402 



PLANT PHYSIOLOGY 



at this end, though the force of gravity will act under these 

 circumstances in the opposite direction (Fig. 124). When sec- 

 tions of axial organs react differently at their two morpho- 

 logically opposite ends, the phenomenon is known as ''polarity." 

 Polarity manifests itself clearly in the distribution of roots 

 and the development of buds of a cutting. If a cutting is sus- 

 pended in normal position in a humid atmosphere, the roots will 



be formed largely at its lower end, 

 and the shoots will be produced from 

 buds situated at its upper end. But 

 when the shoot is suspended in an 

 inverted position, the roots will never- 

 theless rise from the morphologically 

 lower end, though it will now be at the 

 top; while the shoots will develop at 

 the morphologically upper end, now 

 at the bottom. If the center of the 

 cutting is girdled, the effect will be the 

 same as that of the division of the cut- 

 ting into two separate parts. This 

 leads to the conclusion that polarity 

 is probably caused by the accumula- 

 tion of some substances that move in 



sus^P^ndef iT'Tu^M "^^ the bark in a definite direction and 

 phere. On the left, in normal that stimulate at the point of their 

 r^ettd po'^sHio'!:: shot„J"po- Concentration the development of 



larity in callus formation {after both the Callus and the rOOts. Sachs 



' '^'^^^ '• has named these ''rootforming sub- 



stances." Some recent investigations, especially those by Loeb, 

 have led to the conclusion that these are hormones. According 

 to Loeb, the root-forming hormones originate in the leaves, 

 whence they descend through the stem. As long as a branch 

 is attached to the tree, no formation of roots takes place; but 

 if the vessels through which the downflow of these hormones 

 proceeds are cut, then the latter will accumulate at the surface 

 of the cut and cause the formation of roots. 



If a willow cutting is ringed, i.e., if a ring of bark about 1 to 

 2 cm. wide is removed, and the cutting is then placed in water 

 in such a way as to leave the ringed part in a moist atmosphere, 

 the adventitious roots w^U develop much better and in greater 



