CHAPTER IX 



THE INFLUENCE OF THE LEAF ON THE REGENERATION 



IN THE STEM 



Not every case of regeneration is polar in character. The 

 heahng of a wound is not a polar phenomenon; nor is the 

 formation of roots and shoots in an isolated leaf of Bryophyllurn. 

 The final distribution of roots and shoots in a defoliated piece of 

 stem of Bryophyllum suspended vertically and upright in moist 

 air is, however, a pronounced example of the polar form of regen- 

 eration. It is our purpose in this second part of the book to 

 find out to what extent the mass relation can aid us in the investi- 

 gation of the cause of polar character of regeneration in the stem 

 of this plant. 



There is little doubt that the formation of roots at the base and 

 of shoots at the apex of a defoliated piece of stem is determined by 

 the descending and ascending sap respectively, since the quantity 

 of both root and shoot formation increases with the mass of the 

 stem, provided that the stem is exposed to light. 



The question is then: What determines the difference in the 

 action of the descending and ascending sap? There are, as far 

 as the writer can see, two possibilities. The polar character of 

 the regeneration in the stem is either due to chemical differences 

 in the ascending and descending sap; or the polar character of the 

 regeneration is due to the fact that the roots and shoots do not 

 originate from the same type of cells or anlagen and that the cells 

 or anlagen for root formation are reached exclusively or prefer- 

 ably by the descending sap, while the anlagen for shoot formation 

 (two of which exist in each node) are reached exclusively or pref- 

 erably by the ascending sap. Either of these two assumptions 

 might explain the polar character of regeneration in the stem of 

 Bryophyllum from the point of view of the mass relation. 



There can be no doubt that the ascending and descending sap 

 in a normal plant growing in the soil are chemically different. 

 The root takes up the nitrogen from the soil in the form of 

 KNO3 or NH3 and KNO3 is reduced to the nitrite in the root or 



67 



