THE PHOTOPERIODIC PROCESS 247 



long-night treatment of the single leaf and flowers systemically (Ham- 

 ner and Bonner, 1938). This also holds true for long-day plants 

 C Knott, 1934). Since it is the bud that flowers in response to photo- 

 periodic treatment of the leaf, it is obvious that there must be some 

 transmission of stimulus between leaf and bud. This stimulus could be 

 either something that promotes flowering and travels from leaf to bud. 

 or something that inhibits flowering, which is normally stored in the 

 bud and which is somehow absorbed by the leaf as the result of photo- 

 periodic treatment. It is now generally assumed that in short-day plants 

 something that promotes flowering is produced by the leaf as the result 

 of appropriate photoperiodic treatment and then travels from leaf to 

 bud. We know some of the characteristics of the transport of this 

 stimulus (Moshkov, 1937; Kuiper and Wiersum, 1936). Thus we 

 know that transport of the flowering stimulus takes place only in 

 living tissue. It may travel either up or down the stem, and it moves 

 in general in the direction of flow of photosynthate, from photo- 

 synthesizing leaves to growing, photosynthate-consuming organs 

 (Stout, 1945). For this peripatetic stimulus the name "florigen" was 

 proposed by Cajlachjan in 1936, and this is the term we use today. That 

 the florigen which is produced in long-day plants as the result of 

 exposure of their leaves to long days is physiologically identical with 

 the florigen produced by short-day plants as the result of exposure of 

 their leaves to long nights has been shown by reciprocal grafting 

 experiments summarized by Lang (1952). Thus, if we graft together 

 the long-day plant Hyoscyamus niger and the short-day plant Mary- 

 land Mammoth Tobacco, and if we keep both plants on long days, the 

 Maryland Mammoth Tobacco flowers in response to something which 

 it receives from the Hyoscyamus donor (Lang and Melchers, 1948). 

 Reciprocally, if we keep both partners on short days, the Hyoscyamus 

 flowers in response to something which it receives from the Maryland 

 Mammoth Tobacco donor (Melchers and Lang, 1941). Similar graft- 

 ing experiments have been carried out with the long-day Callistephus 

 and the short-day Xamhium (Thurlow, 1948). 



The photoperiodic response is in a sense a qualitative one in short- 

 day plants. Each bud and each plant is either reproductive or vegeta- 

 tive. And in this sense the photoperiodic response is all or none. But in 

 another sense the response of plants to photoperiodic treatment is a 



