Groivth Substances and Plant Tissue Cultures 



683 



some not atfected in their growth. However, the color of all the bean 

 cultures is darkened, from white to light yellow, from light yellow to 

 dark yellow, from dark yellow to brown, depending on the color of 

 the untreated tissue. Apparently there is an effect on one or more 

 enzvme systems involved in pigment formation that is unrelated to 

 the growth effects. 



Radley and Dear (9) have shown that coconut milk contains gib- 

 berell in-like substances as determined by the dwarf pea test. This is 

 mentioned because many of the tissues used in the present work were 

 grown on media containing coconut milk. The levels reported by 

 Radley and Dear are for concentrated coconut milk. When the gib- 

 berellin-like activity of unconcentrated coconut milk is calculated 

 from their data, the level is below the amount necessary to obtain a 

 growth response by a standard dwarf pea test. This level is also be- 

 low that to which the tissues respond. 



It is possible that the gibberellin response of plants is related to 

 the auxin state of the particular plant, plant organ, or tissue to which 

 it is applied. Vlitos and Meudt (12) have emphasized this by con- 

 trasting the responses obtained with intact plants and isolated plant 

 parts. Intact plants respond more to applied gibberellins, whereas 

 the auxin response is exaggerated in isolated plant parts. In addition 

 to the effect of isolation from the parent plant, the tissue culture 

 responses discussed here are obscured somewhat by the presence of 

 plant hormones in many of the media. Moreover, the tissues on media 

 that lack such hormones are, with one exception {Melilotus root. 



Table 5. The effect of gibberellin and 2,4-D on the 

 growth of avocado and broad bean tissue cultures in White's 

 medium plus coconut milk. 



* Ratio of fresh weight at end of test over initial fresh 

 weight. 



