220 PHYTOHOHMONES 



halves bound nrmly together, and after 22 days the region 

 of contact was sectioned and examined. Many cambial 

 layers had formed in and between the bundles in the Helian- 

 thus hypocotyl, although the tissues had not (with a few 

 exceptions) grown together. Controls not in contact showed 

 no such effect, so that the stimulus from the Pisum seedUng 

 had passed across a protoplasmic discontinuity to act upon 

 a plant of quite another family. In other experiments the 

 upward- and downward-pointing portions were both from 

 the same plant, Vicia Faba, but the stimulus passed across 

 a piece of linen inserted in a cut in the stem. 



Snow and Le Fanu (1935) then found that the cambium 

 in the Helianthus hypocotyl may be activated by applying 

 urine, or the ether extract of urine, and finally Snow (1935, 

 1935a) obtained excellent activation by pure auxin a and 

 indole-3-acetic acid. As stated above, in high concentra- 

 tions auxin may cause cell division in a variety of tissues, 

 but in this case it is of considerable importance that the 

 activation is brought about by low concentrations of auxin 

 comparable with those obtaining in the normal plant. 

 Thus, comparing his results with the auxin determinations 

 of Thimann and Skoog (1934), Snow found that moderate 

 cambial activity is induced by an amount of auxin a about 

 2.5 times that produced hourly by the terminal bud of 

 Vicia Faba. There can thus be no doubt that the auxin 

 formed by buds and leaves is responsible for the cambial 

 activation below them; the polar movement of this cambial 

 stimulus is then due to the polar transmission of the auxin. 

 In Snow's experiments cambium was activated for 2-3 cm. 

 below the point of application. The exact tissue in which 

 the auxin travels in such structures as stems has not yet 

 been determined; Cooper's experiments (1936) indicate 

 that it must be Hving tissue, possibly the cambium itself. 

 The stimulus does not seem to travel very far, although 

 the calculations and observations of Biisgen-Mlinch (1929) 

 indicate that it takes several weeks to travel from the 

 growing buds of trees down to the trunk, which would give 



