316 



Morpho genetic Factors 



Fig. 13-4. Above, long-day plant, Nicotiana sylvestris. Below, short-day plant, Nico- 

 tiana tahacum, var. Maryland Mammoth. At left, under long days; at right, under short 

 days. ( From Melchers and Lang. ) 



young but fully developed leaves. Evidence indicates that a flower-form- 

 ing substance is produced in these which then diffuses through living 

 cells to the meristem and there stimulates the formation and growth of 

 floral primordia (p. 397). This substance can be transmitted by grafting 

 from a plant in flower to one which is not, and the latter plant, even 



