CHAPTER XIII 



CALLUS FORMATION 



In order to explain the inhibitory effects of the descending sap 

 on shoot formation in its path in the way suggested at the end of 

 Chapter XII, it would be necessary to show first that the descend- 

 ing sap causes a growth in the stem which the ascending sap 

 does not produce. The callus formation can be used for this proof 

 since it occurs only at the basal, never at the apical end of a 

 stem of Bryophyllum. It can be shown that there is a parallelism 



^4l3 



Fig. 84. — Influence of leaf on callus formation in stem. Stem a, with leaf, 

 forms callus, while stem b, without leaf, does not during the same time. 

 Nov. 15 to Dec. 12. 



between the conditions which inhibit shoot formation and those 

 which favor callus formation in a stem. 



A small piece of stem without leaf forms little or no callus while 

 at the same time a piece of stem of the same mass with a leaf 

 attached to it forms considerable callus (Fig. 84). In this experi- 

 ment the piece of stem without leaf had even the advantage of 



109 



