4Q 



BOTAXICAL GAZETTE 



[JANUARY 



It is highly probable, if not certain, that the increase in length 

 on the lower side of the horizontally placed stem takes place pri- 

 marily in the cortex of the bending region and not in the pith 

 or wood. This follows from the behavior of these 2 parts when 

 the cortex of a bent (split or whole) stem is removed, and the 

 rigidity of the cortex is compared with that of the pith and 

 wood taken out. 



TABLE II 





Stem I 



Stem II 



Stem III 



Stem IV 



Region of stem 

 measured 



Begin- 

 ning of 

 experi- 

 ment 



End of 

 experi- 

 ment 



Begin- 

 ning of 

 experi- 

 ment 



End of 

 experi- 

 ment 



Begin- 

 1 ning of 

 experi- 

 ment 



End of 

 experi- 

 ment 



Begin- 

 ning of 

 experi- 

 ment 



End of 

 experi- 

 ment 



A : non-bent api- 

 cal part 



B: bent central 

 part 



3.0 cm. 



4.0 



2.0 



3 . 2 cm. 



4-9 

 2.0 



3.0 cm. 

 3-0 



3.0 cm. 



5 7 

 3.0 



4.0 cm. 

 6.0 



5.0 



4.0 cm. 



7.0 



50 



4.0 cm. 

 4.0 



4.0 



4.1 cm. 

 4.85 



C: non-bent 

 basal part .... 



415 



If we remove the cortex on the lower (convex) side of a split 

 geotropically bent stem, like that in fig. 17, we find that the rigidity 

 of the cortex in the bent region is much greater than that of the 

 wood or pith ; the latter appears soft in comparison with the cortex 

 of the bent region on the convex side of a geotropically bent stem. 

 It is possible also that the increase in the rigidity of the cortex in 

 this region may be due to a thickening of the cortex, a point which 

 needs further investigation. Whatever the cause of this increase 

 in rigidity may be, we reach the following conclusion regarding the 

 mechanism of the geotropic bending of a horizontally suspended 

 stem of Bryophyllum calycinum. 



On the lower side of such a stem in a region the location of 

 which depends upon the presence or absence, and, in the former 

 case, upon the location of the leaf in the stem, the cortex begins to 

 grow in length (and possibly in thickness). The wood, pith, and 

 cortex on the upper side undergo no such growth. This increase 

 in length (in one region) of the cortex on the lower side leads to 

 a bending of the stem in which the lower side of a horizontally 

 suspended stem becomes convex, the upper side concave. 



