154 THE FLOWERING PROCESS 



period of time involved surely does not sound like a nervous impulse, 

 and it would be quite amazing if the results could be explained on 

 the basis of nutritional substances. Again we are left with the idea 

 of a hormone, as first suggested by the Russian, Chailakhyan. 



A striking aspect of the experiment is that transmission through 

 the graft union will only occur if there is an actual living contact 

 formed between the cells of one plant and those of the other. If the 

 plants are separated by any boundary which prevents living union, 

 the experiment fails even though diffusion of substances through the 

 boundary is possible. The question of why the flowering hormone 

 should move only through living cells has yet to be answered. 



Many experiments have been done in which plants of different 

 response types were grafted to each other to see if the products of 

 induction in one type would cause another type to flower. There 

 have been a few failures, but the number of successes is quite 

 astonishing. Thus induced long-day plants will cause vegetative 

 short-day plants to flower on long days, and vice-versa. Day-neutral 

 plants will cause day-length sensitive plants to flower. Present 

 evidence seems to indicate that the end product of induction, the 

 flowering hormone, is at least physiologically the same in virtually 

 all response types. 



3. Translocation Rate of Flowering Hormone out of the Leaf may 

 be Measured 



Short-day plants which respond to a single night are given a long 

 dark period. Following this treatment one representative group has 

 its leaves removed. A few hours later leaves are removed from another 

 group. This goes on with different groups of treated plants (at 

 perhaps 4-hr intervals) for a few days following the long dark period. 

 When the plants are examined about 9 days after the original dark 

 period, it is found that plants which had their leaves removed 

 immediately following the dark period are quite vegetative. Plants 

 with leaves removed 35 hr or so after the end of the dark period 

 flower nearly as well as plants which never had their leaves removed. 

 Plants defoliated at intermediate times flower to an intermediate degree. 

 This is shown for three experiments with cocklebur plants in Fig. 9-1. 

 It may be possible to imagine other explanations, but surely the 

 logical one is that when leaves were removed immediately following 



