CHAPTER XIX 

 CONCLUDING REMARKS 



It is the purpose of this httle volume to show that a simple 

 mass relation can be used as a guide through the bewildering 

 maze of the phenomena of regeneration. This mass relation 

 states that equal masses of isolated sister leaves produce, under 

 equal conditions of illumination, temperature, etc., approximately 

 equal masses of shoots and roots in equal time. With this relation 

 it was possible to explain why isolation of a leaf leads to regenera- 

 tion; namely, because the material available in the leaf for growth 

 flows normally into the stem where it is used for the growth of 

 roots, shoots, and of the stem itself. When a piece of stem 

 inhibits the regeneration in a leaf the stem gains in dry weight to 

 an amount about equal to the diminution in the dry weight of 

 shoots and roots in the leaf due to the stem. It was shown that 

 this mass relation holds also for the formation of roots and shoots 

 in an isolated piece of stem. Equal masses of stems produce 

 under equal conditions of illumination, temperature, etc., 

 approximately equal masses of roots and shoots in equal time. 



In both cases a second physiological factor was revealed which 

 must be considered ; namely, that the flow of sap in a leaf or stem 

 is secondarily directed towards that part of a leaf or stem where 

 the more rapid growth of shoots or roots occurs. This explains 

 why only some but not all of the anlagen for roots or shoots in 

 a leaf or stem will persist in growing out; while the growth of the 

 others will stop. This again would have remained merely a 

 hypothesis had it not been possible to prove it quantitatively on 

 the basis of the mass relation. 



A further complication arises in the fact of the polar character 

 of the regeneration of roots and shoots in the stem of Bryophyl- 

 lum. Two possibilities presented themselves for the explanation 

 of this phenomenon: the polar character of the regeneration in 

 the stem is either due to a difference in the chemical character of 

 the ascending and descending sap or to a difference in the nature 

 of the cells or anlagen which are primarily reached by the ascend- 

 ing and descending sap. It was possible to decide between the 

 two hypotheses by using the mass relation, inasmuch as it could 



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