Regeneration 163 



plant, while if the leaf is not cut into pieces, and a few 

 notches only grow out, their growth is much more rapid. 

 In all these experiments the idea that the "isolation" 

 in itself is responsible for the growth still presents itself. 

 It can be disposed of by the following experiment which 

 never fails. Three leaves of Bryophyllum calycinum 

 are suspended in an atmosphere saturated with water 

 vapour but their tips are submersed in water (Figs. 

 1 8, 19, 20). The first leaf, Fig. 20, is entirely separated 

 from its stem, the second leaf, Fig. 19, remains connected 

 with the adjacent piece of stem, and the third leaf, 

 Fig. 1 8, remains also connected with this piece of stem 

 but the latter still possesses both leaves. The first leaf, 

 Fig. 20, produces new roots and shoots in the submerged 

 part in a few days; the second leaf, Fig. 19, produces 

 no roots or shoots for a long time. This might find its 

 explanation by the assumption that the first leaf, being 

 more isolated than the second, regenerates more quick- 

 ly. But this explanation becomes untenable owing to 

 the fact that the third leaf, Fig. 18, being less isolated 

 than both (possessing a second leaf in addition to 

 the stem), forms new roots and shoots also more 

 quickly than the second leaf. The phenomena become 

 intelligible in the following way. The fact that in the 

 second leaf shoots and roots are formed very late, if at 

 all, finds its explanation not in the lessened isolation 

 of this leaf, but in the fact that the formation of a 

 new shoot or of a callus in the piece of stem takes place 



