Concluding Remarks • 97 



One further study on vegetative growth should be considered 

 since it bears comparison with the quantitative yet long-lived 

 induced state which seems so puzzling in Xanthium. The reader 

 whose sensibilities were disturbed by "flowering as a virus disease" 

 will have to make the best of another similar analogy, this time to 

 the plant disease crown-gall. In many ways resembling cancer in 

 animals, crown-gall is brought about by a bacterium; following 

 infection, the tissues become tumorous, growing rapidly in a 

 disorganized fashion, and continue to do so even when the bacteria 

 are no longer present. Pieces of such bacteria-free tissue grow 

 rapidly in culture on a simple mineral medium with sucrose and 

 a few vitamins, whereas normal callus tissue from the same plant 

 fails to grow under the same conditions. Braun (1958) has been 

 able to make a whole series of tissue clones intermediate between 

 typical crown-gall and typical normal tissues in their growth rate 

 on the basic medium. This was done by letting the bacterial infec- 

 tion proceed for different lengths of time before a heat treatment 

 that stops it without harming the tissue. In order to make normal 

 tissue grow as fast as fully tumorous crown-gall tissue in culture, 

 one must add to the basic medium 6-furfuryl amino purine, 

 guanylic and cytidylic acids, asparagine, glutamine, inositol, and 

 naphthaleneacetic acid. If the tissue has been exposed to infection 

 for a short time, the first compound may be omitted; if it has been 

 exposed for a longer time, the first four may be omitted, without 

 reducing the rate below that of the fully tumorous tissue. 



Each strain of tissue maintains its particular nutritional re- 

 quirements in culture and does not revert to normal. Braun con- 

 cludes that "a series of quite distinct, but well-defined, growth- 

 substance-synthesizing systems becomes progressively activated" 

 during the crown-gall induction. In short, a quantitative gradation 

 exists as a result of several qualitative changes in metabolism. 

 Perhaps photoperiodic induction in some plants is a process of this 

 kind, with many intermediate stages, and not a unitary process 

 at all. 



With such work as background one might envision florigen 

 as either a single substance, or a combination of substances, 

 normally occurring in many plant cells, but frequently present 

 in insufficient quantities or improper balance for the meristem 

 to proceed to reproductive development. If production in another 



