OTHER ACTIVITIES OF AUXINS 219 



downward direction, no effect being observable above the 

 leaf. In 1910, when phytohormones were not yet recog- 

 nized, Keeble ascribed this transmission of cambial activity 

 to "chemical stimulators," while the first suggestion that 

 the activation is due to a true hormone— produced in grow- 

 ing parts and transmitted in the morphologically down- 

 ward direction — was made by Kastens (1924). 



Subsequently, in an extensive survey of cambial activity 

 in tropical trees. Coster (1927, 1928) came to the conclusion 

 that the young developing buds, and to a lesser extent the 

 leaves, produce hormones which activate the cambium. 

 These hormones may even be produced shortly before the 

 first visible signs of bud development can be detected. In 

 general it is observable that in trees the cambial acti\aty 

 in the branches begins in spring at about the time the buds 

 begin to develop. Snow (1932) mentions that growing in- 

 florescences and fruits of Agrimonia, Spiraea, and Scrophu- 

 laria stimulate the activity of cambium below them, and 

 Priestley (1930) ascribes the same effect to the flowers, 

 buds, and growing leaves of Fraxinus. Gill (1933) finds 

 that the inflorescences of Populus serotina and Salix caprea, 

 which are within the bud at the beginning of the spring, 

 activate the cambium immediately below them as they 

 expand in spring; trees whose catkins are exposed through- 

 out the winter show no such effect in spring. 



Experimental evidence that cambial activation is due to 

 a hormone was first brought by Snow in 1933. His experi- 

 ments were apparently suggested by the work of Simon 

 (1930) who found that, in grafting unrelated plants, vessel 

 differentiation appeared to be stimulated in the neighbor- 

 hood of the graft before the tissues had actually grown 

 together. Snow therefore split longitudinally the stems 

 of a Pisum and a Helianthus seedling for several cm., and 

 severed half the Pisum stem at the lower end of the split, 

 and half the Helianthus stem at the upper end. The down- 

 ward-pointing half of the former was then brought into 

 contact with the upward-pointing half of the latter, the two 



