THE STEM 179 



various organs of the plant move through the I'ind. 

 But the rind, as we have already seen, presents a 

 complicated structure ; we differentiate in it the 

 primary and secondary rind ; by way of which of 

 these two systems does the nutrient sap move ? 

 Again let us repeat our girdling experiment ; but 

 this time we carefully cut only through the external 

 part, the bark proper, being careful not to injure the 

 secondary rind, i.e. the bast of the vascular bundles. 

 We get results similar to those of the former experiment, 

 i.e. roots are formed at the base of the branch. This 

 proves that the movement takes place by way of the 

 secondary rind. Let us venture one step further, and 

 determine by which elements of the secondary rind this 

 sap moves. We know that they are mainly two in 

 number : bast fibres and sieve-tubes. A comparison of 

 the mere form of these two elements makes it probable 

 that the latter fulfil the functions we are considering, 

 because the fibres have very thick walls with almost 

 no cavities, whereas the sieve-tubes have broad canals, 

 communicating with each other by means of open pits, 

 through which not only liquid and semi-liquid substances 

 but even minute grains of starch can pass. This 

 probability changes into certainty after the following 

 experiment. We take an oleander branch and manipu- 

 late it in the same way as we manipulated the willow 

 branch in our second experiment, i.e. we cut off a 

 complete ring of the bark right down to the cambium. 

 A wholly unexpected result follows. Roots are formed 

 not only at the edge of the cut, but also at the base of 

 the branch — it follows that the nutrient substances 

 pass otherwise than by way of the rind. This apparent 

 contradiction is fully explained when we learn that the 

 stem of the oleander shows a deviation from the typical 

 structure of the stem above described. Besides the 

 sieve-tubes in the bark, bundles of these elements are 

 also found in the pith ; and it is these which, in spite of 



