CALLUS FORMATION 



113 



first node below the leaf and only after more than a month's 

 delay a shoot may arise in the axil of the leaf. But instead a 

 vigorous callus is formed at the base of the stem. This result, 

 which is typical, suggests that the greater part of the material 

 sent down by the apical leaf is consumed in callus formation. 

 Had a shoot been formed in the stem this callus formation would 

 have been less. 



Fig. 89. — Stem with an apical leaf in which shoot formation is suppressed forms 

 large amount of callus. Oct. 16 to Nov. 14, 1923. 



The formation of callus may be considered as the result of the 

 growth of certain tissues of the stem under the influence of the 

 descending sap. It is possible that these tissues are also those 

 which give rise to roots and perhaps to the growth in the length 

 and possibly thickness of the stem. We can understand that 

 this growth of the stem might indirectly inhibit the growth of 

 shoots in the same part of the stem, though for the explanation 

 of the mechanism of such an inhibition further experiments will 

 be necessary. 



