" 

 cells, the phloem companion cells, which retain a 

 characteristic meristematic appearance long after 

 the neighboring cells have "matured," and are 

 capable of resuming growth when freed from the 

 inhibitory effects of the neighboring tissues. Cells 

 of the pith, of the cortex, of the epidermis, and 

 other tissues may also, under special and at pres- 

 ent little understood conditions, resume a meriste- 

 matic function (Reiche, 1924, 369; Okado, 1920, 

 312; Wilhelm, 1930, 338; Winkler, 1902, 339; Sin- 

 nott and Bloch, 1941, 324; Stingl, 1909, 326), but 

 until a great deal more is known about them than 

 at present they cannot be looked upon as satisfac- 

 tory material for plant tissue cultures. 



