CHAPTER VII 



THE VALIDITY OF THE MASS RELATION FOR REGENER- 

 ATION IN A DEFOLIATED PIECE OF STEM OF 

 BRYOPHYLLUM CALYCINUM 



1. Experiments on Short and Long Pieces of Stem. — In experi- 

 ments on the stem we encounter the second problem of regener- 

 ation; namely, its polar character. Regeneration has not 

 infrequently a polar character; that is, different organs are formed 

 at the opposite poles of a fragment from a mutilated organism 

 (Figs. 5, 7, 9 and 11). While the polar character of regeneration 

 is not uncommon, it is not universal. Thus in the case of regen- 

 eration in the leaf of Bryophyllum we do not notice that shoots 

 arise from the notches at one end of the leaf while roots arise 

 from the notches at the opposite end, but we notice that both roots 

 and shoots arise always from the same notch, the formation of 

 roots preceding that of shoots. It is quite possible that one side 

 of the embryonic tissue of the notch (possibly the dorsal) gives 

 rise to the root and the other (possibly the ventral) to the shoot, 

 and if this is true it is also a case of polar regeneration. The polar 

 character of regeneration in a defoliated piece of stem of Bryophyl- 

 lum is, however, different since shoots and roots originate at a 

 great distance from each other, shoots growing from the most 

 apical node of the stem and roots from the base (Fig. 35). The 

 problem is, What causes this specifically polar character of regen- 

 eration in the stem? Bonnet and Sachs^ tried to explain this 

 polarity on the assumption of a chemical difference between the 

 ascending and descending sap; the latter being supposed to con- 

 tain specific root-forming svibstances and the former specific 

 shoot-forming substances. These specific organ-forming sub- 

 stances are as yet purely hypothetical though they may actually 

 exist. If they exist they must be present everywhere in the sap of 

 the leaf since every notch of an isolated leaf can give rise to both 

 roots and shoots. We shall not deal with the problem of polarity 



1 Sachs, J.: Stoff und Form der Pflanzenorgane, Arbeiten d. hot. Inst, in 

 Wurzburg, vol. ii, pp. 452, 689, Leipzig, 1882. 



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