56 THE FLOWERING PROCESS 



results have now been obtained with a few species, but often this 

 experiment fails. 



In virtually all of these grafting experiments, the donor plant was 

 flowering at the time the graft was made. In such cases it is possible 

 that the product of the cold treatment was further elaborated into 

 some final flowering stimulus such as the hormone produced in 

 response to day-length (see Chapter 9), which will readily pass a 

 graft union. Melchers performed one experiment, however, which 

 cannot be interpreted in this way. He made a vegetative biennial 

 henbane plant flower on long days, by grafting onto it a vegetative 

 short-day tobacco plant (Maryland mammoth). The implication is 

 that vernalin is a necessary precursor (in a physiological sense, at 

 least) of the flowering hormone, and that flowering in henbane is 

 blocked because this material is not present until after a cold treat- 

 ment, while in tobacco the vernalin is present, but short-days are 

 required to elaborate it further into the flowering hormone. It is a 

 neat explanation, but we require more biochemical knowledge of 

 what is going on before it can be accepted without reservation. 



7. Substrate Requirements 



A number of experimenters have demonstrated the need for 

 oxygen in the vernalization process. De vernalization, on the other 

 hand, can be carried out in a pure nitrogen atmosphere. Plants are 

 converted back to the unvernalized condition, but they may not be 

 damaged in any other apparent way. 



The English workers have carried out a number of experiments 

 with embryos from rye seeds, separated from the rest of the seed. 

 These embryos may be cultured on an agar medium and their 

 nutrient requirements can thus be investigated. Such excised embryos 

 will respond to the vernalization treatment on a relatively inert 

 medium (containing the essential mineral nutrients but no sugar), 

 although the rate and extent of response is reduced. However, if the 

 medium is supplied with a sugar such as sucrose, the response is 

 delayed about 2 weeks but is otherwise similar to the response of 

 intact seeds and seedlings (see Fig. 4-2). A number of sugars have 

 been used and many of them are eff'ective (for some unknown reason, 

 certain ones are not). The details become involved, but it does seem 

 clear that sugar or other carbohydrates, present in the storage parts 



