246 



TEXTBOOK OF PLANT PHYSIOLOGY 



swelling or callus (Fig. 99), while no annual rings will be formed 

 below the cut. Finally, if the trunk of a tree is ringed, it will 

 remain fresh for a long period, because the water rises through 

 the wood, which is not injured by the ringing. But, in the end, 

 unless this ring heals over, the tree will die, since the ring has 

 broken the path of movement of nutritive substances to the roots, 

 the activity of which will soon stop. It is worth noting that in 



Fig. 98. — A ringed twig of willow 



showing root development primarily 



above the ringed portion. 



Fig. 99. — A branch showing develop- 

 ment of callus at the upper edge of 

 the ringed region (after Brown). 



plants having sieve tubes, not only near the outside, but also on 

 the inside of the xylem ring (Oleander), experiments with ringing 

 have given much less definite results, since the translocation of 

 organic substances along the inner phloem will continue after the 

 cutting of the bark. 



Ringing experiments, first conducted by Hales in the seven- 

 teenth century, have led to the established view that substances 

 move along two paths in a plant, an ascending and a descending 

 one. The upward flow proceeds through the wood and carries 

 with it inorganic salts, which are obtained by the roots from the soil. 

 The downward flow takes place through the bark and moves with it 



