dC Ai\i\ALES DE L'INSTITUT PASTEUIi 



lelt iiilact while tlie mass of thft Jeaves of the second set was 

 reduced by cuUing pièces oui from the middlo ot the leaf. It 

 was foiind that the rate of geotropicbending in the latter leaves , 



was considerably slower than in the leaves with larger mass 

 (Fig. 3). No matter in which way the mass of the leaf was 

 reduced tlie rate of bending varied in the same sensé as the 

 mass of the leaf. Fig. 3 is a photographie reproduction of 

 such an experiment. The leaves of the stems on the left side 

 were intact, the mass of the leaves of the stems on the right 

 side of the photograph was diminished by cutling olî the front 

 part of the leaves. Both sets of stems were straight at the 

 beginning of the experiment and both sets were suspended | 

 simultaneously. The stems on the left, with the larger leaves, 

 are bent more strongly than those with smalier leaves on the 

 right. We hâve seen in the first part of this paper that the j 

 shoot production in a leaf varies with its mass. The geotropic j 

 curvature of a stem of Bryophylhnn also varies with the mass 

 of the apical leaf attached to it, no matter which part of the 

 leaf is eut off. This supports the idea that the geotropic cur- 

 vature of the stem occuis at the expense of that material which | 

 exists or is formed in the leaf and serves for the formation oi 

 shoots by the leaf when the latter is detached from the stem. | 



We can now anderstund why « régénération » occurs when | 

 the leaf of Bryophyllum is separated from the plant and why it \ 



cannot occur as long as the leaf forms part of the whole ! 



I 

 organism. 



The writer is under the impression that further quantitative 

 experiments will put (he problem of régénération into the ] 

 category of the phenomena of nutrition or mass action. Rege- j 

 neration appears mysterious only as long as we do not perceive | 

 or measure the change in the distribution of the masses of the 

 nutritive material, which must occur when a part is eut ont 

 from a whole organism. 



