CHAPTER VIII 



THE ROLE OF THE INHIBITING EFFECT OF RAPIDLY 

 GROWING PARTS IN THE REGENERATION OF THE STEM 



The preceding chapter has shown that the mass relation deter- 

 mines the quantity of regeneration in a piece of defohated stem. 

 When roots grow only at the basal and shoots only at the apical 

 end of the piece it follows that all the material available for 

 shoot and root formation in the stem must collect at the ends of 

 the piece; since only in this way can it happen that the total 

 amount of shoot formation at the apex of a long piece of stem is 

 in direct proportion to the mass of the stem. 



The polar character of the regeneration is in a stem of Bryo- 

 phyllum not the direct and primary phenomenon which it is 

 generally assumed to be, but is more complicated. Roots and 

 shoots can appear in all nodes of a piece of stem and the fact 

 that finally roots are left only at the base and shoots only at the 

 apex is due to a secondary effect; namely, to the fact that the 

 more rapidly growing shoots or roots inhibit the growth of 

 the more slowly growing shoots or roots. This phenomenon of 

 inhibition has already been described for the leaf where the rapid 

 growth of shoots in one part of the leaf causes the flow of all the 

 sap of the leaf to that part, thereby inhibiting the growth in the 

 other notches. 



Stems with two leaves at the base were suspended vertically 

 in moist air (Fig. 43). Within 5 days roots and shoots com- 

 menced to form, but the root formation was not confined to 

 the base appeared in all the nodes (1 to 4, Fig. 43), with the 

 exception of the most apical node; and shoots began to form not 

 only in the most apical node but also in the node 1 below. 

 There is little suggestion of a polar character of the regeneration 

 in Fig. 43. Six days later (Fig. 44) the most apical shoots had 

 grown more rapidly than the shoots in the nodes below, the growth 

 of which was now completely inhibited. Furthermore, roots 

 began to grow at the extreme base of the stem. These latter 

 roots grew more rapidly and in greater numbers than the other 



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